Vitamin D, as one of the fat soluble vitamins can be stored in the body.
Its main role in the body is calcium homeostasis, or keeping the calcium levels at a constant level. It also influences the growth and differentiation of cells to some degree.
When managing the calcium levels, D most often works in the digestive tract. It helps calcium to be absorbed from the intestine and helps it be reabsorbed from the kidney.
However, it also helps calcium be released from bones, as well as be taken back up into bones. This is why most calcium supplements on the market also contain Vitamin D.
In the horse diet, D is relatively low. The only place it is really found naturally is sun-cured alfalfa.
However, it is not an issue that D is not found in the diet, because all normal, healthy horses (and humans for that matter) synthesize D in their body through exposure to sunlight.
The only time low dietary inclusion would be a problem is if the body cannot synthesize it, or the horse is deprived of sunlight.
The only symptom of Vitamin D toxicity is hypercalcemia, or calcification of soft tissue.
However, since horse diets are low in D, and the body only makes as much D as it needs, D toxicity is pretty rare in horses.
The only time it would be a concern is if your horse is being fed multiple supplements or fortified feeds containing D. In that instance, it would be prudent to figure out how much D he is getting, and ensure it is below the upper safe limit of 3,300 IU/day.
Vitamin D deficiency is also not a huge concern in horses, due to the fact that they need very little compared to most animals.
Rickets is the disease that develops if a horse does not get enough D. Rickets is also called soft-bone disease, and the main symptom is bone deformities, especially in the limbs.
However, in studies done on horses, visible outward signs of rickets were never reported, though bone growth and development was affected in ponies that were deprived of sunlight and had no D supplementation.
In most practical horse-keeping situations, assuming the horse gets at least some exposure to sunlight, or some supplemental D in the diet, D deficiency is not going to be a problem.
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