Vita means "life" in Latin, and vitamins are essential for life. The World Health Organization calls them the "magic wands" used by the body to synthesize enzymes, hormones and other chemical necessities. Unable to create vitamins from scratch, the body must fetch them from outside sources—typically food. But do the pills many pop for health deliver the same benefits?

Humans need 13 vitamins to survive. Vitamins, also called "micronutrients" because they are required in minute quantities, can be grouped in two categories. There are those that dissolve in fat—A, D, E and K—and can accumulate in the body when consumed in excess. And there are those that are water soluble—C and B—which are easily excreted, as anyone who takes large quantities of vitamin C and riboflavin (B2) can testify. (Their urine is bright yellow or orange.)

The best way to get vitamins is through food, not vitamin pills, according to Susan Taylor Mayne, a professor at the Yale School of Public Health's Division of Chronic Disease Epidemiology. A major problem with supplements is that they deliver vitamins out of context, she says. The vitamins found in fruit, vegetables and other foods come with thousands of other phytochemicals, or plant nutrients that are not essential for life but may protect against cancer, cardiovascular disease, Alzheimer's disease and other chronic ailments. Carotenoids in carrots and tomatoes, isothiocyanates in broccoli and cabbage, and flavonoids in soy, cocoa and red wine are just a few examples.

The combined effect of all these vitamins and phytochemicals seems to have much greater power than one nutrient taken alone, Mayne explains. For example, lycopene—the carotenoid that gives tomatoes their red hue—has been associated with a lower risk for prostate cancer, causing many supplement makers to rush to market pills bearing this healthy stuff. But research suggests that taking it in supplement form does not confer the same benefit as eating tomatoes or tomato products, such as pasta sauce and ketchup, that preserve some of the tomatos chemical integrity.

A healthy diet is paramount, but is there ever a time for supplements? Meir Stampfer, professor of nutrition and epidemiology at the Harvard School of Public Health in Boston, recommends that healthy adults take a multivitamin and extra vitamin D, if they don't get a lot of sun. Taking more than the Institute of Medicine's recommended daily allowance (RDA) of certain vitamins may lower one's risk for certain chronic diseases, he says. For example, Stampfer's research suggests that men and women taking vitamin E supplements for years at a time have a lower risk for heart disease. "The evidence for benefit is weak," but there is also "good evidence for no harm" associated with taking 200, 400, or even 600 IUs (international units) per day, Stampfer explains. (The RDA levels for vitamin E are 22.5 IUs, or 15 milligrams, for men and women.)

Mayne disagrees, pointing to a recent meta-analysis suggesting that vitamin E supplementation increases mortality of all causes. "We can debate" whether this analysis shows that vitamin E supplements are harmful, she says, but "there certainly wasn't any benefit shown." With the possible exception of vitamin D, there is no need to consume more than the RDA of vitamins, Mayne contends. In fact, there is increasing evidence that excessive intake of certain micronutrients is deleterious.

Stampfer acknowledges that overdosing on certain vitamins can be dangerous. "The most common one to look out for is preformed [active form] … vitamin A. It does not take too much to get too much," he says. Try to avoid retinol, retinyl palmitate and retinyl acetate, which may increase the risk of hip fracture and certain birth defects when taken at levels exceeding 10,000 IUs.

But Mayne and Stampfer both agree that more randomized clinical trials are needed to determine the health effects of vitamin supplements—and that such supplements are critical for certain people. Many African-Americans and people living in sun-deprived areas are vitamin D–deficient and could benefit from supplements, Mayne explains. Pregnant women, and even women who might want to get pregnant, should be taking folic acid supplements to help prevent serious birth defects in their babies. People over 50 years of age can benefit from B12 supplementation because absorption of this vitamin in the digestive tract becomes less efficient with age, says Roberta Anding, spokeswoman for the American Dietetic Association. Finally, HIV-positive patients should take multivitamins to boost immunity and slow the rate of disease progression, says Wafaie Fawzi, professor of nutrition and epidemiology at the Harvard School of Public Health.

Ironically, "the people who are most likely to take vitamin supplements are the people who least need them," Mayne says. The affluent and health conscious are popping supplements faster than anyone. It may not be doing any good, and it could be harming them, she says. Anding concurs: "If you eat well, you probably don't need a multivitamin."

On the other hand, as Stampfer, who takes vitamin supplement himself, notes: "If I'm wrong, then I've wasted a few dollars. If I'm right, I've lowered my risk for some diseases I don't want to have."