Vitamin D is a steroid hormone involved in the intestinal absorption of calcium and the regulation of calcium homeostasis. There are two different forms of vitamin D, named D3 and D2, which are very similar in structure. The D2 is a synthetic product, which is predominantly absorbed by fortified food.

Vitamin D3 levels result not only from dietary uptake but can also be produced from a cholesterol precursor, 7-dehydrocholesterol, in the skin during sun exposure. In the liver, the vitamin is hydroxylated to 25-hydroxyvitamin D (25(OH)-vitamin D), the major circulating metabolite of vitamin D. 1,25-(OH)2 vitamin D portrays the biological active form of vitamin D and is synthesized in the kidney.

Research areas include bone and muscle strength, cancer, bone metabolism, diabetes, and cardiovascualr disease.