Vitamin D is a group of fat-soluble vitamins responsible for intestinal absorption of calcium and phosphorous. Although vitamin D is commonly called a vitamin, it is not actually an essential dietary vitamin, as it can be synthesized in adequate amounts by most mammals exposed to sunlight. Cats and dogs however, cannot synthesise vitamin D efficiently and must receive it in their diet.Vitamin D was discovered in an effort to find the dietary substance that was lacking in a disease, namely, rickets, the childhood form of osteomalacia
A little background information
The rickets-fighting power of fish liver oil was discovered in 1782 and vitamin D was isolated in 1932. This liposoluble vitamin is provided in the diet. It is naturally synthesised by humans and herbivores by the conversion of skin sterols under the action of solar rays. To be active in the body it must be modified, first in the liver, then in the kidney.
Vitamin D toxicity is rare although hypercalcaemia is generally a good indicator of hypervitaminosis D. The main symptoms of toxicity include anorexia, nausea, and vomiting, polyuria, polydipsia, weakness, insomnia, nervousness, pruritus, and, ultimately, renal failure (as a result of calcium deposition within the renal medulla).
Vitamin D plays an essential role in the regulation of calcium and phosphorus metabolisms :
Vitamin D deficiency can cause:
Marine fish and fish oils are the richest natural sources of Vitamin D. Other sources include fresh water fish and eggs (particularly the yolks). Beef, liver and dairy contain smaller amounts of vitamin D.