Thee are two sources of vitamin D: diet and exposure to sunlight. Vitamin D3 (cholecalciferol) is a prohormone produced endogenously from the action of ultraviolet light on its precursor (7-dehydrocholesterol) in the skin. Another source of Vitamin D3 is supplements.Vitamin D2 (ergocalciferol) does not occur naturally but is manufactured commercially and can be obtained by prescription. Both are hydroxylated in the liver to form their 25-hydroxy metabolites, the major circulating form of vitamin D in the blood.Some recent studies have shown that vitamin D supplementation with D3 more effectie than supplementation with D2. Vitamin D deficiency is linked with many chronic diseases, including autimmune diseases, diabetes, cardiovascular disease, and cancer. Optimal levels of 25hydroxyvitamin D3 are 32-100ng/mL.