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)
“Maternal Obesity increases resistance, low-grade inflammation, and osteochondrosis (OCD) lesions in foals and yearlings until 18 months of age” (Robels et al. 2018)
“Impact of Maternal Exercise during pregnancy on offspring chronic disease susceptibility” (Blaize, Pearson, and Newcomer, 2015)
“Physical activity patterns during pregnancy” (Borodulin et al. 2008)
“Aerobic exercise during pregnancy influences fetal cardiac autonomic control of heart rate and heart rate variability” (May et al. 2010)
“Maternal and foetal heart rates during exercise in horses” (Lehnhard et al. 2008)
“Maternal investment results in better foal condition through increased play behavior in horses” (Cameron et al. 2008)
“Effects of exercise or oocyte heat shock on embryo development and gene expression in the horse” (Mortensen 2008- unpublished dissertation)
“Effects of physical inactivity and obesity on morbidity and mortality: current evidence and research issues” (Blair & Brodney 1999)
“Impact of Cardiorespiratory Fitness on All-Cause and Disease-specific mortality: Advances since 2009” (Harber et al. 2017)
“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)
Equine Metabolic Syndrome (Frank 2010)
“Equine Grain Associated Disorders” (Kronfeld & Harris 2003)
“Metabolic Predispositions to Laminitis in Horses and Ponies: Obesity, Insulin Resistance and Metabolic Syndromes” (Raymond et al. 2008)
“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)