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There is controversy in the American horse racing industry after a handful of trainers face severe fines and suspensions after their horse(s) tested positive for metformin. The frequency of metformin positives has increased since HISA/HIWU has been acting as the governing body of drug testing and welfare in horse racing (https://www.hiwu.org/cases/pending) over the last year. All trainers claim they did not purposely administer metformin to their horses, affirming the positives must have been due to environmental contamination from the handlers of the horses. This article will cover what metformin is, how it may impact racehorse performance and recovery from exercise, and how it is metabolized in the horse which may result in a positive drug test. What is Metformin? Metformin is a human drug that is prescribed to treat Type II diabetes/metabolic disorder by helping control high blood sugar. Its mechanisms to reduce high blood sugar include reducing the absorption of sugars/carbohydrates in the intestines and reducing the creation of sugar/glucose in the liver. Metformin may help weight loss due to these mechanisms. Over 90 million people are prescribed metformin in the United States, as it is currently the primary treatment for type II diabetes. In horses, metformin has been used to treat metabolic syndrome and prevention of laminitis. This drug may not be as effective in horses as it is in humans because the absorption of metformin in the equine intestines is poor. There are mixed results as to how effective metformin is to increase insulin sensitivity in horses with metabolic dysregulation (Tinworth et al. 2012; Hustace et al. 2009; Durham et al. 2013; Colmer et al. 2024). Metformin is most commonly given orally to horses and humans. However, an injectable version of metformin may also be used. Is Metformin actually performance enhancing? There is some evidence to support that metformin can enhance acute athletic performance. In Learis et al. (2015), “fit” male subjects were given 500mg of Metformin (on the smaller side of a typical daily dosage of this medication for those with metabolic disorder) 1 hour before a supramaximal exercise test to exhaustion on a cycle ergometer (a stationary bike). “Supramaximal” means at exercise intensities above someone’s VO2max, or maximum oxygen consumption. In this case, the exercise intensity was 110% of the intensity that elicited each subject’s VO2max and the subjects had to maintain that intensity for as long as possible (a very painful exercise test). This intensity is actually pretty comparable to the intensities thoroughbred racehorses may elicit during 5-8 furlong races depending on the horse’s fitness. Every subject did this test- one with metformin and the other a placebo, in a blinded-randomized order. During the supramaximal test to exhaustion, every single one of the subjects were able to exercise 24 seconds longer on average after taking Metformin compared to a placebo for the exercise test to exhaustion. Statistically, this difference was very strong (p = 0.001), especially because all ten of the subjects in this study were able to exercise longer after taking Metformin vs the placebo (for my stats nerds out there, the Effect Size was 0.70, pretty high for a sample size of 10 subjects). Also keep in mind these trials were randomized, meaning some subjects did these tests with Metformin first, others did placebo first; this is to reduce the bias of the study, strengthening its results. Overall, this study means Metformin may in fact enhance performance in intense, short duration exercise– very similar to the exercise intensities and duration thoroughbred racehorses endure for racing. Researchers theorize this phenomenon is due to metformin’s potential to enhance the alactic (phosphocreatine) energy system. This study also found metformin had no effects on the aerobic system, contrary to other performance enhancing drugs like EPO (a blood builder). Another study performed in rats found very similar results, finding that time to exhaustion significantly increased after the rats were given a very high dosage of metformin for 10 days compared to a placebo (Araujo et al. 2020). Metformin and Fitness Gains Whereas metformin has been shown to enhance short term performance in prolonged “sprint-like” exercise tests, repeated use of metformin in the long term may inhibit training adaptation or fitness gains. Prolonged metformin use (1,700mg/day) has been shown to blunt muscle growth and strength gains in older adults performing a strength training program due to its ability to reduce post exercise inflammation (Walton et al. 2019) (click here to read why post-exercise inflammation is important to stimulate fitness gains). This has been shown for cardiovascular fitness as well, with Moreno-Cabenas et al. (2022) finding that metformin reduced VO2max (aerobic capacity) gains by almost half compared to a placebo treatment following a 16 week long high-intensity interval training program (a common training program that has already strongly proven to significantly increase aerobic capacity in the exercise science field for all specimens). Those taking the placebo enhanced their aerobic capacity by 25.3% after 16 weeks, while those on metformin only increased their aerobic capacity by 12.7%. This effect metformin has on inhibiting training adaptation is similar, albeit not as severe, as Statins (a common medication to improve cholesterol). Now lets interpret all of this from a racehorse training-performance perspective. Metformin may enhance exercise performance during short-high-intensity exercise, similar to what thoroughbred racing demands. However, metformin may also inhibit fitness gains if taken continually in the long term. This does not mean an older racehorse that has been racing and training for several months or years will suddenly become less fit after taking metformin. Metformin may not reduce fitness that is already obtained. But, metformin may make it a lot harder for a horse to gain fitness when coming back from a layup or during the early stages of their training/racing. Metformin and Exercise Recovery Inflammation To add from the previous section, for racehorses that are older and have been racing/training for several months/years and are just trying to maintain their current fitness, metformin may be beneficial to help not only enhance acute racing performance, but also enhance recovery post exercise. As previously mentioned, the mechanism behind why metformin inhibits exercise gains is because it reduces the inflammatory response exercise causes (Khodadadi et al. 2022). Damage from exercise causes inflammation, and inflammation stimulates the body to recover and adapt to exercise to get stronger. However, if inflammation is artificially reduced (such as with metformin), exercise stimuli is reduced and so are the fitness gains. For horses that are already near their peak fitness and are simply trying to maintain that fitness, the anti-inflammatory effects of metformin may allow those horses to bounce back from races and workouts faster in order to train and race more frequently. Glycogen Replenishment Metformin may also enhance muscle recovery through its enhancement of GLUT-4 (Lee et al. 2012). GLUT-4 is a transport molecule in bodily tissues (particularly muscle) that helps take glucose/sugar from the blood and store it in the tissues as glycogen (the storage form of carbohydrates in tissues that serve as a primary energy source during high-intensity exercise). Exercise depletes muscle glycogen, and this depletion can inhibit exercise performance. Muscle glycogen replenishment post exercise is much slower in horses than it is humans, even when given high carbohydrate diets. Thus, glycogen replenishment post exercise is a significant factor in preventing training fatigue and reduced racing performance in racehorses (Lecombe 2003). Due to metformin’s ability to enhance GLUT-4, it may also speed up glycogen replenishment in racehorses following exercise, enhancing exercise recovery. This can allow horses to train harder more frequently. This effect is similar to Thyroxine, now a banned substance under HISA/HIWU. In humans, metformin has been shown to significantly accelerate glycogen replenishment after prolonged exercise compared to a placebo (Scalzo et al. 2017). Grain is essential for horses undergoing daily high intensity exercise in order to maximize muscle repair and glycogen storage replenishment. However, not all horses are able to metabolically handle high amounts of grain in one sitting and are at higher risk for laminitis. Metformin may be effective in increasing insulin sensitivity, allowing horses to eat higher quantities of grain more safely. Weight Loss In humans, it has been reported that metformin can induce a 2-3% body weight in the first year of taking the drug. Although the reasoning for this is not fully understood, research has found metformin can reduce appetite which may be associated with this weight loss (Stanford Medicine). As any trainer will tell you, reducing appetite is not exactly beneficial for racehorses, unless they are in a paddock 24/7 on a layup with pastures rich in carbohydrates. Metformin’s effect to increase insulin sensitivity can also reduce fat storage after eating a high carbohydrate meal, possibly further contributing to weight loss. There is little research on metformin’s effects of weight loss in horses. The loss of fat mass in overweight horses can be advantageous in enhancing racehorse performance to reduce energy cost while racing to some extent. Although, this effect may not be as potent as Thyroxine- which was once a commonly prescribed oral medication given to racehorses before it was banned by HISA/HIWU. Thyroxine is also much more easily absorbed by the horse and is likely much easier to evade drug tests after administration compared to metformin. Why Metformin is a Banned Substance Under The Horseracing Integrity and Safety Act/The Horse Racing Integrity and Welfare Unit (HISA/HIWU), any drug/substance that does not have FDA approved use in horses is considered a banned substance (with a few exceptions). Metformin is not FDA approved for horses, thus is a banned substance. Furthermore, the only legitimate reason to give metformin to a horse is to treat insulin dysregulation and metabolic syndrome (proven with a glucose tolerance test, fasted blood sugar/insulin levels, etc) if diet change alone and other FDA approved medications are ineffective. That being said, metabolic dysregulation is extremely unlikely, if not unheard of, in young, fit thoroughbreds undergoing racehorse training. Therefore, there should be little reasoning to give metformin to a racehorse. Lisa Laurzus, the media spokesperson for HISA, has gone on to say “We have intelligence that suggests that some trainers believe it is performance-enhancing, whether it is or it isn’t,” in a 2024 Paulick Report article after being questioned why metformin is a banned substance. Despite the findings that metformin has the capacity to enhance athletic performance in prolonged sprints and improve muscle recovery, the World Anti-Doping Agency does not ban metformin for professional athletes. The reasoning for this is not clear. Metformin Absorption in Horses Hustace et al. (2009) has shown metformin is poorly absorbed in horses, meaning in order for there to be significantly high concentrations of metformin in the blood, the horse must be given high amounts of metformin in order to get the desired effects. In this study, horses were given 6,000mg of metformin orally with feed or without feed. The half-life of the drug is fairly fast, reducing its concentration in the blood in half about every 25 minutes. As estimated, only 7.1% of the 6,000mg of metformin given without feed was actually absorbed by the horse. When horses were given the drug orally with feed, only 3.9% was absorbed (this is extremely low. To compare, Flunixin, a common non-steroidal anti-inflammatory given to horses, when fed orally has an absorption rate of 86%, Equine Emergencies 2014). The peak concentration of metformin in the blood found was 0.4 ug/mL at almost an hour after the 6,000mg was administered, thereafter the concentration reduced by half about every 25 minutes (per its ‘half-life’). For those that are not familiar with pharmacology kinetics, that is an extremely low peak concentration after having given a horse 6,000mg of metformin. Under HISA/HIWU, there is no threshold for metformin, for it is a banned substance. This means that metformin, unlike ‘controlled substances’, cannot be given to racehorses at any time and that if they test positive for any amount of metformin, it is a positive test and the trainer may be fined and/or suspended based on those findings. This is where the controversy of environmental contamination comes into play. 1 in 12 people in the U.S. are prescribed metformin. Thus, it is very plausible a groom, trainer, or hotwalker is prescribed the medication. Conversely, metformin is very poorly absorbed by the horse with a very short half-life (meaning it is metabolized quickly). Furthermore, we have not seen as frequent positive drug tests in racehorses from other more commonly prescribed medications in the U.S. that are also ‘Banned Substances’ under HISA/HIWU. These substances include Lisinopril (88 million prescriptions between 2004-2021), Amlodipine (70 million), Losartan (55 million), and Hydrocodone (70 million). Lisinopril has a much higher bioavailability (absorption capacity) in horses compared to metformin (60-86% vs ~7.0%; Afonso et al. 2013). Amlodipine bioavailability has not been studied in horses, but in humans, dogs, cats, and rats, absorption rates range from 63-100% (Stopher et al. 1988). Losartan also has not been studied in horses, but in pigs and humans, absorption rates are about 33% (Rudy & Kostis 2005). Keep in mind that the equine digestive tract is not comparable to other species, but these substances may have the potential to have higher bioavailability to equines than metformin. More research warranted It is unknown what incriminating details HISA knows about these metformin cases that the general public does not know. Furthermore, more research on the contamination risks of metformin and other substances in racehorse stables is needed. In a study by Fenger et al. (2017), Metformin was found to contaminate two out of twenty one "ship in" horse stalls at Charles Town Racetrack that were swabbed for potential environmental contamination risks for racehorses. Brewer et al. (2024) further concludeds that the amount of Metformin contaminated in these stalls are "fully consistent with its occational identification at trace levels in racehorse blood and urine samples." This article, Brewer et al. (2024), performed an expert analysis on Metformin's risk of environmentally contaminating racehorses. The researchers reccomend a 'Screening Limit of Detection' of 5ng/ml in blood. Keep in mind, the racehorses that have tested positive for Metformin in the past year have been testing for levels ranging from 162-630 pg/ml in blood. A nanogram (ng) is equal to 1000 picograms (pg). The article goes on to mention reasonable possibilities for Metformin to be environmentally exposed to racehorses through human contact of those taking Metfomrin and possibly through tap water. However, it is clear metformin may have acute performance enhancing effects for racehorses and may improve recovery post exercise. Although it may actually inhibit long term training gains in fitness if given frequently. Albeit, the study mentioned prior, Learis et al. (2015), found that when 500mg was administerd 1 hour before a high intensity exercise test to fatigue, time to fatigue was significnatly increased compared to a placebo. These male subjects were likely around 60kg (the study does not state), meaning the dose was likely around 8.3mg/kg of body weight. If we were to apply this to a 450kg thoroughbred, the equivalent dose would be 3,735mg. If we add in the factor of the very low bioavailability of about 7% on averags as listed by Hustace et al. (2009), to get the effects from that dosage (if the performance enhancing effects were similar in horses as it is humans as shown in Learis et al.), you would need to administer 53,357mg of metformin to said horse roughly 1 hour before post time. With that, the horse would most definitely test well over 5ng/ml (the recomended screening limit from Brewer et al. 2024) in post blood samples. This is where the side of a trainer delibrately giving metformin to a racehorse for acute performance enhancement becomes questionable. Once again, more research is clearly needed before major regulatory decisions are made. Especially in regards to the absorption/metabolism of metformin in equines and whether or not it has the potential to be performance enhancing to young fit racehorses. Additionally, future research should entail what the post blood sample concentration level of metformin would test for if the minimum performance enhancing dose (if any) was given before exercise in young thorougbreds. However, this complex research may require much time and funding to perform. Final Thoughts I personally cannot conclude whether these horses that have tested positive for metformin were deliberately treated with the substance or were truly from environmental contamination. Both are plausible events. I am simply here to provide the facts from an exercise physiologist’s point of view that also has experience working within the horse racing industry. References: “Metformin improves performance in high-intensity exercise, but not anaerobic capacity in healthy male subjects” (Learsi et al. 2015) https://pubmed.ncbi.nlm.nih.gov/26250859/#:~:text=Metformin%20improved%20performance%20and%20anaerobic,anaerobic%20capacity%20in%20healthy%20subjects. “Chronic metformin intake improves anaerobic but not aerobic capacity in healthy rats” (Araujo et al. 2020) https://pubmed.ncbi.nlm.nih.gov/31577914/ “Metformin blunts muscle hypertrophy in response to progressive resistance exercise training in older adults: A randomized, double-blinded, placebo-controlled, multicenter trial: The MASTERS Trial” (Walton et al. 2019) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6826125/ “Effects of chronic metformin treatment on training adaptations in men and women with hyperglycemia: A prospective study” (Moreno-Cabenas et al. 2022) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9321693/ “An update on mode of action of metformin in modulation of meta-inflammation and inflammaging” (Khodadadi et al. 2022) https://link.springer.com/article/10.1007/s43440-021-00334-z “Metformin Regulates Glucose Transporter 4 (GLUT4) Translocation through AMP-activated Protein Kinase (AMPK)-mediated Cbl/CAP Signaling in 3T3-L1 Preadipocyte Cells*” (Lee et al. 2012) https://www.jbc.org/article/S0021-9258(20)41650-3/pdf#:~:text=Metformin%20also%20ameliorates%20insulin%20resistance,role%20in%20cellular%20energy%20homeostasis. “Muscle Glycogen Metabolism in Horses: Interactions between substrate availability, exercise performance, and carbohydrate administration” (Lecombe 2003) https://etd.ohiolink.edu/acprod/odb_etd/etd/r/1501/10?clear=10&p10_accession_num=osu1041621577 “Pharmacokinetics and bioavailability of metformin in horses” (Hustace et al. 2009) https://pubmed.ncbi.nlm.nih.gov/19405907/ “Disposition and excretion of flunixin meglumine in horses” (Soma et al. 1988) https://pubmed.ncbi.nlm.nih.gov/3247913/#:~:text=Flunixin%20meglumine%20was%20rapidly%20excreted,bound%20to%20protein%20in%20plasma. “The effect of oral metformin on insulin sensitivity in insulin resistant ponies” (Tinworth et al. 2012) https://pubmed.ncbi.nlm.nih.gov/21349749/ “Effects of metformin hydrochloride on blood glucose and insulin responses to oral dextrose in horses” (Durham et al. 2013) https://pubmed.ncbi.nlm.nih.gov/23600690/#:~:text=Reasons%20for%20performing%20study%3A%20Metformin,responses%20to%20oral%20carbohydrate%20ingestion. “The effect of pre-dosing with metformin on the insulin response to oral sugar in insulin-dysregulated horses” (Colmer et al. 2023) https://pubmed.ncbi.nlm.nih.gov/37545128/ “Ergogenic properties of metformin in simulated high altitude” (Scalzo et al. 2017) https://pubmed.ncbi.nlm.nih.gov/28394459/#:~:text=Metformin%20augments%20glucose%2Fglycogen%20regulation,glucose%2Fglycogen%20dependence%20is%20increased. “Weight loss caused by common diabetes drug tied to ‘anti-hunger’ molecule in study” (Stanford Medicine) https://www.hiwu.org/cases/pending “Keeping Pace: ‘More Drugs’ is never the right answer in Horse Racing” (Paulick Report 2024) https://paulickreport.com/features/keeping-pace/keeping-pace-more-drugs-is-never-the-right-answer-in-horse-racing “Pharmacodynamic evaluation of 4 angiotensin-converting enzyme inhibitors in healthy adult horses” (Alfonso et al. 2013) https://pubmed.ncbi.nlm.nih.gov/23952255/ “The metabolism and pharmacokinetics of amlodipine in humans and animals” (Stopher et al. 1988) https://pubmed.ncbi.nlm.nih.gov/2467130/ “Losartan Potassium - an overview: Angiotensin II Receptor Antagonist” (Rudy & Kostis 2005) https://www.sciencedirect.com/topics/medicine-and-dentistry/losartan-potassium Flunixin Overview - Bioavailability and Half-life: Equine Emergencies 2014 https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/flunixin#:~:text=Flunixin%20is%20rapidly%20absorbed%20after,serum%20levels%20within%2030%20minutes.&text=Absorption%20is%20delayed%20by%20feeding.&text=The%20Vd%20is%200.1%20to,is%201%20to%202%20hours. "Metformin as an environmental substance transfering to horses -- a case report and analysis" (Brewer et al. 2024) https://scholars.uky.edu/en/publications/metformin-as-an-environmental-substance-transferring-to-horses-a- "An In-Depth Look at Stall Contamination" (Fenger et al. 2017) https://nationalhbpa.com/an-in-depth-look-at-stall-contamination/ HIWU Controlled Substance List and Substance Thresholds in Blood and Urine: https://assets.hiwu.org/a/hisa_controlledprohibitedlist_report_2.02.23_opt.pdf?updated_at=2023-02-13T14:41:22.400Z HIWU Banned Substance List: https://assets.hiwu.org/a/hisa_bannedprohibitedlist_report_012723a_opt.pdf?updated_at=2023-02-13T14:38:13.254Z ARCI Banned Substance List: https://www.arci.com/docs/Uniform-Classification-Guidelines-Version-17.0.pdf

Exercise is extremely important for health in both humans and animals. Exercise can reduce risk of obesity, cardiovascular disease, type 2 diabetes, cancer, and more in humans (Blair & Brodney 1999). In humans, the number one predictor of your risk for disease and how long you will live (all-cause mortality) is your maximum oxygen consumption (VO2max), which can only be enhanced through exercise and genetics (Harber et al. 2017). Exercise can increase the body’s insulin sensitivity in both humans and horses, thus reducing the risk for insulin resistance, metabolic disease, laminitis, and obesity (Frank 2010). In growing horses, insulin dysregulation can increase risk of OCD and poor hoof formation (Fitzgerald et al. 2004; Kronfeld & Harris 2003; Frank 2010; Raymond et al. 2008). With all these benefits of exercise in mind, much research has been conducted on both humans and animals on whether exercise performed by a pregnant mother can benefit, or harm, their offspring. It is already well established that exercise during pregnancy (termed ‘maternal exercise’) can increase fitness of the mother, reduce excess weight gain from the mother, reduce risks of gestational metabolic disease, and improves the recovery of the mother following birth (Borodulin et al. 2008). This growing mountain of research has shifted the habitual recommendations for pregnant individuals to perform more exercise before and during pregnancy to improve the health outcomes of the mother and offspring. A review paper by Blaize, Pearson, and Newcomer (2015) analyzed over a dozen studies looking at how maternal exercise impacted the offspring in rats, mice, swine, and humans. They concluded maternal exercise at moderate or high intensities has been shown to: Prevent metabolic dysregulation and disease (by increasing insulin sensitivity) in mother and offspring. Increase cardiovascular health of the mother and offspring, reducing risk of cardiovascular disorders. Reduce risk of tumor and cancer development in mothers and offspring. It is important to note that most of these benefits found in the offspring lasted all the way into the offspring’s adulthood. Another interesting finding from this review was that even pregnant mothers that exercised, but had restricted protein intake throughout pregnancy, had a significantly lower risk of fetal/offspring underdevelopment and lack of growth compared to mothers that had restricted protein intake but did not exercise. High quality protein intake in the proper quantities from the mother is essential for proper fetal development. Thus, it is fascinating to see that maternal exercise protects the offspring’s development despite the mother being under-nourished compared to well-nourished mothers that do not exercise. This is of course also seen on over-nourished mothers (with excess carbohydrate and fat intake in their diet and are overweight), with maternal exercise reducing the risk of the offspring being overweight and/or has metabolic dysregulation. Mothers that are both over-nourished and under-nourshed have similar risk for disease and growth impairment. Exercise can prevent these unwanted risks in both over- and under-nourished dams. Maternal Exercise and OCD Development in Offspring If there is one thing breeders wish to avoid in their young growing horses, it is osteochondritis dissecans (OCD) formation. There are many cited causes for OCD development such as genetics, insulin dysregulation, mineral-vitamin imbalance, rapid growth rates, and bone injury (Bourebaba et al. 2019). A study by Robels et al. (2018) found that the offspring (from foal to yearling) of overweight broodmares were significantly more likely to develop excess inflammation, have insulin resistance, and have more OCDs compared to broodmares of normal weight. The growth rates of the offspring were not different between overweight and normal weight broodmares. According to the research previously shown, exercise of the mother could offset these negative health outcomes of the offspring, thus possibly reduce the risk of OCD formation in the offspring. Reducing OCDs in offspring can greatly reduce that offspring’s risk for injury and improve their auction prices at the sales. Myth of exercise-induced hypoxia in the placenta For years physicians and veterinarians believed if a pregnant mother underwent exercise, blood flow would be diverted away from the placenta and fetus, thus causing hypoxia and stress to the fetus, impairing the development of the offspring. However, much research has disproved these claims in horses, humans, rats, and many other animals (Lehnhard et al. 2008; May et al. 2010; Blaize et al. 2015). However, high intensity exercise in extremely hot/humid conditions (resulting in heat stress) may impair reproductive efficiency in pregnant mares (Mortensen 2008- unpublished dissertation). A study by Anton and colleagues (2014) found that exercising a pregnant mare at moderate intensities 16 to 80 days of gestation was not detrimental to the pregnancy. In this study, all broodmares delivered healthy foals and suffered no difficulties during birth. The American Association of Equine Practitioners recommends treating your pregnant mare as you would a non-pregnant one during the first 7 months of pregnancy, unless there are special circumstances. They also note that pregnant mares will benefit from moderate riding or exercise. There is not much research on the extent of what intensities of exercise can impact the offspring’s health, such as whether high intensities are more beneficial than moderate intensities. However, recommendations based on the research currently published, like any human or animal you are exercising, avoid too quick of a progression of exercise duration and intensity that will cause over-training. This will indeed increase risk of stress and injury to the mare, and may negatively impact the offspring. There are some extreme examples of successful pregnancies while a horse is undergoing racehorse training and racing. Beautatitsbest, a mare trained by Kentucky Derby-winning trainer Eric Reed, underwent typical racehorse training until unexpectedly having a healthy foal just 2 days after working the mare an easy 3f in 40 seconds. No one was aware the mare was pregnant, and therefore underwent racehorse training prepping the mare up to a race like any other racehorse. The mare nor the foal had no difficulties during birth and remained healthy. Spain was one month pregnant with a foal by Storm Cat when she won the $300,000 Fleur de lis Handicap (G2) and got third 14 days later in the $300,000 Molly Pitcher Stakes (G2). Spain successfully gave birth to a colt later named Carpocrates that was sold at auction for $2.4 million as a weanling. More examples of mares producing healthy foals while being pregnant during a duration of their racing career include Indian Queen (won the Royal Ascot Gold Cup 1991), Chinese White (won the Pretty Doll Stakes 2010), Fit For A Queen (Turfway Breeders' Cup Stakes 1992), Cassandra Go (Temple Stakes and King's Stand Stakes 2001), Redstone Dancer (Brownstown Stakes and Minstrel Stakes 2007), and Grecian Dancer (Ridgewood Pearl Stakes 2008). More Research Needed There is a concerningly lack of research on how exercise in pregnant horses can impact their offspring’s health in the short and long term. However, the evidence that maternal exercise can positively influence the offspring’s health is similar across humans, rats, swine, and mice. Therefore, it is possible exercise of broodmares may also have similar positive outcomes, but more research is needed. Breeding horses for athletic disciplines is a multi-billion dollar industry, with major breeding operations netting incomes well over millions. It would not be a waste of money by any means for wealthy breeding farms to invest in performing their own research studies on how exercising their pregnant broodmares impacts the metabolic health and risk of OCD lesion in their offspring, as well as the broodmares’ health and recovery from pregnancy. Exercising the pregnant broodmare may also impact the offspring’s long term fitness capacity and athletic performance. Research on to what extent the intensity, duration, and frequency of maternal exercise impact offspring health/fitness outcomes also need to be explored. Hopefully this article can inspire those who have the ability and resources to research such topics. References “Impact of parental exercise on epigenetic modifications inherited by offspring: A systematic review” (Axsom & Libontani 2019) https://physoc.onlinelibrary.wiley.com/doi/full/10.14814/phy2.14287 “Maternal Obesity increases resistance, low-grade inflammation, and osteochondrosis (OCD) lesions in foals and yearlings until 18 months of age” (Robels et al. 2018) https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0190309 “Impact of Maternal Exercise during pregnancy on offspring chronic disease susceptibility” (Blaize, Pearson, and Newcomer, 2015) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4575625/ “Physical activity patterns during pregnancy” (Borodulin et al. 2008) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3319731/ “Aerobic exercise during pregnancy influences fetal cardiac autonomic control of heart rate and heart rate variability” (May et al. 2010) https://pubmed.ncbi.nlm.nih.gov/20356690/ “Maternal and foetal heart rates during exercise in horses” (Lehnhard et al. 2008) https://www.cambridge.org/core/journals/comparative-exercise-physiology/article/abs/maternal-and-foetal-heart-rates-during-exercise-in-horses/E91220F308828CFFC9E9328BE3DF75DE “Maternal investment results in better foal condition through increased play behavior in horses” (Cameron et al. 2008) https://www.sciencedirect.com/science/article/pii/S0003347208003291?casa_token=fsaPYjd83s8AAAAA:TWMf0uogUV5YkZPWvcGIR2mZATLjmbguSBT6nSEdc3ePc_Ab8T0-43mE74-R9AoeA1FP3R3A-A “Effects of exercise or oocyte heat shock on embryo development and gene expression in the horse” (Mortensen 2008- unpublished dissertation) https://oaktrust.library.tamu.edu/bitstream/handle/1969.1/ETD-TAMU-1232/MORTENSEN-DISSERTATION.pdf?sequence=1 “Effects of physical inactivity and obesity on morbidity and mortality: current evidence and research issues” (Blair & Brodney 1999) https://pubmed.ncbi.nlm.nih.gov/10593541/ “Impact of Cardiorespiratory Fitness on All-Cause and Disease-specific mortality: Advances since 2009” (Harber et al. 2017) https://pubmed.ncbi.nlm.nih.gov/28286137/ “Osteochondritis dissecans (OCD) in Horses” (Bourebaba et al. 2019) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6534522/#:~:text=Involvement%20of%20diet%2C%20growth%20rate,develop%20OCD%20pathology%20%5B19%5D. “Exercising the pregnant mare from day 16 to day 80 of Gestation” (Anton et al. 2014) https://www.sciencedirect.com/science/article/abs/pii/S0737080613004978 Equine Metabolic Syndrome (Frank 2010) https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1939-1676.2010.0503.x “Equine Grain Associated Disorders” (Kronfeld & Harris 2003) http://vetfolio-vetstreet.s3.amazonaws.com/mmah/1e/12618156b0423b88c3bd9d566fac59/filePV_25_12_974.pdf “Metabolic Predispositions to Laminitis in Horses and Ponies: Obesity, Insulin Resistance and Metabolic Syndromes” (Raymond et al. 2008) https://www.sciencedirect.com/science/article/pii/S0737080608003468?casa_token=sFvNCRO0icEAAAAA:cRkv9YEXPzwImR4SQ0eFHSGu7KVzDsBwFJAlmPvPAGD4hCfeMFPsXNduya-kKv7S33EqMPwU2g “The relationship between insulin status and OCD occurrence in thoroughbred yearlings” (Dobbs et al. 2010 - Abstract Proceedings) “Insulin Resistance in the Horse” (Fitzgerald et al. 2004 - Abstract Proceedings) “Hyperglycemia hyperinsulinemia after feeding a meal of grain to young horses with osteochondritis dissecans (OCD) lesion” (Raltson 1996)

The Myostatin Gene (MSTN), also known as “the speed gene”, regulates how large a muscle fiber can grow and what type of muscle fibers (fast or slow twitch) a muscle can have. This gene has been shown to determine sprinting ability and determine the best racing distance for thoroughbred racehorses. The myostatin gene also influences other variables that will be discussed in this article. The myostatin gene influences the cell to produce myostatin (a protein). This myostatin protein controls the growth of tissues throughout the body, particularly muscle. Myostatin limits the capacity of how much the muscle can grow in size. When myostatin is deficient or absent, muscle is able to grow limitlessly. The mouse and the cow on the right does not have the myostatin gene, showing excessive muscle mass, whereas the mouse and cow on the left is a regular mouse. (Photo from John Hopkins Medicine & Reconminetics) The ability for the myostatin gene to influence muscle size is somewhat influenced by the number of fast twitch muscle fibers a horse’s muscles can have. For those who are not familiar with muscle physiology, to explain it simply, there are “slow twitch” and “fast twitch” muscle fibers. Slow twitch muscle fibers (Type I) are lean and not able to produce a lot of power upon muscle contraction. However, these muscle fibers have a lot of endurance (being able to produce near its maximal contractile force repeatedly for a long period of time without getting tired or fatigued). These fibers prefer aerobic metabolism to produce energy. Fast twitch muscle fibers (Type IIA or IIB) have the ability to generate high levels of power upon contraction as well as contract very rapidly due to their large size. However, the price of producing high power outputs over a short period of time makes these muscle fibers fatigue quickly. These fibers prefer anaerobic metabolism to produce energy. Fast twitch muscle fibers have a higher capacity to grow larger in size than slow twitch muscle fibers, especially with training. Diagram comparing fast twitch (Type IIA/IIB) to slow twitch (Type I) muscle fibers. (Photo from Parallel Coaching) Each muscle has a certain ratio of fast twitch to slow twitch muscle fibers. For instance, a horse’s gluteus medius (the giant muscle along the horse’s hind end) is composed of about 90% fast twitch muscle fibers and 10% slow twitch because of its need to produce powerful muscle contractions at the gallop or when jumping. Meanwhile, the triceps brachii of the same horse may have about 35% slow twitch and 65% fast twitch muscle fibers (due to the triceps brachii’s function of stabilizing the limb during movement with smaller contractile forces but stay contracted for long periods of time to stabilize the limb) (Hoven et al. 1985). The image above shows a microscopic view of the different muscle fibers within a muscle sample. Each fiber is stained a certain color in relation to the type of muscle fiber it is. Notice it shows “Type IIA”, “IIB” and “Type I”. I am not going to go too much into detail about these different fiber type names in this article, but just know that “Type IIA and IIB” are fast twitch and “Type I” are slow twitch muscle fibers. (Photo from Bakhsh et al. 2019) However, genetics and training can influence those ratios. There is controversy as to whether training influences the ratio of fiber types more so than genetics, but the research tends to favor genetics (Miyata et al. 2018). Especially in regards to higher ratios of fast twitch over slow twitch muscle fibers. Fast twitch muscle fibers have a larger capacity to grow larger in size compared to slow twitch muscle fibers. As you could probably now conclude, those who have an absent myostatin gene have higher quantities of fast twitch muscle fibers, thus why these animals are able to grow larger muscles. This leads us to our next section. The Myostatin Genotypes and Muscle Fiber Composition There are three different myostatin genotypes: CC- myostatin is mostly not present. CT- myostatin is partially present. TT- myostatin is largely present. A study conducted by Rooney et al. (2017) investigated the variation of fiber type ratio of the horse’s gluteus medius in relation to their myostatin genotype in untrained thoroughbred horses (with an average age of 21 months). Image of the gluteus medius of the equine. They found that horses’ with a CC genotype were composed of about 4% slow twitch muscle fibers and 95% fast twitch muscle fibers on average in their gluteus medius muscle. CT genotypes had about 10% slow twitch and 89% fast twitch. TT genotypes had about 14% slow twitch and 85% fast twitch. Photo demonstrating the muscle fiber type distribution in untrained thoroughbred’s gluteus medius across the different myostatin genotypes found from Rooney et al. (2017). Notice from the photo above that CC genotypes have almost 10% higher abundance of fast twitch muscle fibers (Type IIA/IIB) than TT genotypes. Keep in mind that the gluteus medius muscle both in humans and horses is already a fast twitch fiber dominant muscle due to its function of being one of the primary power sources of strength and power during galloping/running and jumping. (the human gluteus medius has a composition of about 40% slow twitch and 60% fast twitch for comparison; Sirca & Michieli 1980). So these ratios of fiber types are not true for every muscle in the horse’s body. But we can rightfully assume that most muscles in the body will follow a similar pattern of CC genotypes overall having more fast twitch muscle fibers in most muscles on average and TT genotypes having more slow twitch muscle fibers on average, with CT genotypes lying somewhere in between. This study also looked at other physiological differences within the muscle fibers of different myostatin genotypes such as citrate synthase enzymes, mitochondrial volume, specific fiber types, etc. However, I am not going to overcomplicate this article for readers that do not have a background in physiology. If you are curious about the other results of this study and what it means for racehorse performance and training, please feel free to email me gallopscience@gmail.com. A study by Miyata et al. (2018) found very similar associations in fiber type ratio and the myostatin genotypes yearling and 2yo thoroughbred horses before and after these horses were given a similar 5 month training regimen (ranging from 1000m hill gallops, to 1600-2400m canters, and 500m high speed works twice a week). This study shows that even when horses are given the same training regimens (which can change fiber type somewhat), CC genotypes still remain more dominant in fast twitch muscle cells over TT genotypes, again with CT lying between CC and TT. The overall result of these studies are that CC myostatin genotypes have muscle physiology that are more suited for power, acceleration, and speed whereas TT myostatin genotypes have muscle physiology that are more suited for endurance. CT genotypes lie somewhere in between. With this information, racehorses that are CC may have higher potential to perform better in short distance races (“sprint” races) and with TT having higher potential to perform better in longer distanced races (“route” or “two turn” races). This is backed by many studies that will be discussed in the next section. Preferred Racing Distance Based on Myostatin Genotype There are quite a few studies that actually show the myostatin gene influences which distance thoroughbred racehorses are best suited for. In these studies, researchers found the myostatin genotype of hundreds of both elite and average thoroughbred racehorses and tried to find associations between their genotype (CC/CT/TT) and the race distances that horse best competed in. These results were taken from thoroughbred racehorses in North America, Ireland, Great Britain, New Zealand, Australia, and East Asia. The combined results of these studies (Farries et al. 2018; Hill et al. 2010; Bins et al. 2010; Hill et al. 2019) indicate the following race distances each genotype is best suited for: CC - 8 furlongs (<1600m or 1 mile) or less CT - Between 7-12 furlongs (1400m-2200m or 1 to 1.5 miles) TT - 10 furlongs (2000m+ or 1.25 miles) or longer This does not mean that CC horses only race well in races less than 8 furlongs. Just that CC horses are more likely to race better at those distances. This is the case for other genotypes as well. This is demonstrated in the following figure from Hill et al. (2010): Distribution of best race distance in accordance to each horse’s myostatin genotype in 179 elite thoroughbred racehorses: CC- blue, CT- red, TT- green. (Photo modified from Hill et al. 2010) Notice how CC horses (more fast twitch muscle fibers) have best racing distances ranging from 5 to 8 furlongs, but the vast majority of CC horses seem to have a best racing distance of 5-6 furlongs. There are no CC horses in this dataset that have their best race over 10 furlongs. Notice an opposite trend for TT horses (less fast twitch with more slow twitch muscle fibers), where TT horses in this data set have had their best racing distances ranging from 7 to 13 furlongs. However, a much larger portion of TT horses have had their best races at distances of 10 furlongs or longer. Notice the trend with CT horses. CT horses clearly have the most range when it comes to best racing distances. Unlike CC and TT horses that do not appear on their opposite ends of the racing distance spectrum, CT horses show up in all racing distances. However, the bulk of CT horses have their best racing distances between 8 and 11 furlongs. Whether the horse is bred for turf or dirt likely does not affect these optimum race distances. Many good trainers already know whether their horse will be a sprinter or stayer simply by looking at their pedigree and watching them handle training. However, there are a lot of horsemen that rely simply on racing their horses 2-turns (longer distances) for the first time to see whether or not their horse runs well at those distances. This can waste a lot of money (jockey mount fees, pre-race vet work, shipping, etc) and time (waiting for the race, the horse’s need to recover from the race, waiting for the next race in the condition book, etc.). By knowing your horse’s myostatin genotype, you can make more accurate training and racing decisions earlier on to save the trainer, owner’s time and money as well as maximize your racehorse’s success. Best Racing Age The study by Farries et al. (2018) was unique in that it also analyzed the association between each horse’s myostatin genotype and the age in which each horse’s first race occurred, in addition to the age in which each horse’s best race occurred. CC horses competed in their first race by about 1 month on average earlier than did CT horses, and CC horses completed their first race over 2 months on average earlier than TT horses. As for when each horse raced their best racing distance, CC horses competed in their best race over 2 months on average earlier than CT horses and over 4 months on average earlier than TT horses. The results by Farries et al. (2018) and Bailey et al. (2022) indicate that CC horses are more likely to perform well early on in their career, such as in their 2yo year, and TT horses are more likely to perform well later on in their career, such as in their mid to late 3yo year. CT horses can go either way, but on average may perform better later than CC horses but much earlier than TT horses. It is unclear why the myostatin gene determines this “late development” of performance occurs with CT and TT horses in comparison to CC horses. However, we do know that longer distanced races do not occur until late into a horse’s 2yo year or until their 3yo year. It is likely that trainers already sense their horses will prefer longer distance races (potentially being CT or TT) and are waiting until the longer races occur. Also it is common for trainers to start their horse’s first race over shorter distances (4.5-6.5f) and progress them into racing longer distances (7f or longer). So while CC horses start in their first race already at their optimum racing distance, CT and TT horses usually are not. Also, most trainers understand that it takes a longer amount of time to get a horse fit for longer distance races than sprint races. Therefore, the delayed first race and best race for CT and TT horses. Myostatin Gene and Potential for Performance The scientific literature is clear that the myostatin gene does not determine whether or not the horse will be a high quality or a low quality racehorse. Only that the myostatin gene can predict what distance that horse may best compete at. However, if you have a CC horse that races poorly in sprint races, it is likely that horse is simply just not a good racehorse or they have other underlying issues inhibiting their performance. The same goes for TT horses; if a TT horse races poorly in longer distanced races, they are likely just not good racehorses or they have other underlying issues inhibiting their performance. As for CT horses, again they can go either way. If your CT horse does not compete well in sprint races, they may have the potential to race better in longer distance races. If not, then again, they may simply not be good racehorses. In North America, the vast majority of races are held between 5 and 9 furlongs. Races longer than 10 furlongs are rare, especially at certain racetracks. Therefore, if you are a North American based owner or trainer and have a TT horse, they may not race as frequently and their earning potential may not be as high as CC and CT horses. The American Triple Crown is composed of the Kentucky Derby (10f), The Preakness (9.5f), and The Belmont (12f) in the span of 6 weeks. Clearly, a CT horse is best suited for running at all of these racing distances. This is even more so for The Kentucky Derby due to the race’s points qualification system via The Kentucky Derby prep races. The Kentucky Derby prep races are held during horses’ 2yo year and early 3yo year range from 8 to 9 furlongs. The top 3 horses in each race earn a certain amount of points. The top 20 horses that accumulate the most points qualify for the Kentucky Derby. So not only does the horse have to race and compete competitively earlier on in their career, they also must compete competitively in those middle distance races in order to even qualify for the Kentucky Derby. These qualities most definitely suit CT horses (and possibly early developing TT horses) based on the research. Again, the myostatin gene does not determine overall racing quality or class, just optimum racing distance of that horse. Conformational Characteristics A study by Tozaki et al. (2011) measured different confirmation characteristics of over 100 thoroughbred yearlings’ (born between March and April), such as wither height, body weight, chest circumference, and cannon bone circumference. The researchers then tested their myostatin gene to find associations with the different myostatin genotypes and growth rates and conformational differences. These horses’ confirmation was remeasured every month for 6 months as they endured training for racing. Body weight, wither height, cannon bone circumference, and chest circumference significantly increased with age in all horses. Cannon bone circumference was significantly greater in males than females at all times. Chest circumference was significantly higher in females than males until December of the horses’ yearling year. Body weight and wither height was not significantly higher in males over females until the horses reached February of their 2yo year. There was no association between any of these horse’s conformational characteristics and each horse’s respective myostatin genotype except for body weight. Horses with CC genotypes were significantly heavier than CT and TT genotypes, and CT genotypes were heavier than TT genotypes. This is the case for both male and female horses. This is likely due to, as previously mentioned, CC horses having more muscle mass due to the higher portion of fast twitch muscle fibers (which are generally larger in size than slow twitch fibers). These differences in weight may also be due to, although not mentioned by the researchers, higher fat build up in CC horses than TT horses. The fat composition of horses can be influenced by several factors, but it is important to note fast twitch muscle fibers are not nearly as adequate to burn fat compared to slow twitch muscle fibers (McAinch et al. 2012). This may predispose CC horses, having more fast twitch muscle fibers, to have a harder time burning fat off the body than TT horses, being more abundant in slow twitch fibers. Heart Size Kis et al. (2023) measured different conformational parameters of the heart of over 60 Hungarian Thoroughbred horses and tried to find associations with the myostatin genotypes. The size of each horse’s chambers of the heart (ventricles and atria) were not significantly different between CC, CT, and TT. However, aortic diameter at the sinus of Valsalva (end- diastole and end-systole) and aortic diameter at the valve (end-systole) was significantly greater in CC horses than CT and TT horses. Image of the heart and the aortic diameter at the sinus of Valsalva and the aortic valve. This finding suggests CC horses may have higher cardiac output (how much blood that can be pumped through the heart over a given period of time, a variable in oxygen delivery throughout the body). However, this is ironic given that higher cardiac output is usually a characteristic seen in more endurance-based athletes of humans and horses (Weber et al. 1987; Wilmore et al. 2000), which would characterize CT and TT horses. The evidence shown prior clearly shows CT and TT horses have superior endurance over CC horses. Therefore, it is hard to say whether CC horses truly have higher cardiac output than CT and TT horses. If so, it likely does not play a role in their endurance because even though horses with higher cardiac output have higher ability to deliver oxygen to its muscles and most fast twitch muscle fibers generally do not have the means to take up that oxygen (usually due to lack of blood vessels surrounding fast twitch muscle fibers, lack of oxygen carrying enzymes, and lack of mitochondria that can actually metabolize that oxygen to create energy). However, given CC horse’s higher content of fast twitch muscle fibers, they are able to produce high amounts of acidity in the blood over a short period of time during intense exercise. This increase in blood acidity (caused by hydrogen ions, not ‘lactic acid’ - see my article “Lactic Acid does not cause Fatigue or Muscle Soreness in horses”) reduces the red blood cell’s ability to bind to oxygen. Thus, if CC horses truly have increased cardiac output, this may be a genetic adaptation to compensate for the acidity produced by the fast twitch muscle fibers by increasing the capacity for oxygen levels in the blood to be higher. Overall, further research is needed to truly determine whether CC horses truly have higher cardiac output than CT and TT horses. Breeding for the Myostatin Gene If you remember “punnett squares” from your high school biology class, how to breed for the myostatin gene is relatively easy to understand. Remember that horses are either CC, CT, or TT. Each of these letters are an “allele” on the horse’s DNA. As previously explained, the “C allele” is the myostatin gene that allows a horse to have speed and power. The “T allele” allows the horse to have more endurance. If you tested for the myostatin gene in a mare and a stallion, you can predict the likelihood of whether the offspring will be a CC, CT, or TT. For example, if the dam is CT, and the sire is CT, the following chances of what genotype the offspring will have will be the following: 25% CC, 50% CT, and 25% TT. (Image on right) Meanwhile, if you have a dam that is CC and a sire that is CC, the offspring will have an 100% likelihood of also being a CC; a sprinter. (Image on left) A dam that is TT and a sire that is CC will have a 100% likelihood of producing an offspring that is CT; a middle or classic distance racehorse. (Image on right) Breeders that know the myostatin genotype of their mares and stallions can make more accurate breeding decisions depending on whether you want a sprinter, classic/middle distance horse, or long distance horse. Smart breeders can also pair in preferred conformational traits for a certain distance to combine with the myostatin gene. For those that are able to gene test their broodmare and not the stallion they plan to breed with, understanding the characteristics of the different myostatin genotypes may help a breeder accurately guess what gene the stallion may have. For instance, stallions that were “late developers” and ran their best races at 10 furlongs or longer (and some of their offspring show similar traits) are likely TT genotypes. If a stallion ran their best races early on as a 2 or 3yo and ran their best race distances at 7f or shorter, then they are likely a CC genotype. Stallions that ran successfully in both sprint and route races are likely CT genotypes. Myostatin Gene Origin in Thoroughbreds No American racing fan today could even recognize how horse races were conducted prior to the 1900’s. Race distances ranged from 1 to 4 miles long, in ‘heats’. A horse had to win two races (or heats) in the same day at the same, or longer, distance in order to win prize money. If there was a tie win between two horses in heat wins, those two horses would race a third time to decide the winner. This means horses could be racing up to 2 to 12 miles in a single day. Today, there is not a single thoroughbred race at a designated racetrack that is held over 2.5 miles in America today. Races 1.5 miles or longer are extremely rare in America. As one could imagine, those horses racing 1 to 4 miles one to three times a day near maximal effort back in the 18th and 19th century needed extreme endurance to not only race at these longer distances, but to recover from them in order to compete in the following heats. Because of this, speed was not emphasized and therefore the myostatin gene was not very present in the breed. Eclipse, one of the most renowned racehorses in American history during the 1700’s (beating horses in consecutive 4 mile races), has been confirmed to be a TT myostatin genotype. Researchers have concluded that the “C” allele in the myostatin gene was present, but very rare prior to the 1900’s (Bower et al. 2012). Genetic samples from a dozen museums of historically influential thoroughbred stallions born between 1764 and 1930 all tested as TT, no myostatin gene “C” allele. It wasn't until the horse racing industry in both North America and Great Britain began to change following 1860, where 1-4 mile “heat” races disappeared and racing horses at 2yo became more emphasized. Race distances gradually became shorter to allow the horses to recover quicker from their races and race more frequently, increasing the earning potential for racehorse owners. Due to these industry changes, speed in the racehorse became more sought out for over endurance. But the myostatin speed “C” allele did not yet become very widespread in the thoroughbred breed until Northern Dancer (born in 1961). Northern Dancer held a “C” allele, which was passed down from his sire Nearctic who also had a C allele. Nearctic obtained his C allele from his dam, Lady Angela (originated from Great Britain). Northern Dancer would become the most influential thoroughbred sire in not just North America, but the entire world. He passed on his C allele to many thoroughbred progeny, giving them superior speed. Through Northern Dancer, the “C” allele spread rapidly throughout the thoroughbred breed, now producing not just CT, but CC genotypes. Over 97% of thoroughbreds today have Northern Dancer present in their pedigrees (McGivney et al. 2020). Myostatin Gene Prevalence The myostatin gene C allele is abundant in different parts of the world today. Whereas prior to 1950, the vast majority of racehorses were TT, today the vast majority of horses are CC. In Europe, based on genetic testing, it is estimated that 14% of the European thoroughbred population is TT, 51% is CT, and 35% is CC. In North America, it’s estimated 14% is TT, 53% is CT, and 33% is CC. In Australia/New Zealand, 7% is TT, 43% is CT, and 48% is CC. (Hill 2019) Prevalence of myostatin genotypes among thoroughbred racehorses around the world. Information taken from (Hill 2019) Implications for Trainers and Owners The earlier you know whether your horse is a sprinter or long distance horse, the sooner you can individualize their training to race at those respective distances. Owners can accurately choose which trainer will be best for their racehorse based on their myostatin genotype. For instance, if you know your horse is CC, then it would be most effective to send that horse to a trainer that is very successful with 2yos and/or sprinters. If your horse is TT, then it would be most effective to send your horse to trainers that takes their time with late developers and are known to produce successful older horses that prefer long distance. How to test for the Myostatin Gene: PlusVital: https://www.plusvital.com/products/speed-gene-test Etalon: https://www.etalondx.com/horse-dna-tests/ Conclusion Key points: The Myostatin gene regulates muscle growth and the ratio of fast to slow twitch fibers. The Myostatin gene can accurately predict best racing distance for thoroughbred racehorses (CC: 4-7f, CT: 8-12f, TT: 10f+) The Myostatin gene can accurately predict early or late developing racehorses (CC early developers, TT late developers, CT in between CC and TT). The myostatin gene does not determine a horse’s conformation other than body weight/muscle mass. The vast majority (over 80%) of thoroughbred racehorses are CC or CT around the world. The myostatin gene can help owners, trainers, and breeders make more accurate training, racing, and breeding decisions to boost their success as well as their racehorses. References “Variation of fiber types in the triceps brachii, longissimus dorsi, gluteus medius, and biceps femoris of horses” (van den Hoven et al. 1985) https://europepmc.org/article/med/4014843 “Skeletal muscle mitochondrial bioenergetics and associations with myostatin genotypes in the Thoroughbred horse” (Rooney et al. 2017) https://pubmed.ncbi.nlm.nih.gov/29190290/ “Selective type II fibre muscular atrophy in patients with osteoarthritis of the hip” (Sirca & Michieli 1980) https://pubmed.ncbi.nlm.nih.gov/6444440/ “Effect of Myostatin SNP on muscle fiber properties in male Thoroughbred horses during training period” (Miyata, 2017) https://pubmed.ncbi.nlm.nih.gov/29058242/ “Sequence Variants at the myostatin Gene Locus Influence the Body Composition of Thoroughbred Horses” (Tozaki et al. 2011)- Japanese Study https://www.jstage.jst.go.jp/article/jvms/73/12/73_11-0295/_article/-char/ja “Association of myostatin gene polymorphism with echocardiographic and muscular ultrasonographic measurements in Hungarian thoroughbreds horses” (Kis et al. 2023) Myostatin gene CC causes larger hearts and weight/height https://www.sciencedirect.com/science/article/pii/S0034528823001479 “The contribution of myostatin (MSTN) and additional modifying genetic loci to race distance aptitude in Thoroughbred horses racing in different geographic regions” (Hill et al. 2019) Europe/Middle-East, Australia/New Zealand, North America and South Africa, average race distance, best race distance for elite, nonelite and all winning horses” https://pubmed.ncbi.nlm.nih.gov/30604488/#:~:text=Conclusions%3A%20MSTN%20is%20the%20single,genetic%20potential%20for%20distance%20aptitude. “A genome-wide SNP-association study confirms a sequence variant (g.66493737C>T) in the equine myostatin (MSTN) gene as the most powerful predictor of optimum racing distance for Thoroughbred racehorses” (Hill et al. 2010) https://bmcgenomics.biomedcentral.com/articles/10.1186/1471-2164-11-552 “A sequence polymorphism in MSTN predicts sprinting ability and racing stamina in thoroughbred horses” (Hill et al. 2010) https://pubmed.ncbi.nlm.nih.gov/20098749/ “Identification of the myostatin locus (MSTN) as having a major effect on optimum racing distance in the Thoroughbred horse in the USA” (Bins et al. 2010) https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2052.2010.02126.x?casa_token=pOAbnsRJeNYAAAAA%3ArsQqs0dBCoTmQRTMfDD3sHmM682qELnGv3okN6zQxWgbVHfV4buArQQFe0OqoUjlPpn7yqwFXVyIEC4 “The genetic origin and history of speed in the Thoroughbred racehorse” (Bower et al. 2012) https://www.nature.com/articles/ncomms1644 “Speed Gene Background Essay” (Hill 2019) https://www.education.sa.gov.au/sites/default/files/learning-at-home/speed_gene_essay_-_pbs_org.pdf?v=1586909082 “Cardiac output and oxygen consumption in exercising Thoroughbred horses” (Weber et al. 1987) https://pubmed.ncbi.nlm.nih.gov/3425767/ “Cardiac output and stroke volume changes with endurance training: The HERITAGE Family Study” (Wilmore et al. 2000) https://d1wqtxts1xzle7.cloudfront.net/44781152/Cardiac_output_and_stroke_volume_changes20160415-24637-1vhcqcu-libre.pdf?1460783038=&response-content-disposition=inline%3B+filename%3DCardiac_output_and_stroke_volume_changes.pdf&Expires=1706146809&Signature=NB4kMb7hqvBbm57sd2Lazvt6caGxaQLJcPfRCRkYPSzUPEATE9LilecPU6zaFBMDzuH1PIURGedYnAGa9lsyoHepHUA9V61HYeCiY761bjbKIN8BNMLT5bThfSiJ6hnHs4U0KqpzKIezCT8I44G5ALyW9PGZ7L91--NhEGwHA2unSc2u~J7VaFB3NXBQHusPUcmCp-rIq6wRYXzHf9AoZ8j8wlIu0XGUkdF-~4UrodLxcjgPKee0f8PUkb13-kMhT7uBwXrVIbTw2AHJK59cK6MkMUF9Th6zhASrISASkBOFZy4ifM5a2j08l~e0yAFx4EYQSNy4Q8SGhw5LxOVQJQ__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA “Genetics of Thoroughbred Racing Performance” (Bailey et al. 2022) https://pubmed.ncbi.nlm.nih.gov/30604488/#:~:text=Conclusions%3A%20MSTN%20is%20the%20single,genetic%20potential%20for%20distance%20aptitude. “Genetic contributions to precocity traits in racing Thoroughbreds” (Farries et al. 2018) https://pubmed.ncbi.nlm.nih.gov/29230835/#:~:text=Here%20we%20have%20identified%20variants,with%20precocity%20than%20with%20distance. “Genomic inbreeding trends, influential sire lines, and selection in the global Thoroughbred Population” (McGivney et al. 2020) https://pubmed.ncbi.nlm.nih.gov/31949252/ “Dietary regulation of fat oxidative gene expression in different skeletal muscle fiber types” (McAinch et al. 2012) https://onlinelibrary.wiley.com/doi/full/10.1038/oby.2003.197

Exercise Induced Pulmonary Hemorrhage (EIPH) occurs when the tiny blood vessels (capillaries) within the lungs of a horse burst during exercise, causing bleeding in the lungs. “Exercise-Induced”, meaning caused by exercise. “Pulmonary”, meaning the lungs. “Hemorrhage”, meaning bleeding. Basic Lung Anatomy and Physiology: Before we dive deeper into EIPH, let's review the basic anatomy and physiology of the lungs. Image of air pathway of horse. A horse’s lungs inhale oxygen first through the nostrils, then past the epiglottis, then trachea, then bronchioles, and then alveoli (alveolar sacs). Once the oxygen reaches the alveolar sacs, oxygen is diffused into the blood across an extremely thin layer of tissue that lines the blood vessels within the lungs. The layer between the open space in the lungs and the blood vessel wall is about 1/100th the width of a human hair– once again, an extremely thin layer. This thin layer is important because it allows oxygen to pass through this layer from the lung space and into the blood more easily than a thicker layer. However, this reduces the durability of these blood vessels within the lungs. Prevalence of EIPH Between 40-75% of thoroughbred racehorses experience EIPH during racing (Sullivan & Hinchcliff, 2015; McGilvary & Cardwell, 2021). But EIPH is not just limited to racehorses. Barrel racers (66%, Gold et al. 2018), show jumpers (50%, Bonomo et al. 2019), polo ponies (30%, Sullivan & Hinchcliff 2015), endurance horses (50%, Tarancon et al. 2019), draft pulling horses (26%, Burns et al. 2023), Three-Day Event horses (40%, Ainsworth & Hackett 2004), pony club event horses (10%, Ainsworth & Hackett 2004)  experience EIPH during competition. The incidence of EIPH is likely higher in these different sport horses, but the data is limited due to the lack of research of EIPH incidence in these different sport horses compared to thoroughbred racehorses. Data taken from studies cited above (references linked at bottom of article). The main method to determine the presence and severity of EIPH is via pulmonary endoscopy (otherwise known as “scoping”). A vet or trainer may take an endoscope (a thick snake-like wire with a camera on the end) up a horse’s nostril, past their epiglottis, and down their trachea. With endoscopic exams, the severity of EIPH (if any) can be determined and monitored in a specific horse following exercise. Image of endoscopic examination of lungs. There are different grades of severity of EIPH in horses, as shown in the image below. The distribution of EIPH severity are as follows: Grade 0 - No bleeding visible in trachea. Grade 1 - Less than quarter of trachea covered with visible blood. Grade 2 - Between quarter and half of trachea covered with visible blood. Grade 3 - More than half of trachea covered with blood. Grade 4 - More than 90% of trachea covered with blood (This grade is not depicted in the  image below) Epistaxis - The worst grade of EIPH is epistaxis, when a horse’s lungs bleed so severely that blood is present coming out of the nostrils of the horse. Sometimes it may take 30-60 minutes following hard exercise while their head is down to eat or graze when blood can be visible dripping out of the nostril. These cases of epistaxis are often not life threatening and it is extremely rare a horse develops further complications such as infection or severe blood loss where the horse’s life is threatened. In some very severe cases, epistaxis occurs in the middle of hard exercise where blood is violently blown out of the horse’s nostrils with each breath (it is these cases that often result in death of the horse). Keep in mind that these grades of EIPH are not universal and may vary depending on the study you are reading. Many studies use a scale of 4 grades in addition to Epistaxis. It is likely that the “Grade 3” shown in the image above would likely be a grade 4 (“severe”) in some studies. The prevalence of each grade of EIPH varies across studies. This data across different sport horses is likely not reliable enough to be worth reporting as of now due to small sample sizes and lack of studies overall– except thoroughbred racehorses because EIPH in those sport horses have been extensively studied for decades. However, there is a consistent trend of the majority of EIPH cases being grade 2 or less. The more severe the EIPH, the less common the prevalence. In Hinchcliff et al. (2005), thoroughbred racehorses in Australia (where Lasix is banned on race day) were scoped after their races to measure the severity of their EIPH. 44.6% had no EIPH. 36.7% had grade 1 EIPH. 13.5% had grade 2 EIPH. 3.3% had grade 3. 1.7% had grade 4. 1.7% had epistaxis. Epistaxis seems to occur in about 1-3% within all EIPH cases in thoroughbred racehorses according to numerous studies (Hinchcliff 2015). Data taken from Hinchcliff et al. (2005) in Australian thoroughbred racehorses following race without Lasix. The harder the horse exercises, the more likely the horse will experience more severe grades of EIPH, if any EIPH is present. (If you want to see the prevalence of the different grades of EIPH in the other sport horses I mentioned, I have linked the studies below for you to read yourself. If you have trouble accessing the full article, feel free to email me - gallopscience@gmail.com) The big takeaway here is that EIPH in horses is fairly common across all disciplines. Additionally, the vast majority of EIPH cases (over 97% of cases) cannot be seen unless you do an endoscopic examination of your horse’s trachea/airways. This is important for horse owners and trainers to understand because your horse may be bleeding every time they exercise and you won’t ever know it. Veterinarians and researchers have concluded EIPH is not painful due to the lack of nerves in the alveoli of the lungs, but in severe EIPH cases may invoke the horse to coughing, respiratory distress, and frequent swallowing. How much does EIPH affect Athletic Performance How common EIPH actually is in horses that exercise may be frightening to many horse owners and trainers. However, EIPH grades of 1 or less (which are the most common grades of EIPH cases across sport horses including racehorses) do not seem to affect performance or the horse’s career longevity. Once again, this area of research with other sport horses is very small, so I am not able to say how much EIPH affects other sport horse’s performance. However, this topic has been extensively studied in thoroughbred racehorses, which can provide some insight for owners and trainers of other sport horse disciplines. According to Sullivan & Hinchcliff (2015), a single episode of an EIPH grade 1-3 does not seem to impact career longevity in thoroughbred racehorses, whereas a grade 4 or epistaxis may. In Hinchcliff et al. (2005), racehorses with an EIPH grade of 2 or higher were 4-times less likely to win and 2-times less likely to win or finish in the top 3 of their race compared to horses with an EIPH grade of 1 or less. Racehorses with an EIPH grade of 1 or less were 3-times more likely to be in the top 90th percentile for race earnings. Meanwhile, a study by Crispe et al. (2019) with a large sample size found that only a grade 3 or higher of EIPH had a significant impact on performance whereas a grade 2 or less did not. In an article from the Paulick Report, Dr. Warwick Bayly comments on the results of an ongoing epidemiological study of EIPH prevalence in two year old thoroughbred racehorses racing without Lasix across the United States he and his colleagues have been conducting: ‘“If you're worried about EIPH from the perspective of interfering with performance, we found it had some impact on performance just like what's been shown before [in previous research], but only on really severe EIPH cases,” he said. “Less than 10 percent of horses had any association with not running well as a result of EIPH.”’ (Paulick Report 2023). If you want more details on the results of this study involving 2 year old racehorses racing without Lasix, check out this article: https://www.thoroughbreddailynews.com/from-dust-to-dust-do-terrible-racetrack-barns-exacerbate-eiph/ It is important to note that there are horses that experience severe EIPH, even epistaxis, but still perform well and even win races. Although, one can guess on how much better than horse can be if they did not experience EIPH. Horses that experience moderate to severe EIPH are more likely to have worsened EIPH the next time they undergo hard exercise. This is because when the blood vessels in the lungs break, they are not able to completely be repaired. Scar tissue may develop in replacement of the blood vessels, which is a weaker tissue. Some treatments involving stem cells have been promising in enhancing the recovery of the lungs following an EIPH episode. But more research is needed. Because EIPH can get progressively worse with repeated severe episodes, prevention of bleeding not only becomes critical for short term athletic performance, but also long-term performance. If you suspect your horse bled during a race, usually they will have abnormal fatigue and drop back from their position quickly about halfway through their race. The horses that pass them in the final 400-600m of the race are less likely to be bleeders. These findings have also been found in scientific literature, where Crispe et al. (2017) found that horses that had higher faster average speed in the early/mid portion of the race were more likely to be grade 3 or higher bleeders. Meanwhile, horses with grades 1 and 2 are more likely to run down their competitors from the back or mid-pack in the final 400m of the race compared to horses without any EIPH. If you monitor your horse’s heart rate after a workout or race, if the horse’s heart rate takes a longer time to drop following that exercise, they may be experiencing EIPH. Potential Causes of EIPH The concept many people fail to realize is that EIPH is a symptom caused and influenced by several factors. EIPH is likely not caused by one single variable. Those factors that influence EIPH prevalence and severity will be covered in this article. Why Pulmonary Hemorrhage is “Exercise-Induced” When a horse exercises, their heart rate rises and their stroke volume (the amount of blood pumped per heart beat) increases. As heart rate increases, the speed of flow of blood increases, which then increases the stress on the blood vessel walls. The body can help blunt these stresses by dilating (increasing the width of) the blood vessels in the body. However, how readily the blood vessels dilate and the max width of each blood vessel is limited. The harder the horse exercises, the higher their heart rate will get, the faster the blood will flow through the blood vessels, and the more resistance/stress is put upon the walls of the blood vessels. Given the blood vessels of the lungs are very small and frail as previously explained, they are more likely to rupture during exercise. Additionally, as the horse exercises, they are taking deeper breaths, and have a higher breath-rate the faster they gallop– in other words “pleural pressure” within the lungs increases the harder the horse exercises. With each breath, the alveolar sacs within the lungs expand and deflate. These mechanisms are constantly stretching and compressing these tiny blood vessels within the lungs, further increasing the stress on these blood vessels. This environment the blood vessels of the lungs are experiencing during exercise increase the risk of them breaking, causing the lungs to bleed. With all of this in mind, it makes it all the more incredible that there are sport horses, especially racehorses, undergoing vigorous exercise and do not bleed at all (at least visibly in the trachea). It is important to note that EIPH does not occur only during maximally hard exercise. It can occur during submaximal, easy-moderate exercise intensities as well in some horses. The Spleen The horse is unique in that they can store up to 30% of their red blood cells in their spleen. During fight or flight response, or during hard exercise, the spleen will release these red blood cells into the spleen. With more red blood cells in circulation, oxygen delivery from the lungs to the muscles is enhanced, enhancing the horse’s endurance. However, when the spleen releases red blood cells into the overall circulation, blood volume can increase to as much as 20%. This sudden and large increase in blood volume can drastically increase blood pressure. The release of more red blood cells into blood circulation also increases the blood’s viscosity (thickness, stickiness) which puts more resistance on the blood vessel walls. This combination of increased blood viscosity and blood pressure adds more stress on the blood vessel walls. Making them more prone to breaking. Because of this, researchers found an association between higher hematocrit (the proportion of red blood cells to blood plasma) and a higher risk of developing EIPH in Standardbred horses exercising on a treadmill (Lo Feudo et al. 2022). Lung Inflammation and Asthma Lung inflammation has been associated with EIPH in racehorses (Newton & Wood 2002; Michelotto et al. 2011; McKane & Slocombe 2010). However, whether the inflammation was a cause or effect of EIPH is unclear. Other studies have failed to find this association (Silva et al. 2017; Hinchcliff et al. 2015). How asthma or lung inflammation is measured varies across studies, so this may have contributed to these contradicting results. Lung inflammation can weaken the integrity of the blood vessel walls within the lungs. Lung inflammation can also constrict the airways, increasing pleural pressure. These factors can possibly increase the risk of EIPH. As for the mixed results of whether lung inflammation increases the risk of EIPH, these findings further emphasize that EIPH is multifactorial. One study found that mild equine asthma was present in 80% of racehorses and negatively impacted racing performance (Ivester et al. 2018). Another study also found that lung inflammation has the ability to decrease exercise capacity and performance in standardbred horses (Stucchi et al. 2020). So whether EIPH is influenced by lung inflammation or not, this variable should not be ignored. Temperature A handful of studies have found that horses racing in colder temperatures were associated with higher prevalence of EIPH (Crispe et al. 2016; Crispe et al. 2018; Lapointe et al. 1994; Hinchcliff et al. 2010; Leguillette et al. 2016). It is unknown if temperature acclimation plays a role. Hinchcliff et al. 2010 found that horses racing in <20 C° (68 F°) were 1.9 times more likely to have an EIPH grade greater than 1 and a two-fold increase in having a grade 2 or higher. Exercising in lower temperatures has been shown to cause airway inflammation and increase blood pressure in the pulmonary circuit in humans, horses, and other animals (Giesbrecht 1995; Davis et al. 2006; Chauca & Bligh 1976; Davis et al. 2007), which may serve as a possible explanation for this phenomenon. Cardiac Arrhythmias (Atrial fibrillation) Cardiac arrhythmias (abnormal heart rhythms) during exercise has been associated with increased risk of developing EIPH– particularly atrial fibrillation (AF). Nath et al. (2022) found that AF was associated with EIPH prevalence and severity. This area of research is relatively new and needs more evidence to definitively conclude a direct causal relationship between cardiac arrhythmias and EIPH risk. The idea is that during exercise, if the heart has several abnormal heart beats in a period of time, the blood flow from the lungs to the heart becomes interrupted. This can cause a “back-up” of blood flow within the lungs, drastically increasing the pressure on the blood vessel walls in the lungs and can cause them to burst. There are several causes of cardiac arrhythmias such as fatigue, lung inflammation, unsoundness, stress, genetics, etc. Horses are unique in that they seem to have cardiac arrhythmias more frequently during exercise than humans. In many horses, these arrhythmias seem to occur without cause and do not impair health or performance of the horse. Experts in equine cardiology are still trying to define what is considered “abnormal/concerning” cardiac arrhythmias and “normal” cardiac arrhythmias. Genetics There have been a few studies that have shown epistaxis is heritable, likely suggesting that EIPH overall is genetic (Velie et al. 2014; Weideman et al. 2004; Welsh 2014). However, these genetic links do not explain all EIPH cases. More definitive research is needed in this area. Speed and Racing Strategy As mentioned before, horses that had higher faster average speed in the early/mid portion of the race were more likely to be grade 3 or higher bleeders. Meanwhile, horses with grades 1 and 2 are more likely to run down their competitors from the back or mid-pack in the final 400m of the race compared to horses without any EIPH (Crispe et al. 2018). This is observed by racehorse trainers as well, solidifying this finding. Scientific literature has also shown that a rapid acceleration of speed causes higher pressure in the blood vessels of the lungs compared to a more gradual increase of speed over the course of an exercise to the same speeds (Manohar 1994). This suggests that faster acceleration may increase the likelihood of a horse experiencing more severe EIPH. Lasix Lasix has been repeatedly shown to decrease the severity of EIPH and is the only substance to have repeatedly shown such significant reductions in EIPH severity in research (Sullivan et al. 2014). But how does Lasix work? Remember back to “The Spleen” section above. Again emphasizing that during hard exercise the spleen releases a large portion of red blood cells into circulation. This causes a large and sudden increase in blood volume which increases stress on the blood vessel walls within the lungs. Lasix (also known as Furosemide or Salix) is a diuretic, meaning it reduces blood volume by forcing the body to urinate by excreting water from the blood (keep in mind that about 70% of the blood is water).  Lasix works by saturating sodium transporters in the horse’s blood so electrolytes like sodium, along with chlorine and calcium, are urinated out along with water. About 3cc of Lasix can cause the horse to urinate over 20lbs of urine from their body in the span of 4 hours. This reduction in blood volume can decrease blood pressure at rest and during exercise. The decreased blood volume Lasix causes puts much less stress on the blood vessel walls, reducing their likelihood to burst. In other words, Lasix can counteract the mechanisms of the spleen that contributes to EIPH. Keep in mind, Lasix does not eliminate EIPH, it only reduces the severity of it. This once again emphasizes that EIPH does not have one lone cause (such as high blood volume). Lasix only has the capacity to eliminate EIPH if the horse is already a grade 1-2 bleeder. Horses can still have severe EIPH with Lasix. In some horses, Lasix does not reduce EIPH severity at all. There are still some horses that can have a grade 4 or even epistaxis with Lasix despite receiving Lasix before exercise. This once again furthers the point that EIPH has multiple causes. Many racehorse trainers in countries where Lasix is not allowed before racing (Europe, Australia, Japan, etc) will prevent their horses from drinking water the day they race. This is for the same reason Lasix is used, to reduce the amount of water (within the plasma) in the blood to reduce the horse’s blood volume. Some trainer’s will “sponge” the horse’s mouth with water to still eliminate their sensation of thirst leading up to their race. Despite Lasix being banned on race-day in these countries, many trainers still give their horse’s Lasix before high-speed workouts during training (which is legal) to prevent them from bleeding. This is to prevent excessive long-term lung damage from EIPH in these horses. Some trainers only give Lasix to known “bad-bleeders” during training. Others give it to all their horses no matter what out of prevention. During the early to mid 1900’s, some veterinarians and trainers found that draining a bucket or two of the blood from the horse’s jugular vein the day before a race boosted some of those horse’s performances. This is likely due to, again, reducing blood volume and thus reducing the severity of EIPH. This practice has obviously not been used in decades with advancements in horse welfare. The more you use Lasix on a horse, the more the horse’s body may build a tolerance to the pharmaceutical. Smart veterinarians and trainers will start out giving horses small doses of Lasix over time (around 3cc). A horse that has been racing for years and given Lasix frequently throughout their career may be given 10cc prior to a workout. Electrolytes Electrolytes include calcium, potassium, sodium, chlorine, and magnesium. These molecules are important for the body’s hydration because they help retain water within the cells and blood. Electrolytes also have other mechanisms such as aiding in muscle contractions and maintaining the pH (acidity) of bodily fluids. What do electrolytes have to do with EIPH? Again, electrolytes help retain water within the blood, thus helping retain blood volume. Higher blood volume can cause higher blood pressure during exercise and put strain on the blood vessels within the lungs as previously explained. This is not to say that you should withhold essential electrolytes from your horse to prevent or reduce their EIPH, but maybe not overload them with it. Many old-timer racehorse trainers have observed that alfalfa can increase risk of EIPH. The original thought is that alfalfa can be dusty, which can inflame the airways. But this observation is more likely due to alfalfa’s high calcium content (an electrolyte that retains water in blood). Other trainers have been known to withhold salt and other electrolyte supplements from horse’s on race day because of their own experiences with high blood pressure; with their doctors advising they should reduce salt from their diet to improve their blood pressure. Again, this is because excessive salt (containing two electrolytes, sodium and chlorine) intake causes water retention, increasing blood volume, and has the potential to increase blood pressure. It is important to mention racehorse trainers that do retain water and/or electrolytes prior to races or give lasix will often give their horse’s electrolyte IV’s, supplements, or pastes following their races to aid in their recovery and welfare. Electrolytes are necessary for optimum athletic performance, but overdoing them may not be beneficial due to these reasons. Many fortified grains already contain the optimum balance of electrolytes your horse needs depending on the horse’s workload and sweat-rate. There are many equine supplements on the market claiming to reduce or eliminate EIPH without any scientific basis backing their product or their ingredients. If a supplement claims to reduce EIPH and contains any electrolytes (calcium, potassium, sodium, chlorine, magnesium), be suspicious. In fact, be suspicious over any supplements with ingredients that have little scientific basis behind what the supplement company claims they do. Lasix is the only substance that has proven through dozens of peer reviewed research papers to legitimately reduce EIPH severity (Sullvian et al. 2014). FLAIR Nosestrips Image of FLAIR nosestrip on racehorse. Remember back to when I mentioned “pleural pressure.” Pleural pressure is the amount of pressure put upon the walls of the lungs (and their blood vessels) when the horse is breathing. Given how rapidly horses are capable of breathing during exercise (taking over 2 breaths per second during a high speed gallop), the size of the horse’s lungs, and that the horse can only breathe through their nostrils, a horse’s pleural pressure compared to other animals is extremely high. This pressure alone can put much strain on the lung’s blood vessels. I mentioned Lasix is the only scientifically proven “substance” to reduce the severity of EIPH. Nose strips are also the only proven “product” that has been proven to reduce the severity of EIPH in a few scientific research articles. Nose strips allow a wider opening for air to flow through the nostrils, also preventing the nostrils from collapsing during inhalation. This allows pleural pressure within the lungs to be reduced. There have been three studies performed on nasal strips effectiveness of reducing EIPH: “Effect of an external nasal dilator strip on cytologic characteristics of bronchoalveolar lavage fluid in Thoroughbred race- horses” (Valdez et al. 2004) https://avmajournals.avma.org/view/journals/javma/224/4/javma.2004.224.558.xml “Effects of an external nasal strip and frusemide on pulmonary haemorrhage in Thoroughbreds following high intensity exercise” (Geor et al. 2001) https://beva.onlinelibrary.wiley.com/doi/abs/10.2746/042516401776563490?casa_token=OQXzMnM7-7EAAAAA:TnjP9QJFBzSJVF8hrUfh8Mfz5c6e66I_N5ztW0_gEvGpx93ZItM4N-rJDWvo6x6GnwLSpJ_y8ya1njsP “Effect of furosemide and the equine nasal strip on exercise-induced pulmonary haemorrhage and time-to-fatigue in maximally exercising horses” (McDonough et al. 2007). https://www.cambridge.org/core/journals/equine-and-comparative-exercise-physiology/article/abs/effect-of-furosemide-and-the-equine-nasal-strip-on-exerciseinduced-pulmonary-haemorrhage-and-timetofatigue-in-maximally-exercising-horses/9750879B5AEEEE10CEC43D265212D3B5 All three of these studies demonstrate nasal strips have the ability to reduce EIPH severity. However, it is important to mention that the study by Geor et al. (2001) and McDonough et al. (2007) were funded by CNS, Inc.- the company that patented and owns FLAIR Nosestrips (https://www.sec.gov/Archives/edgar/data/814258/000089710101500081/cns010440_10k.htm). The study by Valdez et al. (2004) does not specify their funding but did use FLAIR Nosestrips in their study. All of these studies have small sample sizes (which is common in equine research), so achieving statistical significance can be challenging unless the effectiveness of the treatment is very strong. Take whatever you want with this information. Then again, like Lasix, horses can still experience severe EIPH, even epistaxis, with FLAIR nose strips. Image of racehorse experiencing epistaxis with FLAIR nosestrip (this horse did not receive Lasix). Miscellaneous Since Lasix was banned in 2 year old and stakes races in thoroughbred racing within the United States, some trainers and vets have looked for non-medication alternatives to help EIPH that are legal before racing. Two of these include a vitamin C and/or L-arginine IV solution (or “jug”). Both this vitamin and amino acid alone can cause a diuretic effect when given in large quantities, which may help against EIPH the same way Lasix does. Although these natural substances may not be as strong of a diuretic as Lasix. Additionally, vets are not allowed to administer IV’s within 24 hours of a race, so the effect of the diuretic may wane over the 24 hours as opposed to Lasix being administered 4 hours before the horse races. Abnormal Throat Function Various throat dysfunctions, such as recurrent laryngeal dysfunction, dorsal displacement of soft palate, and entrapped epiglottis have been shown to impair horse performance, especially racehorses (Erck-Westergren et al. 2017). Photos of different types of airway dysfunction in the horse. (Photo from FLAIR equine nasal strips) These throat abnormalities are fairly common amongst performance horses, but many cannot be identified unless a throat endoscopy is performed while the horse is exercising (when the function of the airways is challenged) compared to analyzing throat function when the horse is at rest. ​​Upper airway obstruction has been shown to increase the risk of EIPH in several studies (Courouce-Malblanc et al. 2010; Joo et al. 2021; Mucciacito Junior et al. 2021; Mason et al. 2012; Ducharme et al. 2010). However, Davidson et al. (2011), Burns et al. (2023), and Lo Fuedo et al. (2022) found no significant association between airway function and EIPH risk. In the study by Mucciacito Junior et al. (2021), researchers analyzed over 3,000 thoroughbred racehorses’ upper airways for throat abnormalities and EIPH severity post-race (some were treated with lasix, others not). Researchers found that horses with dorsal soft palate displacement were significantly more likely to experience more prevalent and severe EIPH than horses with no throat abnormalities. However, there was no difference in the prevalence or severity of EIPH in horses with left-sided laryngeal hemiplegia (LLH) than horses with no throat abnormalities. The results from this study does not necessarily conclude that LLH does not put a horse more at risk for EIPH at all. This is also the case with the other studies that did not find a significant association with airway abnormalities and EIPH risk (Davidson et al. 2011, Burns et al. 2023, and Lo Fuedo et al. 2022). Again, as the theme of this entire article, EIPH is multifactorial. Other variables may have increased the risk for EIPH, thus causing a lack of statistical significance in this study to show LLH alone increases EIPH risk. That being said, it is even more interesting that racehorses with horses with dorsal soft palate displacement did in fact have significantly higher EIPH risk. Impaired airway function may increase the pressures within the lungs, thus increasing the risk for the blood vessels within the lungs to burst, causing EIPH. How about horses that underwent surgery to correct their airway abnormalities? Mason et al. (2012) found that thoroughbred racehorses that underwent prosthetic laryngoplasty with ventriculocordectomy (PLVC) surgery to treat left-sided laryngeal hemiplegia (LLH) were more likely to experience more severe EIPH (especially epistaxis) and tracheal mucus following races compared to racehorses with assumed normal airway function and did not undergo throat surgery. The researchers hypothesize that the surgery may have been inefficient to restore airway function and airway pressures within the respiratory tract and may increase airway inflammation (Ramzan et al. 2008; Tetens et al. 1996). As a result, the pressures within the alveoli are higher and blood vessels within the lungs are more likely to break, thus increasing the risk of EIPH (McKane & Slocombe 2010; Ducharme & Hiraga 1999; Jackson et al. 1997; West et al. 1993). It is not clear whether EIPH was worse pre vs post the surgery in these racehorses. Also keep in mind surgeries for different airway disorders have different effects on performance and the original severity of the airway dysfunction as well as the skill of the veterinarian performing the surgery also factor into post-surgery performance outcomes. Conclusion: In every equine discipline, there are horses that experience bleeding in their lungs due to exercise (EIPH). EIPH has several potential causes such as high blood pressure, high pleural pressure, high blood volume, respiratory inflammation, cold air temperature, cardiac arrhythmias, throat abnormalities, and genetics. Over 75% of thoroughbred racehorses without Lasix have an EIPH grade of 1 or less following a race, which may not impact athletic performance. Lasix is a very scientifically proven option to reduce severity, but not always completely eliminate, EIPH in horses. Flair Nosestrips and other non-pharmascudical diuretics (legal for competition) may also be helpful investments. Don’t fall for sham supplements. References: “Associations between Exercise Induced Pulmonary Hemorrhage (EIPH) and Fitness Parameters Measured by Incremental Treadmill Test in Standardbred Racehorses” (Lo Feudo et al. 2022). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8868235/ "Association between exercise-induced pulmonary hemorrhage and performance in Thoroughbred racehorses” (Hinchcliff et al. 2005) https://www.researchgate.net/profile/Kenneth-Hinchcliff/publication/7586365_Association_between_exercise-induced_pulmonary_hemorrhage_and_performance_in_Thoroughbred_racehorses/links/00b7d532895db91289000000/Association-between-exercise-induced-pulmonary-hemorrhage-and-performance-in-Thoroughbred-racehorses.pdf “Exercise Induced Pulmonary hemorrhage; risk factors, clinical signs, and prevention” (Rendel 2016) https://www.vettimes.co.uk/app/uploads/wp-post-to-pdf-enhanced-cache/1/exercise-induced-pulmonary-haemorrhage-risk-factors-clinical-signs-and-prevention.pdf “Prevalence of Exercise-Induced Pulmonary Hemorrhage, Tracheal Mucus and Recurrent Laryngeal Neuropathy in competitive Draft Pulling Horses” (Burns et al. 2023) https://www.sciencedirect.com/science/article/abs/pii/S0737080623007074 “Occurrence of exercise-induced pulmonary haemorrhage in show jumping horses”  (Bonomo et al. 2019) https://www.sciencedirect.com/science/article/abs/pii/S1090023318306944 “Exercise-Induced Pulmonary Hemorrhage in barrel racing horses in the Pacific Northwest region of the United States” (Gold et al. 2018) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5866954/ “Update on Exercise-Induced Pulmonary Hemorrhage” (Sullivan & Hinchcliff 2015) https://pubmed.ncbi.nlm.nih.gov/25770069/ “Prevalence of exercise induced pulmonary hemorrhage in competing endurance horses” (Tarancon et al. 2019) https://avmajournals.avma.org/view/journals/javma/255/6/javma.255.6.710.xml “Training related risk factors for exercise induced pulmonary haemorrhage in British National Hunt Racehorses” (McGilvary & Cardwell, 2021) https://beva.onlinelibrary.wiley.com/doi/full/10.1111/evj.13448 “EIPH: postrace endoscopic evaluation of standardbreds and thoroughbreds” (Birks et al. 2002) https://doi.org/10.1111/j.2042-3306.2002.tb05451.x “Bar shoes and ambient temperature are risk factors for exercise-induced pulmonary haemorrhage in Thoroughbred racehorses” (Crispe et al. 2016) https://doi.org/10.1111/evj.12458 “Evidence of an association between inflammatory airway disease and EIPH in young Thoroughbreds during training” (Newton et al. 2002) https://doi.org/10.1111/j.2042-3306.2002.tb05459.x “Exercise-induced pulmonary haemorrhage in Thoroughbred racehorses: a longitudinal study” (Crispe et al. 2018) https://doi.org/10.1111/evj.12957 “The Relationship between Lung Inflammation and Aerobic Threshold in Standardbred Racehorses with Mild-Moderate Equine Asthma” (Stucchi et al. 2020) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7459439/#:~:text=Mild%E2%80%93moderate%20equine%20asthma%20(MEA,important%20cause%20of%20poor%20performance. “Descriptive analysis of longitudinal endoscopy for exercise-induced pulmonary haemorrhage in Thoroughbred racehorses training and racing at the Hong Kong Jockey Club” (Preston et al. 2015) https://doi.org/10.1111/evj.12326 “Pulmonary inflammation due to exercise-induced pulmonary haemorrhage in Thoroughbred colts during race training” (Michelotto et al. 2011) https://pubmed.ncbi.nlm.nih.gov/22108190/ “An observational study of environmental exposures, airway cytology, and performance in racing thoroughbreds” (Ivester et al. 2018) - Dr. Laurent Couëtil https://onlinelibrary.wiley.com/doi/full/10.1111/jvim.15226 “Experimental mild pulmonary inflammation promotes the development of exercise-induced pulmonary haemorrhage” (Mckane and Slocombe 2010) https://pubmed.ncbi.nlm.nih.gov/21059012/ “A systematic review and meta-analysis of the efficacy of furosemide for exercise-induced pulmonary haemorrhage in Thoroughbred and Standardbred racehorses” (Sullivan et al. 2014) https://beva.onlinelibrary.wiley.com/doi/10.1111/evj.12373 “Heritability of epistaxis in the Australian Thoroughbred racehorse population” (Velie et al. 2014) https://www.sciencedirect.com/science/article/abs/pii/S1090023314002664 “A genetic analysis of epistaxis as associated with EIPH in the Southern African Thoroughbred” (Weideman et al. 2004) https://www.bloodhorse.com/pdf/WeidemanPaperEIPHandGenetics.pdf “Heritability analyses of musculoskeletal conditions and exercise-induced pulmonary haemorrhage in Thoroughbred racehorses” (Welsh 2014) https://theses.gla.ac.uk/4862/1/2014WelshPhD.pdf “Associations between postrace atrial fibrillation and measures of performance, racing history and airway disease in horses” (Nath et al. 2022) https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/jvim.16878 “The association between exercise-induced pulmonary haemorrhage and race-day performance in Thoroughbred racehorses” (Crispe et al. 2019) https://beva.onlinelibrary.wiley.com/doi/full/10.1111/evj.12671 “Risk factors for exercise-induced pulmonary haemorrhage in Thoroughbred racehorses” (Hinchcliff et al. 2010) https://beva.onlinelibrary.wiley.com/doi/full/10.1111/j.2042-3306.2010.00245.x?casa_token=zeD67ScB53QAAAAA%3AOt3KaN1T9gD8l5n3SQWukmBRBz5esYObXl-gXzW10b19Dj8BAZzxJyNz6fso3ez1D6Dx2eP9zOpRKQM “A survey of exercise-induced pulmonary haemorrhage in Quebec standardbred racehorses” (Lapointe et al. 1994) https://pubmed.ncbi.nlm.nih.gov/7889923/ “Tracheobronchoscopic Assessment of Exercise-Induced Pulmonary Hemorrhage and Airway Inflammation in Barrel Racing Horses” (Leguillette et al. 2016) https://onlinelibrary.wiley.com/doi/full/10.1111/jvim.13959 “Influx of neutrophils and persistence of cytokine expression in airways of horses after performing exercise while breathing cold air” (Davis et al. 2007) https://pubmed.ncbi.nlm.nih.gov/17269885/ “Evaluation of race distance, track surface and season of the year on exercise-induced pulmonary haemorrhage in flat racing thoroughbreds in Brazil” (Costa & Thomassian 2006) https://beva.onlinelibrary.wiley.com/doi/abs/10.1111/j.2042-3306.2006.tb05592.x?casa_token=JnJfGuQfiYMAAAAA:WPS9aaBQMDWiBn3agbQIv9pzgwEP_Mdqq3dbB-q8snVmjR1fyFL5d5zOTb4wr4hxwExqX-mFTLGi3Ao “Risk factors for exercise-induced pulmonary haemorrhage in Thoroughbred racehorses” (Hinchcliff et al. 2010) https://beva.onlinelibrary.wiley.com/doi/full/10.1111/j.2042-3306.2010.00245.x?casa_token=zeD67ScB53QAAAAA%3AOt3KaN1T9gD8l5n3SQWukmBRBz5esYObXl-gXzW10b19Dj8BAZzxJyNz6fso3ez1D6Dx2eP9zOpRKQM “Exercise-Induced Pulmonary Hemorrhage in the racing apollosa horse” (Hillige et al. 1985) https://www.sciencedirect.com/science/article/abs/pii/S0737080685800095 “Frequency of and risk factors for epistaxis associated with exercise-induced pulmonary hemorrhage in horses” (Takahashi et al. 2001) https://avmajournals.avma.org/view/journals/javma/218/9/javma.2001.218.1462.xml “Pulmonary vascular pressures of Thoroughbreds increase rapidly and to a higher level with rapid onset of high-intensoty exercise than slow onset” (Manohar 1994) https://beva.onlinelibrary.wiley.com/doi/abs/10.1111/j.2042-3306.1994.tb04057.x “The respiratory system in a cold environment” (Giesbrecht 1995) https://pubmed.ncbi.nlm.nih.gov/7487830/#:~:text=Acute%20or%20chronic%20cold%20exposure,secretions%20and%20decreased%20mucociliary%20clearance. “Cold air-induced late phase bronchoconstriction in horses” (Davis et al. 2006) https://pubmed.ncbi.nlm.nih.gov/17402479/ “Influx of neutrophils and persistence of cytokine expression in airways of horses after performing exercise while breathing cold air” (Davis et al. 2007) https://pubmed.ncbi.nlm.nih.gov/17269885/ “An additive effect of cold exposure and hypoxia on pulmonary artery pressure in sheep” (Chauca & Bligh 1976) https://pubmed.ncbi.nlm.nih.gov/951524/ “Respiratory diseases and their effects on respiratory function and exercise capacity” (Erck-Westergren et al. 2013) https://pubmed.ncbi.nlm.nih.gov/23368813/ “Cohort study examining long-term respiratory health, career duration and racing performance in racehorses that undergo left-sided prosthetic laryngoplasty and ventriculocordectomy surgery for treatment of left-sided laryngeal hemiplegia” (Mason et al. 2013) https://pubmed.ncbi.nlm.nih.gov/22812572/ “Experimental mild pulmonary inflammation promotes development of exercise-induced pulmonary hemorrhage” (McKane & Slocombe 2010) https://pubmed.ncbi.nlm.nih.gov/21059012/ “Efficacy of prosthetic laryngoplasty with and without bilateral ventriculocordectomy as treatments for laryngeal hemiplegia in racehorses” (Tetens et al. 1996) https://pubmed.ncbi.nlm.nih.gov/8915450/ “Lower respiratory tract disease in Thoroughbred racehorses: analysis of endoscopic data from a UK training yard” (Ramzan et al. 2008) https://pubmed.ncbi.nlm.nih.gov/18083654/ “Upper airway disease: does it affect lower airway mechanisms and pulmonary haemodynamics?” (Ducharme & Hiraga 1999) https://pubmed.ncbi.nlm.nih.gov/10659336/ “Effects of airway obstruction on transmural pulmonary artery pressure in exercising racehorses” (Jackson et al. 1997) https://pubmed.ncbi.nlm.nih.gov/9256978/ “Stress failure of pulmonary capillaries in racehorses with exercise induced pulmonary hemorrhage” (West et al. 1993) https://pubmed.ncbi.nlm.nih.gov/8226517/ “Relation between Exercise-induced pulmonary hemorrhage and findings in upper airway and trachea in Thoroughbred racehorses” (Mucciacito Junior et al. 2021) https://periodicos.pucpr.br/cienciaanimal/article/view/27894/pdf “Upper and Lower Airways Evaluation and Its Relationship with Dynamic Upper Airway Obstruction in Racehorses” (Lo Fuedo et al. 2022) https://www.mdpi.com/2076-2615/12/12/1563 “Physiological measurements and prevalence of lower airway diseases in Trotters with dorsal displacement of the soft palate” (Courouce-Malblanc et al. 2010) https://pubmed.ncbi.nlm.nih.gov/21059014/ “Asthmatic Disease as an underlying cause of dorsal displacement of the soft palate in horses” (Joo et al. 2021) https://pubmed.ncbi.nlm.nih.gov/33349416/ “Dynamic endoscopy of the upper-airway– What is significant?” (Trope 2013) https://pubmed.ncbi.nlm.nih.gov/23667075/ “Upper airway obstruction partial asphyxia as possible cause of exercise-induced pulmonary hemorrhage in the horse: An hypothesis” (Cook 1988) https://www.sciencedirect.com/science/article/abs/pii/S0737080688801035 “Exercising blood gas analysis, dynamic upper respiratory track obstruction, and post exercising bronchoalveolar lavage cytology: A comparative study in poor performing horses” (Davidson et al. 2011) https://www.sciencedirect.com/science/article/abs/pii/S0737080611000955 Extra: EIPH Risk Factors- epidemiology https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/exercise-induced-pulmonary-hemorrhage EIPH is Genetic - https://www.bloodhorse.com/horse-racing/articles/135743/study-eiph-is-an-inherited-trait?fbclid=IwAR2irUwPTnS4rV3FGjYv0bIoQ-svrm3OszXXsRyaKn7kvb0bAX8ZatxYdQI From Dust to Dust: Do “Terrible” Racetrack Barns Exasperate EIPH? -TDN https://www.thoroughbreddailynews.com/from-dust-to-dust-do-terrible-racetrack-barns-exacerbate-eiph/ Study shows less than 10% of 2yo runners experience severe EIPH regardless of Lasix administration- Paulick Report, Dr. Bayly https://paulickreport.com/horse-care-category/study-shows-less-than-10-percent-of-2-year-old-runners-experience-severe-eiph-regardless-of-lasix-administration/ “Effect of bedding on the incidence of Exercise Induced Pulmonary Hemorrhage  in racehorses in Hong Kong” (Mason et al. 1984) https://europepmc.org/article/med/6495577 “The effect of herbal supplementation on the severity of exercise induced pulmonary haemorrhage” (McDonough et al. 2007) Yunnan Paiyao and Single Immortal were not any more effective to a placebo (corn starch) in reducing EIPH. https://www.cambridge.org/core/journals/equine-and-comparative-exercise-physiology/article/abs/effect-of-herbal-supplementation-on-the-severity-of-exerciseinduced-pulmonary-haemorrhage/E141F8C64193637B6F0B8755D5D38839 “New Furosemide research reveals unexpected impacts of the medication” - Paulick Report - KER research on Lasix urination quanity with 3-10cc of Lasix and effects on drinking dehydration. https://paulickreport.com/horse-care-category/vet-topics/new-furosemide-research-reveals-unexpected-impacts-of-the-medication

Nearly 100 years ago, researchers of exercise physiology found that as the harder the muscles worked, the more fatigued the muscle became, and the more lactic acid accumulated within the muscles. In other words, as the concentration of lactic acid increases in the muscles, the more fatigued the muscles become. This association between lactic acid and muscle fatigue has guided how we perceive exercise fatigue and muscle soreness. However, as any good critical thinker knows, association is not causation. This cause-effect relationship between lactic acid and muscle fatigue has been disproven repeatedly in published research over the last 25 years. Yet, there are still many horse owners, trainers, and even equine veterinarians that believe when a horse experiences exercise-induced fatigue, muscle soreness, or severe muscle cramping (otherwise known as tying-up), this is caused by lactic acid. Once again, these notions are false. This article aims to explain why lactic acid is no longer thought to cause exercise fatigue or muscle soreness, and that lactic acid is actually a beneficial bi-product of exercise metabolism rather than a ‘waste product.’ To clear things up, lactic acid actually isn’t the molecule that accumulates in the muscle cell during hard exercise, lactate is. Lactate is the conjugate base of lactic acid. I will cover more on that later, but for now just associate lactic acid with lactate. How much lactate the body is producing can be measured by the concentration of lactate in the blood, otherwise known as "blood-lactate concentration". So what is lactate exactly? Lactate is the product of anaerobic glycolysis (one of three pathways that makes energy for the muscle cell). When the muscle cell is in demand for energy, the cell may take carbohydrates stored within the cell or blood and break it down into glucose (a sugar molecule). This sugar molecule then goes through a series of chemical reactions that results in 2 molecules called pyruvate. During the process of forming pyruvate, energy (in the form of ATP) is formed for the muscle cell. This process is called glycolysis. Image of aerobic and anaerobic pathways of glycolysis. Pyruvate can then go through one of 2 pathways: 1) Convert into Acetyl-CoA and go into the mitochondria (the ‘powerhouse’ of the cell)  to be further broken down into energy with the assistance of oxygen, Aerobic Glycolysis. Or 2) Pyruvate is converted to lactate, Anaerobic Glycolysis. These processes are shown in the previous image. What dictates whether pyruvate goes to the mitochondria to form more energy or converts into lactate? The muscle cell’s demand for energy. If the muscles need a high amount of energy very quickly, such as during an extended fast gallop, then glycolysis will be repeated rapidly, producing a lot of energy over a short period of time, with lactate being the end product. Thus, lactate accumulates quickly in the cell, regardless of how abundant oxygen is within the muscle cell. If the demand of energy is low, such as a trot, canter, or even a light gallop for fitter horses, then glycolysis will occur at a lower rate and the pyruvate that are formed will be sent to the mitochondria to form more energy rather than converting into lactate. What is the purpose of pyruvate turning into lactate? Wouldn’t it be more efficient for the pyruvate to go to the mitochondria to form more energy for the cell? Not exactly. You see, glycolysis is one of the fastest pathways of creating energy for the muscle cell. But glycolysis has a downside, the process produces hydrogen ions (H+). The concentration of hydrogen ions can determine the pH (acidity or alkalinity) of the muscle cell. As the number of hydrogen ions increases in the cell, the lower the pH and the more acidic the cell becomes. If the cell becomes too acidic, metabolic processes, such as forming energy, become impaired and slow down. Some molecules can even become damaged within the cell if the pH gets too low. During low energy demand, glycolysis produces hydrogen ions at a very low rate. The cell can tolerate this low accumulation of hydrogen ions by removing them out of the cell or taking them up by other molecules. Thus, not significantly changing the pH of the cell. But as mentioned before, in times of high energy demand, glycolysis will repeat rapidly. This will cause a rapid accumulation of acidic hydrogen ions within the cell. This influx of hydrogen ions occurs too rapidly than the cell can handle and pH begins to drop. However, there is a molecule that can help buffer this rapid increase in acidity: lactate! When pyruvate is converted into lactate, it takes up 2 hydrogen ions. By absorbing hydrogen ions formed inevitably by glycolysis, the acidity of the cell can be reduced. Essentially, the conversion of pyruvate to lactate helps reduce the accumulation of acid within the cell, thus improving the capacity of the muscle cell to perform more glycolysis to meet the high demand of energy. This is why it is more energy efficient to produce lactate following glycolysis than have pyruvate enter the mitochondria to form more energy (which is a time consuming process and is not as efficient in times of high energy demand). Glycolysis is an imperative energy pathway to form energy very quickly during high intensity exercise, but if hydrogen ions are not taken up by lactate, acidity accumulates in the muscle cell quickly and glycolysis is slowed down- overall, slowing down energy production and causing muscle acidosis and fatigue. But when pyruvate is converted to lactate, hydrogen ions are able to be taken up and the acidity of the cell is reduced. Image of pyruvate converting to into lactate by taking up two acidic hydrogen ions. Let’s look at more of the science showing why lactate does not cause fatigue but actually aids in preventing fatigue: If you actually look at the acidity of pyruvate vs lactate, pyruvate is actually a more acidic molecule than lactate. In chemistry, a molecule’s acidity can be  measured by using pKa. The smaller the pKa, the more acidic the molecule is. Pyruvate has a pKa of 2.50. Lactate has a pKa of 3.87. Thus, making pyruvate much more acidic than lactate for the cell. If you have a chemistry background and want to learn more about the biochemistry of muscle fatigue, you may be interested in this article: “Biochemistry of exercise induced metabolic acidosis” R Robert’s et al. (2004) https://pubmed.ncbi.nlm.nih.gov/15308499/ This article goes on to state that lactate inhibits, not causes, muscle acidosis. To further prove this point, another very fascinating study performed in 2001 looked at the effects of injecting lactic acid into the muscles of rats during fatiguing muscle contractions. Researchers isolated the muscles of rats and forced it to contract using an electric shock. They repeated these shocks while recording the force production of the muscle over time. As expected, the more the muscle was shocked, the weaker the force the muscles were able to produce over time due to muscle fatigue. The pH of the muscles were also measured over time. As the muscles fatigued, the lower the pH (more acidic) the muscles had. However, once they injected a solution of lactic acid into the fatigued muscles, when they went to shock the muscles again, the force production was able to recover almost as high as the initial force produced at the start of the exercise. The conclusion of this study was that lactate actually has a protective effect against exercise-induced muscle fatigue. So what happens to lactate after it takes up hydrogen ions to help buffer cell acidity? Well it can do one of two things: 1) Be transported out of the cell and into the blood, where it can be transported to the liver and converted back into glucose/sugar for energy. Or 2) The lactate is taken into the mitochondria of the muscle cell and is further broken down into energy. I will explain these mechanisms and what it means for a horse’s performance in future articles. But for now, understand that lactate is not only a buffer, but a source of energy for the cell. Not an acid. Not a waste product. Image of lactate pathways. The “burn” and fatigue your horse’s muscles feel during high intensity exercise is a combination of muscle acidosis caused by the influx of hydrogen ions (formed by glycolysis) and also possibly micro-damage of muscle fibers during high-force muscle contractions depending on the fitness of the horse. Not lactic acid/lactate. So what about muscle soreness following vigorous exercise? This can’t be caused by lactate because lactate concentration in the muscle and blood actually return to normal resting levels within 1 hour of exercise, regardless of how hard the exercise was. This is because lactate is immediately recycled back into glucose or energy as previously mentioned. Research has shown that muscle soreness is caused by micro-tears within the muscle fibers. When these micro-tears occur, a molecule called Creatine kinase (CK), as well as other protein molecules that make up the muscle cell, leaks out into the extracellular space and blood. This is why CK is elevated in the blood days following exercise. Exercise physiologists and vets use blood CK as a measure of muscle damage following exercise. Micro-tears in the muscle following rigorous exercise can also result in a horse tying-up (muscle cramping or rhabdomyolysis) if the damage is severe enough. As mentioned before, this causes CK and damaged muscle proteins to be elevated in the blood. Eventually, the horse’s kidneys filter these proteins out of the blood and into the urine, causing a very dark-smelly urine color. The dark color in the urine are the damaged muscle proteins and CK. This is why it is common to see horses urinate this dark-smelly urine after a rhabdomyolysis episode or severe exercise. Image of muscle micro-tears under microscope. Micro-tears in the muscle can take several days to repair depending on the severity of damage to the muscle, the horse’s diet, and the exercise regimen following the initial exercise. The horse may not actually feel muscle sore until 1-2 days following the initial exercise. This is called "delayed onset muscle soreness" (DOMS). The exact reasoning for why DOMS occurs is not yet well understood. Often horse owners, riders, or trainers will exercise their horse the days following a hard workout with the hopes of “flushing out” the lactic acid from the muscle. But as mentioned before, this concept does not make any physiological sense considering lactic acid/lactate is already “flushed” out of the muscle within an hour following a hard workout. And the acidity level of the muscle caused by hydrogen ions is also recovered within a few hours following a hard workout as well. So the idea that you are helping “flush out” lactic acid out of the horse’s muscles by exercising them in the days following a hard workout is false. However, this does not mean that light exercise the days following a hard workout is not beneficial to overall muscle recovery. Also, lactate can be "flushed out" of the muscles faster with light exercise such as a light canter, trot, or walk- but only within the minutes (not hours or days) following a hard bout of exercise. Additionally, I’m sure many horsemen have heard “horse physical therapists” saying their massaging, PEMF, laser light, cryotherapy, etc. “helps reduce lactic acid from the muscles” several hours or days after exercise. But once again, these claims are not congruent with the physiology. Graph of blood lactate over time before, during, and after exercise. To conclude: Lactic acid does not cause muscle acidosis, hydrogen ions do. Lactic acid, or more correctly termed lactate, is actually a buffer and helps to prevent muscle acidosis. Therefore, lactate is actually protective against muscle fatigue. Lactate does not cause muscle soreness following exercise, muscle damage does. Lactate is brought back to normal levels in the body within 60 minutes following hard exercise. Furthering the point that lactate cannot and does not cause muscle soreness the days following exercise. Muscle cramping is not caused by lactate, but most likely by the acidosis of hydrogen ions and muscle damage. Hopefully you are now better equipped with the knowledge of what actually causes muscle acidosis and have an improved understanding of the protective mechanisms lactate has against muscle fatigue. In future articles, I plan to cover more about how lactate can be used to measure a horse’s fitness as well as the molecule’s important mechanisms during exercise that can dictate exercise performance. If you want more resources on this topic, I have a list of links to peer-reviewed research papers, articles, and videos explaining some of the concepts I mentioned in this article down below. If you have any questions, please feel free to email me at gallopscience@gmail.com. Published Journal Articles: “Biochemistry of exercise induced metabolic acidosis” (R Robert’s et al. 2004) https://pubmed.ncbi.nlm.nih.gov/15308499/ “Protective effects of lactic acid on force production in rat skeletal muscle” (Nielson et al. 2001) https://physoc.onlinelibrary.wiley.com/doi/10.1111/j.1469-7793.2001.t01-1-00161.x “Lactate: metabolic fuel or poison for racehorses?” (Lindinger 2011) https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/expphysiol.2010.056531 “Lactate Doesn't necessarily cause fatigue: why are we surprised?” (Brooks 2001) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2278833/#:~:text=Since%20the%20work%20of%20Fletcher,until%20fatigue%20accumulate%20lactic%20acid. Other Newsletter Articles: “Myths about lactic acid debunked” https://recoupfitness.com/blogs/news/3-myths-about-lactic-acid-debunked “The Lactic Acid Myth” https://drkhorsesense.wordpress.com/2018/09/16/the-lactic-acid-myth/ Videos: “Glycolysis Explained (Aerobic vs Anaerobic, Pyruvate, Glyconeogenesis)” https://youtu.be/f62zuNfU28I?si=q4K0iy0rdGKVB46M “What Lactate is & What it ACTUALLY does: 5 Min Phys” https://youtu.be/QtOcIZqkieA?si=CyTP_M5zQ2tRbE-7