Vitamin D belongs to the group of fat-soluble vitamins. It is taken into the body through a few foods that contain vitamin D, dietary supplements or through the sun's rays that trigger endogenous synthesis in the liver and kidneys. It may be found in two important forms: vitamin D2 (ergocalciferol) and D3 (colecalciferol). Vitamin D is inert in the body until it undergoes two hydroxylations in the liver and kidney and becomes biologically active. Vitamin D is most commonly associated with bone health and preventing the development of osteomalacia and rickets. An important finding is that the vitamin D gene is expressed in all living cells, and about 3% of the human genome is under its control. Therefore, recent research indicates a beneficial effect of vitamin D on the immune and cardiovascular systems and the prevention of autoimmune and malignant diseases. Increased public awareness of the health benefits associated with vitamin D may increase the risk of toxic effects due to self-administration of the vitamin in doses higher than those recommended for age and body weight. Fatty fish meats such as tuna, mackerel and salmon, and fish liver oil are among the richest sources of vitamin D. Small amounts are found in cheese, beef liver, and egg yolk. Some mushrooms also provide vitamin D in various amounts. It is also commercially available in a variety of forms including water and oil soluble capsules, concentrates, tablets, chewing gums, sprays, injections, and oral drops that are the only acceptable dosage form for infants. For children older than 1 year and adults up to 70 years, a daily intake of 600 IU of vitamin D per day is recommended, and for people older than 70 years, a daily intake of 800 IU per day is recommended. A daily intake of 400 IU per day is recommended for infants.