Vitamin D is available to your horse in two forms: vitamin D2 (ergocalciferol) found in plants and vitamin D3 (cholecalciferol), which is synthesized in the skin on exposure to sunlight or provided orally in synthetic form. | Photo: iStock

A: A review article written by Danish researchers and published in the Journal of Equine Veterinary Science in 2015 summarized our current knowledge about vitamin D and horses. The authors’ take-home message: We “know next to nothing about vitamin D in horses.” This makes it hard to give you a concise answer, but from what we do know I do think that provision of supplemental sources of dietary vitamin D may be of benefit to some horses.

What is Vitamin D?

Vitamin D is available to your horse in two forms: vitamin D2 (ergocalciferol) found in plants and vitamin D3 (cholecalciferol), which is synthesized in the skin on exposure to sunlight or provided orally in synthetic form. Neither of these forms of vitamin D are active and both must go through a number of activation steps before they can influence physiologic functions in the body. The first step occurs in the liver where they are enzymatically hydroxylated to 25-hydroxyvitamin D2 (25OHD2) and 25-hydroxyvitamin D3 (25OHD3). These metabolites then circulate throughout the body in blood plasma.

Almost all organs of the body can further activate these two metabolites to their active form however most of the final conversion occurs in the kidneys. Here 25OHD2 and 25OHD3 are converted to 1,α25-dihydroxyvitamin D2 (1,25(OH)2D2) and 1,α25-dihydroxyvitamin D3 (1,25(OH)2D3) the hormonally active forms of the vitamin that can bind to the vitamin D receptors throughout the body, where they influence physiological functions