Vitamin supplements came into vogue in the early 1900s, when it was difficult or impossible for most people to get a wide variety of fresh fruits and vegetables year-round. Back then, vitamin-deficiency diseases weren't unheard-of: the bowed legs and deformed ribs of rickets (caused by a severe shortage of vitamin D) or the skin problems and mental confusion of pellagra (caused by a lack of the B vitamin niacin). But these days, you're extremely unlikely to be seriously deficient if you eat an average American diet, if only because many packaged foods are vitamin-enriched. Sure, most of us could do with a couple more daily servings of produce, but a multi doesn't do a good job at substituting for those. "Multivitamins have maybe two dozen ingredients — but plants have hundreds of other useful compounds," Neuhouser says. "If you just take a multivitamin, you're missing lots of compounds that may be providing benefits."

That said, there is one group that probably ought to keep taking a multivitamin: women of reproductive age. The supplement is insurance in case of pregnancy. A woman who gets adequate amounts of the B vitamin folate is much less likely to have a baby with a birth defect affecting the spinal cord. Since the spinal cord starts to develop extremely early — before a woman may know she's pregnant — the safest course is for her to take 400 micrograms of folic acid (the synthetic form of folate) daily. And a multi is an easy way to get it.

Myth: Vitamin C is a cold fighter
In the 1970s, Nobel laureate Linus Pauling popularized the idea that vitamin C could prevent colds. Today, drugstores are full of vitamin C–based remedies. Studies say: Buyer, beware.

In 2007, researchers analyzed a raft of studies going back several decades and involving more than 11,000 subjects to arrive at a disappointing conclusion: Vitamin C didn't ward off colds, except among marathoners, skiers, and soldiers on subarctic exercises.

Of course, prevention isn't the only game in town. Can the vitamin cut the length of colds? Yes and no. Taking the vitamin daily does seem to reduce the time you'll spend sniffling — but not enough to notice. Adults typically have cold symptoms for 12 days a year; a daily pill could cut that to 11 days. Kids might go from 28 days of runny noses to 24 per year. The researchers conclude that minor reductions like these don't justify the expense and bother of year-round pill-popping (taking C only after symptoms crop up doesn't help).

Myth: Vitamin pills can prevent heart disease
Talk about exciting ideas — the notion that vitamin supplements might help lower the toll of some of our most damaging chronic diseases turned a sleepy area of research into a sizzling-hot one. These high hopes came in part from the observation that vitamin-takers were less likely to develop heart disease. Even at the time, researchers knew the finding might just reflect what's called the healthy user effect — meaning that vitamin devotees are more likely to exercise, eat right, and resist the temptations of tobacco and other bad habits. But it was also possible that antioxidant vitamins like C, E, and beta-carotene could prevent heart disease by reducing the buildup of artery-clogging plaque. B vitamins were promising, too, because folate, B6, and B12 help break down the amino acid homocysteine — and high levels of homocysteine have been linked to heart disease.

Unfortunately, none of those hopes have panned out.

An analysis of seven vitamin E trials concluded that it didn't cut the risk of stroke or of death from heart disease. The study also scrutinized eight beta-carotene studies and determined that, rather than prevent heart disease, those supplements produced a slight increase in the risk of death. Other big studies have shown vitamin C failing to deliver. As for B vitamins, research shows that yes, these do cut homocysteine levels ... but no, that doesn't make a dent in heart danger.

Don't take these pills, the American Heart Association says. Instead, the AHA offers some familiar advice: Eat a varied diet rich in fruits, vegetables, and whole grains.

Truth: A pill that’s worth taking
As studies have eroded the hopes placed in most vitamin supplements, one pill is looking better and better. Research suggests that vitamin D protects against a long list of ills: Men with adequate levels of D have about half the risk of heart attack as men who are deficient. And getting enough D appears to lower the risk of at least half a dozen cancers; indeed, epidemiologist Cedric Garland, M.D., at the University of California, San Diego, believes that if Americans got sufficient amounts of vitamin D, 50,000 cases of colorectal cancer could be prevented each year.

But many — perhaps most — Americans fall short, according to research by epidemiologist Adit Ginde, M.D., at the University of Colorado, Denver. Vitamin D is the sunshine vitamin: You make it when sunlight hits your skin. Yet thanks to sunscreen and workaholic (or TV-aholic) habits, most people don't make enough.

How much do you need? The Institute of Medicine is reassessing that right now; most experts expect a big boost from the current levels (200 to 600 IU daily). It's safe to take 1,000 IU per day, says Ginde. "We think most people need at least that much."

So here's the Reader's Digest Version of the truth about vitamins: Eat right, and supplement with vitamin D. That's a no-brainer coupled with a great bet — and that's no lie.