Vitamin D is a vitamin found in four different chemical forms that acts as a hormone and a steroid in the body. In the body, it regulates the levels of calcium and phosphates in the bloodstream and therefore plays a vital role in the formation and maintenance of bone. A lack of the vitamin leads to rickets in children, osteomalacia in adults, and exacerbates osteoporosis in the elderly.
The body is capable of making its own Vitamin D when skin is exposed to ultraviolet light. However, as it is not clear how much sunlight any given individual needs to meet their Vitamin D needs, it is recommended that all necessary Vitamin D be obtained from dietary sources. Unfortunately, few foods naturally contain Vitamin D. It is found in quantity in most forms of fish, and in lesser quantities in eggs and beef liver. It is found in large quantities in fish liver oil. However, Vitamin D is fairly easy to synthesize and other foods are fortified with it. Milk produces Vitamin D when exposed to ultraviolet light in the same manner that skin does, and most milk sold in the developed world is so fortified. It is also added to most food cereals.
Vegans often have trouble meeting their Vitamin D requirement as no plant naturally contains Vitamin D and most synthesized Vitamin D is from animal sources. However, some mushrooms and brewer's yeast will both produce Vitamin D when exposed to ultraviolet light.
Vitamin D is stored in fat and too much Vitamin D in the diet can be toxic, leading to Hypervitaminosis D. The risk is particularly high in patients with thyroid problems. It causes dehydration, nausea and anorexia, among other problems.