Updated May 22, 2008

Choosing a Multivitamin

by Marie Spano, M.S., R.D.

Vitamin A. Vitamin A is best known for its role in helping the eyes adjust when going from brighter to dimmer light, so that a person can see well in the dark. But it has other roles, as well. It helps promote cell and tissue growth throughout the body and protects skin and other tissues from infection. The carotenoid form of vitamin A, found in plant foods, also functions as an antioxidant.

Multivitamins should contain vitamin A in the form of beta-carotene, which the body converts to vitamin A. Though large doses of vitamin A are stored in the body and are toxic, beta-carotene is not toxic, though high doses (33,000–50,000 IU) may raise the risk of lung cancer in smokers.

B vitamins. The B vitamins, which include biotin, choline, folic acid, niacin, pantothenic acid, B1, B2, B6, and B12, are involved in the metabolism of carbohydrate, fat, and protein and in energy production. Because the B vitamins work together, single sources of a particular B vitamin should not be taken unless prescribed by a physician.

Although deficiencies in most of the B vitamins are not common, taking excess amounts of folic acid can mask a B12 deficiency. A deficiency of either can lead to anemia. People who eat absolutely no animal products (vegans) and who do not take a multivitamin supplement are at risk for B12 deficiencies. Also at risk of a B12 deficiency are people who do not have a protein typically found in the stomach lining called intrinsic factor, which is necessary for the body to absorb B12. These people must receive B12 in shots. The elderly are also at risk for developing a B12 deficiency because their bodies are more likely to stop producing intrinsic factor. For this reason, elderly people should take a multivitamin supplement unless otherwise directed by their physician.

Vitamin C. Vitamin C is well known for its role in immune functioning. However, it also helps produce collagen, keeps capillary and blood vessel walls firm, thereby preventing bruising, keeps skin and gum tissue healthy, and helps the body absorb plant sources of iron (non-heme iron; a form that is not well absorbed overall).

Vitamin D. Vitamin D helps the body absorb both calcium and phosphorus. In addition to obtaining vitamin D from food and beverages, the body can manufacture the active form of this vitamin in the body upon exposure to sunlight.

Vitamin D has been in the news lately due to recent research examining vitamin D intake and a reduced risk of colon, breast, and prostate cancer, as well as research indicating that certain groups of individuals may be deficient in vitamin D. Those likely to be deficient include people living in less sunny climates (especially during the winter) and the elderly (the body's ability to synthesize vitamin D from sunlight decreases with age).

Vitamin E. Vitamin E's main function is as an antioxidant, protecting cells from damage. Vitamin E is actually a group of eight compounds, all with different potencies. Within each one of these eight compounds, there are several forms, called stereoisomers. Vitamin E is one particular micronutrient for which it is near-impossible to meet your daily needs through food intake alone (unless that food is fortified with vitamin E), so supplementation is a viable option. If possible, choose a supplement that contains "mixed tocopherols and tocotrienols." Supplementation with the main form, found in supplements containing alpha-tocopherol, decreases tissue stores of one of the other forms, gamma-tocopherol.

If you take any form of blood thinners or anticoagulant medicines, make sure your doctor knows that you are taking a supplement that contains vitamin E.

Vitamin K. Vitamin K plays a vital role in blood clotting. It also helps the body make proteins for the blood, bone, and kidneys.

If you are taking blood thinners or anticoagulants, check with your doctor if you plan on taking a vitamin supplement that contains vitamin K.

Marie Spano is a registered dietitian and exercise physiologist. She is a freelance writer, consultant, and speaker in the nutrition, fitness, and health industries. In addition, she is a spokesperson for the International Society of Sports Nutrition.

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