Vitamin D is a group of fat-soluble secosteroids. In humans Vitamin D is unique both because it functions as a prohormone and because when sun exposure is adequate the body can synthesize it (as Vitamin D3). Measures of the serum levels refect endogenous synthesis from sun exposure as well as intake from the diet and it is believed that synthesis may contribute generally to the maintenance of adequate serum concentrations. The evidence indicates that the synthesis of vitamin D from sun exposure works in a feedback loop that prevents toxicity but, because of uncertainty about the cancer risk from sunlight, no recommendations are issued by the Institute of Medicine for the amount of sun exposure required to meet vitamin D requirements...