Vitamin D Overview
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 calciferolin their names. In Latin, calciferol means "calcium carrier." Vitamin D 3 (cholecalciferol) is made by the body and is found in some foods. Vitamin D 2 (ergocalciferol) is the form most often added to milk and other foods, and the form you're most likely to use as a supplement.
Strong evidence tells us that the combination of vitamin D and calcium supplements can be quite helpful for preventing and treating osteoporosis. Vitamin D is experiencing renewed attention for many potential health benefits, and research has not yet caught up with clinical evidence for its usefulness in preventing and treating a number of conditions. This is part of the reason why many potential uses of vitamin D currently have little supporting research evidence.
How It Works
Vitamin D has numerous actions in the body, many of which we are just now discovering. Vitamin D enhances the efficiency of the small intestine in absorbing serum calcium and phosphorus. Vitamin D has receptors in the cells of other organs, like the intestine, kidney, stomach, brain, prostate, breast, and white blood cells.1
As with vitamin A , dosages of vitamin D are often expressed in terms of international units (IU) rather than milligrams. The Institute of Medicine's updated (November 2010) recommendations for daily intake of vitamin D44 are as follows. The first number is the Recommended Dietary Allowance (RDA), and the second number is the Upper Intake Level (UL), the highest daily dose that is considered safe:
- Children and Infants:
- 0-6 months: no RDA/1,000 IU
- 6-12 months: no RDA/1,500 IU
- 1-3 years: 600 IU/2,500 IU
- 4-8 years: 600 IU/3,000 IU
- 9-18 years: 600 IU/4,000 IU
- 19-70 years: 600 IU/4,000 IU
- 71 years and older: 800 IU/4,000 IU
- Pregnant/lactating women:
- 14-50 years: 600 IU/4,000 IU
These recommendations were updated because growing evidence suggests that prior recommendations may have been too low. 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. ^ In addition, a 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. ^ 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. ^ The consequences of this deficiency may be increased risk of hypertension , osteoporosis , and several forms of cancer . ^ 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 rifampin may interfere with vitamin D absorption or activity. ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
Vitamin D is typically taken at doses between the Recommended Daily Allowance (RDA) and the Upper Level Intake (UL), the upper limit of what is considered safe. Therapeutic doses are usually closer to the UL. These recommendations were updated in 2010 by the Institute of Medicine, and they vary for people of different ages and pregnancy/lactation status. See Requirements/Sources for more details.
Sometimes, vitamin D is recommended or prescribed at doses that exceed what is typically considered "safe." In these cases, physician supervision is recommended. (See Side Effects & Warnings for more information.)
What Is the Scientific Evidence for Vitamin D?
Individuals with severe osteoporosis often have low levels of vitamin D. ^ Supplementing with vitamin D alone is probably no more than minimally helpful, at best, ^ but the combination of calcium and vitamin D is probably more effective. (See the Calcium article for more information.)
Interestingly, vitamin D may offer another benefit for osteoporosis in seniors: most (though not all) studies have found that vitamin D supplementation improves balance in seniors (especially female seniors) and reduces risk of falling. ^ Since the most common adverse consequence of osteoporosis is a fracture due to a fall, this could be a meaningful benefit. Why vitamin D should offer this benefit, however, remains a mystery.
Supplementation with vitamin D plus calcium may aid healing aftera fracture has occurred. ^
Getting adequate vitamin D may help prevent the development of hypertension.45-49
- Institute of Medicine. Report Brief: Dietary Reference Intakes for Calcium and Vitamin D. Released November 30, 2010. Accessed at the IOM Web site: http://www.iom.edu/Reports/2010/Dietary-Reference-Intakes-for-Calcium-and-Vitamin-D/Report-Brief.aspx?page=1
- Fuller KE, Casparian JM. Vitamin D: balancing cutaneous and systematic considerations. South Med J. 2001;94:58-64.
- Rostand SG. Ultraviolet light may contribute to geographic and racial blood pressure differences. Hypertension. 1997;30(2 pt 1):150-156.
- Krause R, Buhring M, Hopfenmuller W, et al. Ultraviolet B and blood pressure [letter]. Lancet. 1998;352:709-710.
- Scragg R. Sunlight, vitamin D, and cardiovascular disease. In: Crass MF II, Avioli LV, eds. Calcium Regulating Hormones and Cardiovascular Function. Boca Raton, FL: CRC Press; 1995:213-237.
- O'Connell TD, Simpson RU. 1,25-dihydroxyvitamin D3 and cardiac muscle structure and function. In: Crass MF II, Avioli LV, eds. Calcium-Regulating Hormones and Cardiovascular Function. Boca Raton, FL: CRC Press; 1995:191-211.