Vitamin D Biology Cannot Be Extrapolated from Humans to Horses

The daily dose of vitamin D required by horses is largely speculative as the biology of this vitamin remains understudied. What is known, however, is that understanding the ins and outs of vitamin D in horses should not be entirely extrapolated from other species.

In humans, for example, vitamin D deficiency occurs commonly, up to 50% in certain populations, with only the more severe cases presenting with osteomalacia or bone softening. Deficiencies in horses, on the other hand, are not frequently reported, if at all, according to Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research.

Vitamin D Levels in Healthy Horses

Vitamin D exists in two main forms in the body: vitamin D2 and vitamin D3. Fungi on plants produce vitamin D2 that horses consume, whereas nutritional supplements contain vitamin D3. In some species, like humans, vitamin D3 is also produced in the skin after exposure to ultraviolet B rays from sunshine. Whether or not this occurs in horses remains unclear.

In a recent study, vitamin D2 and D3 levels were measured in two groups of horses: (1) those stabled for most of the day and supplemented with vitamin D3 daily, and (2) unsupplemented horses kept on pasture.^* Blood levels of vitamin D2 and D3 were measured intermittently for one year.

Horses supplemented with vitamin D3 had higher levels of vitamin D3 compared with the group of unsupplemented horses kept on pasture. In fact, vitamin D3 levels in pastured, unsupplemented horses were undetectable year-round, even on long summer days. Data from this study also revealed:

• Vitamin D2 levels were significantly higher in horses kept on pasture than in stabled, supplemented horses;
• Vitamin D2 was lower during the winter than in the summer; and
• Total vitamin D levels in all horses was an average of 10.7 nmol/L, which was lower than the level considered a deficiency in humans (less than 25 nmol/L).

“In humans, these levels would constitute a severe deficiency. Yet, in the populations of horses included in the study, no clinical evidence of hypovitaminosis D was appreciated, such as demineralization of skeletal tissue, rickets, or osteoporosis,” Crandell explained.

Vitamin D Levels in Obesity, EMS, and PPID

Studies in humans show that vitamin D deficiency occurs commonly with obesity, metabolic syndrome, and type 2 diabetes. Further, evidence suggests that supplementing with vitamin D may improve insulin sensitivity.

“There is a high prevalence of equine metabolic syndrome (EMS) and pituitary pars intermedia dysfunction (PPID, equine Cushing’s disease) in aged horses. If vitamin D supplementation could help horses with endocrine issues as it does in humans, then it would give us another option for treating these diseases,” Crandell said.

Vitamin D levels were measured in 107 horses with metabolic problems. Thirty-two were diagnosed with PPID, 41 with insulin dysregulation (ID, a central feature of EMS), and 14 with both PPID and insulin dysregulation. Forty of those animals had previously been diagnosed with laminitis, one of the most important sequelae of endocrine abnormalities.

As identified in the first part of the experiment, only vitamin D2, not vitamin D3, was measurable in all 107 horses. Neither ID or PPID were associated with vitamin D2 status in this group of horses; however, obese horses had higher vitamin D2, which is contrary to obese humans, who are reportedly deficient in vitamin D and have a higher risk of developing ID and diabetes. The lower vitamin D levels in obese humans was thought to be due to the fact that vitamin D is a fat-soluble vitamin and is sequestered in the fat rather than circulating in the bloodstream.

Do Horses Need a Supplement?

“Based on these data, the biology of vitamin D appears quite different in horses compared to other species. Total measurable vitamin D levels (both D2 and D3) were well below what is considered a deficiency in humans. These levels would be associated with a high risk for the development of osteomalacia, an occurrence that has not been reported in horses,” reported Crandell.

These data also show that the skin production of vitamin D3 in horses is absent/minimal compared to other species and that consumption of vitamin D—through either dietary vitamin D2 or supplemental vitamin D3—is more important. But, how much vitamin D a horse actually needs still remains unknown.

According to Crandell, “The National Research Council suggests that horses have a daily requirement of 6.6 IU of vitamin D per kilogram of body weight, but does not indicate which form. Further, that recommendation was based on a single experimental study conducted in the late 1970s. “Further investigation is highly warranted,” Crandell noted.

In conclusion, the researchers wrote, “Our data suggest that grazing animals (horses) can remain clinically healthy despite having undetectable levels of vitamin D3 because pasture-derived vitamin D2 meets their vitamin D requirements; supplementation of sedentary animals with access to pasture/forage is therefore unlikely to be necessary for optimal health.”

Careful management of horses can reduce the incidence of obesity, even in those horses genetically engineered to be easy keepers. Management strategies include: (1) suitable forage-based diets with appropriate supplementation of a balancer pellet or a vitamin and mineral supplement; (2) use of a grazing muzzle to limit pasture intake; and (3) a regular exercise program that matches the physical capabilities of the horse. For sound horses, exercise remains an important tool in the fight against certain metabolic diseases.

 ^*Dosi, M.C.M.C., B.C. McGorum, R.D. Kirton, E. Cillan-Garcia, R.J. Mellanby, J.A. Keen, E.A. Hurst, and R.A. Morgan. 2022. The effect of season, management and endocrinopathies on vitamin D status in horses. Equine Veterinary Journal:13873.