Vitamin D2 in Horses

Both vitamin D2 and vitamin D3 have similar activities in horses. The main function of the vitamin is the control of the absorption, transport and deposition of calcium and, to a lesser extent, phosphorus. The first action in this sequence is the promotion of intestinal absorption of calcium. This occurs mainly in the duodenum and many experiments have shown that little calcium is absorbed in the absence of vitamin D.

By far the major activity is the deposition and mobilisation of bone calcium. Calcium is transported in the blood plasma from the intestine via liver and kidneys to bones and other calcified structures and in the opposite direction from bones to the kidneys where some may be excreted. Vitamin D controls the levels of calcium and phosphorus in blood at a super-saturated level which permits deposition in bone. A less than saturated level withdraws calcium from bones. A further related activity is the control of the reabsorption of phosphorus, calcium and sodium in the renal tubules of kidneys.

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