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Most body systems of the horse are driven by light and circadian rhythms, which affect sleep, body temperature, immune function, and digestive activity, among other things. Physical activity and athleticism are also ruled by the sun and the seasons, making transnational travel and jet lag an important consideration for globe-trotting horses participating in international competitions.

“Equine athletes that travel by air to competitions face huge time and seasonal differences that might affect their health, their ability to adjust their circadian rhythms to new time zones and routines, and their performances during the competition events,” wrote a group a veterinary researchers* from New Zealand.

Establishing the rhythm of various hormones and minerals in horses is challenging compared to other mammals, like humans, because their sleep-wake pattern is unique and sleep is not limited to dark hours.

To better characterize how various body processes are impacted by light and season, Azarpeykan and colleagues measured vitamin D metabolites, also called calciotropic hormones, as well as calcium (Ca), magnesium (Mg), and phosphorus (P) on the shortest and longest days of the year. Vitamin D is produced in the skin of animals, and exposure to light could impact its production as well as bone health and levels of Ca and P.

“The study revealed that circulating levels of the calciotropic hormones, Ca, P, and Mg were all significantly lower on the shortest day of the year compared to the longest, and rhythmic fluctuations occurred on the longest, but not shortest, day of the year,” explained Kathleen Crandell, Ph.D., an equine nutritionist for Kentucky Equine Research.

While scientists are still piecing together the overall impact of these biological rhythms, they do suggest that management practices can influence the ebb and flow of various hormones in athletic horses, potentially minimizing the impact of jet lag and seasonal changes.

“Horses that are primarily stabled rather than on pasture most of the day and those fed, watered, and exercised at the same time every day regardless of where they are in the world may be less likely to be negatively impacted by jet lag,” suggested Crandell.

She added, “Of course, ensuring the diet is balanced and provides adequate levels of macro- and micronutrients like vitamin D, Ca, P, and Mg is also imperative.”

*Azarpeykan, S., K.E. Sitmer, E.K. Gee, et al. Circadian rhythm of calciotropic hormones, serum calcium, phosphorus and magnesium during the shortest and longest days of the year in horses in New Zealand. Journal of Animal Physiology and Animal Nutrition. In press.

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