RENEE MONTAGNE, host:

Nutrition experts met earlier this week at the National Institutes of Health and agreed that there is little evidence to prove that taking a daily multivitamin prevents the onset of chronic diseases. NPR's Allison Aubrey reports on how the uncertainties influence the experts' own personal choices about which vitamin supplements to take.

ALLISON AUBREY reporting:

There's no algorithm for devising the perfect multivitamin. The kind of studies needed to prove whether they prevent chronic disease just haven't been done. So what's included in the standard formulation is determined as much by tradition, theory, and a good marketing campaign, as it is by evidence.

Dr. IRWIN ROSENBERG (Professor of Physiology, Tufts University): The whole idea of multivitamins goes back 60 or so years, when Miles Laboratories put together something called One-A-Day Multivitamins. And that idea has stuck.

AUBREY: Irwin Rosenberg directs nutrition research at Tufts University.

Dr. ROSENBERG: With no real rationale, why just take all the vitamins you can think of and put them all into one pill?

AUBREY: Rosenberg's skepticism informs his own behavior. He doesn't take a multivitamin. What he does do is supplement with the individual vitamins that he knows his body doesn't have enough of. At the top of his list is vitamin D.

Dr. ROSENBERG: It's very difficult to get a lot of vitamin D from the diet.

AUBREY: Or from the sun in northern latitudes. Rosenberg says new studies signal that adequate D is key to the functioning of several systems in the body.

Dr. ROSENBERG: Ranging from immunity to proliferation in precancer or cancer, and even some evidence that relates vitamin D to cognitive function.

AUBREY: Another supplement on his list is the B vitamins. With age, a decrease in stomach acid makes it hard to absorb B12 from the diet. Harvard nutritionist Meir Stampfer says he takes a multivitamin that packs in a lot of the Bs and folate.

Prof. MEIR STAMPFER (Professor of Nutrition, Harvard University): I think everybody ought to be getting some folate.

AUBREY: For women trying to become pregnant, folate is a must since it's proven to protect against birth defects. And for the rest of the population, Stampfer says folate seems to offset one risk of drinking alcohol.

Prof. STAMPFER: Even moderate alcohol consumption can raise the risk of certain forms of cancer, and again, this is not proven, but there is some data suggesting that that increased risk can be taken away if you have adequate folate.

AUBREY: There's a continuum of certainty when it comes to the science behind vitamin supplements. Experts presented lots of data at the conference to demonstrate, for instance, the known benefits of calcium. But in cases where the evidence is less compelling, the decision about whether to take a supplement make seem trickier.

Take, for instance, vitamin E.

Prof. STAMPFER: The evidence for vitamin E is not strong.

AUBREY: But Stampfer says he still takes it. Several trials have found that E offers no protection to people who already have signs of heart disease, but Stampfer says one randomized trial of healthy people found that vitamin E may cut the risk of prostate cancer. His thinking is that he's pretty certain the daily dose of E can't hurt him...

Prof. STAMPFER: And so it seems like a good gamble.

AUBREY: Or an educated guess.

The idea that nutrition researchers can't say for certain which vitamin supplements are truly beneficial may come as a surprise, but it also seems to explain why so many people fall back on the idea of a multivitamin as a sort of cheap insurance policy. The payoff may never come, but it gives some peace of mind.

Physician Scott Kahan is a fellow in preventive medicine at Johns Hopkins University. He says a healthy diet and exercise should be the mainstay of disease prevention. But he too has begun supplementing with an iron-free multivitamin and daily fish oil.

Dr. SCOTT KAHAN (Preventive Medicine Fellow, Johns Hopkins University): I have a hectic life. I just don't get enough from food.

AUBREY: Researchers at the conference recommended funding for more studies to nail down some of the promising, yet unproven, correlations. Ultimately, researchers such as Irv Rosenberg hope to see the multivitamin approach give way to a more tailored one - one where people take the precise cluster of just the vitamins they need.

Allison Aubrey, NPR News, Washington.

MONTAGNE: Our experts give more tips on vitamins and how to get the most out of them at npr.org.

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