Vitamin D is both a vitamin and a hormone. It's a vitamin because your body cannot absorb calcium without it; it's a hormone because your body manufactures it in response to your skin's exposure to sunlight.

There are two major forms of vitamin D, and both have the word calciferol in their names. In Latin, calciferol means "calcium carrier." Vitamin D2 (ergocalciferol) and D3 (cholecalciferol) have similar actions in the body and can both be found in fortified food and supplements.

Strong evidence tells us that the combination of vitamin D and calcium supplements can be quite helpful for preventing and treating osteoporosis. Other potential uses of vitamin D have little supporting evidence.

Dosages of vitamin D are often expressed in terms of international units (IU) rather than milligrams. The Institute of Medicine's guidelines for Recommended Dietary Allowance are:108

  • Infants birth to age 1 year: 400 IU
  • 1-70 years: 600 IU
  • 70 years and older: 800 IU
  • Pregnant or nursing women: 600 IU

These recommendations have recently been increased based on the latest evidence. For example, in a study of military personnel in submarines, use of 400 IU of vitamin D daily was inadequate to maintain bone health, while six days of sun exposure proved capable of supplying enough vitamin D for 49 sunless days.89A study of veiled Islamic women living in Denmark found that 600 IU of vitamin D daily was insufficient to raise vitamin D levels in the blood to normal levels.1 The authors of this study recommend that sun-deprived individuals should receive 1,000 IU of vitamin D daily.

There is very little vitamin D found naturally in the foods we eat (the best sources are coldwater fish). In many countries, vitamin D is added to milk and other foods like breakfast cereals and margarine, contributing to our daily intake.

As indicated by the study of submarine personnel noted above, by far the best source of vitamin D is sunlight. However, current recommendations which stress sun avoidance and the use of sunblock may have the unintended effect of increasing the prevalence of vitamin D deficiency. Severe vitamin D deficiency was common in England in the 1800s due to coal smoke obscuring the sun. During that time, cod liver oil, which is high in vitamin D, became popular as a supplement for children to help prevent rickets. (Rickets is a disease caused by vitamin D deficiency in which developing bones soften and curve because they aren't receiving enough calcium.)

Vitamin D deficiency is known to occur today in the elderly (who often receive less sun exposure) as well as in people who live in northern latitudes and don't drink vitamin D-enriched milk.5,7 The consequences of this deficiency may be increased risk of hypertension, osteoporosis, and several forms of cancer.8

Additionally, phenytoin (Dilantin), primidone (Mysoline), and phenobarbital for seizures; corticosteroids; cimetidine (Tagamet) for ulcers; the blood-thinning drug heparin; and the antituberculosis drugs isoniazid (INH) and rifampinmay interfere with vitamin D absorption or activity.9-27