Vitamin requirements of individual horses are often hard to define. Most vitamin requirements represent the amount needed in the horse’s diet to prevent the classic deficiency symptoms. However, that may not be the same as the amount required for optimum health, well-being, or performance.
Most of us are familiar with vitamin D’s role in calcium absorption, and that it is synthesized by the skin when exposed to sunlight. However, the various precursors of vitamin D, and its active and inactive forms may be less familiar.
Vitamin D is actually a steroid hormone. Horses consume vitamin D naturally from plants in the form of ergocalciferol, or vitamin D2. In manufactured diets, vitamin D is typically supplemented in the form of vitamin D3, or cholecalciferol. Horses also synthesize D3 from skin exposure to ultraviolet light. Vitamins D2 and D3 are absorbed out of the small intestine, and converted to calcidiol in the liver. Calcidiol is the compound that is typically used as an indicator of vitamin D status, as it closely reflects both dietary intake and skin synthesis. Horses general have a much lower concentration of calcidiol in the blood in comparison to other animals.
One more reaction must take place in the kidney before vitamin D is in its active form, known as calcitriol. This final reaction is actually tightly regulated according to body needs. More calcidiol will be converted to this active form, calcitriol, when needed.
ACTIVATED D’s ROLE
Activated vitamin D directly regulates the amount of calcium and phosphorous circulating in the blood. It can increase the amount of calcium in the body by increasing its rate of absorption out of the small intestine, and increasing reabsorption by the kidney.
Vitamin D promotes mineralization of the skeleton through its regulation of calcium, and deficiencies of vitamin D result in osteomalacia. In young animals and humans, this is referred to as rickets.
While the function of calcium regulation is commonly known, vitamin D is actually involved in the normal function of a variety of tissues. Beyond bone health, vitamin D also has a role in cell growth and tissue differentiation. Vitamin D receptors have been found in all cell types in the body, emphasizing its much wider role in the physiology of the body.
In human nutrition, vitamin D and its role in other body functions, particularly immune function, has been more fully explored than in any of our animal species. Macrophages, large immune cells capable of engulfing pathogens, produce calcitriol locally. Here vitamin D is used as a cytokine, or a substance released in response to the presence of an antigen, which acts as a cellular mediator and enhances the immune response.
In humans, low vitamin D status has been linked to cardiovascular disease, auto immune disorders, neoplasias, infectious disease and even psychiatric disease. Of the autoimmune diseases linked to vitamin D deficiency, these include type I diabetes mellitus, Crohn’s disease, rheumatoid arthritis and multiple sclerosis. Indeed many cancers have also been linked to hypo-vitamin D status. However, with this said, large scale studies have been inconclusive, yielding conflicting results. Recently, supplementation of vitamin D in controlled studies was found to be ineffective in preventing the common cold or upper respiratory infections . However, the possibility exists that some of the diseases listed above may actually result in the destruction of vitamin D rather than being caused by its deficiency.
It is interesting that here in the US, the only legal claim that can be made in regards to vitamin D supplementation is that it can reduce the risk of osteoporosis; yet in the European Union, products can also state that vitamin D helps with normal function of the immune system, and normal inflammatory response.
Most work in animals has centered on bone metabolism and calcium homeostasis, which is not surprising as the link to overall health and human nutrition is somewhat new. Human nutritionists have now recognized that the amount of vitamin D needed to prevent rickets is inadequate to maintain other vital functions. However, remember that random supplementation is never advised, and results in humans can never be directly extrapolated to animals in general, let alone horses specifically. In addition, over-supplementation is never recommended.
While vitamin D toxicity is unlikely, it has occurred experimentally. Vitamin D toxicity is marked by calcification of the soft tissues, and can be fatal. Interestingly, it is actually used in lethal doses in baits as a rodenticide, when combined with calcium.
So what does all of this mean for your horse?
It has been shown repeatedly that vitamin D in the blood is higher in the summer than the winter, which would certainly make sense as the sun is the principle source of vitamin D for most horses.
Because many performance horses are stalled almost continuously, even more so in the winter, it is important that they receive a feed which contains vitamin D. In the past, the vitamin D requirements of the horse have been stated to be 300 IU of vitamin D per 100 lbs. Currently, the requirement is 6.6 IU/kg body weight for horses not exposed to sunlight.
Most management systems where the horse is regularly pastured or exposed to sunlight will be sufficient to provide enough vitamin D.
Dr. Kris Hiney is the Equine Nutrition Advisor at Omega Fields ( www.omegafields.com ). She trains and shows her own horses in the reining industry.