Vitamin D is a fat-soluble vitamin. Vitamin D and its metabolites have a significant clinical role because of their interrelationship with calcium homeostasis and bone metabolism.
Vitamin D metabolism and action
Best ways to achieve adequate vitamin D intake:
Fatty fish (D3) (400 international units/3.5 oz), fortified milk (400 international units/quart), and cereal products (50 international units/cup) are good dietary sources. Vitamin D2 and vitamin D3 supplements are available over the counter in multiple doses and 50,000 international units of vitamin D2 supplements can be given by prescription. Ten minutes of midday summer sunlight exposure to a fair-skinned person in a tank top and shorts not wearing sunscreen produces 10,000 international units of vitamin D3. Dark-skinned individuals and elderly get less production. However, many individuals wear sunscreen (SPF >8) which prevents vitamin D production by the skin. Therefore, oral vitamin D is necessary for most people. The optimal vitamin D intake is 800 to 1200 international units daily and should not exceed 4000 international units/day chronically.