Vitamin D occurs in ten forms, D1- D10. The two most important forms are D2 and D3. The best food sources of vitamin D are cod liver and fish liver oils, butter, egg yolk, liver, vitamin D-fortified milk, and oily fish such as herring, mackerel, sardines, and salmon. The body can also manufacture vitamin D in the skin when it comes in contact with the sun's ultraviolet rays. People who live in areas of smog or infrequent sunlight, as well as strict vegetarians, should consider daily supplementing with 400 IUs of vitamin D.

Vitamin D is essential for the absorption of calcium, and for regulating the metabolism of calcium and phosphorous, both of which are integral components of healthy bones and teeth. It also aids in regulating the nervous system and maintaining cardiovascular health and normal blood clotting, and is an important nutrient for childhood growth. Because of its ability to aid in the calcification process, vitamin D can also be useful for maintaining bone health during menopause.

In childhood, the primary sign of vitamin D deficiency is rickets, while in adults, lack of the vitamin can result in softening of the bones (osteomalacia). Tetany, a form of muscle spasm, hearing loss, nearsightedness, psoriasi, celicac disease, and osteoporosis can also result from vitamin D deficiency.