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Research indicates that the best approach to vitamins may be to take just those you can't get through diet.
There's no simple algorithm for devising the perfect multivitamin. The kind of studies needed to prove whether they prevent chronic disease haven't been done. What's included in the standard formulation is determined by a mix of tradition, scientific theory, marketing and evidence.
The whole idea of multivitamins goes back 60 years when the Miles Laboratory put together something called "One-A-Day" multivitamins, and the idea has stuck says Dr. Irwin Rosenberg a professor at Tufts University.
"There's no real rationale of why you should just take all the vitamins you can think of and put them into one pill," he says.
Rosenberg doesn't take a daily multivitamin. Instead he supplements with individual vitamins that he knows his body can't get through diet.
For instance, it's very difficult to get a lot of vitamin D from the diet, he says. New studies suggest that vitamin D affects everything from immunity to proliferation in cancer, and brain function.
B12 is another vitamin that's hard to get an adequate supply of through diet alone. This is particularly true for older people who don't make enough stomach acid to absorb the vitamin from food in their diets.
Other experts recommend folate. For women trying to conceive, folic acid is a proven way to protect against birth defects. Epidemiologist and nutritionist Dr. Meir J. Stampfer of Harvard Medical School says folate also seems to offset risks from drinking alcohol.
"Even moderate alcohol increases the risk of some cancers," says Stampfer. "This is not proven, but there's some data to suggest that this increased risk can be taken away if you have adequate folate."
There's a continuum of certainty when it comes to the science behind vitamin supplements. Experts presented lots of data at the conference, such as the known benefits of calcium.
But in cases where the evidence is less compelling, the decision about whether to take a supplement may seem trickier.
Take vitamin E. Several trials have found that E offers no protection to people who already have signs of heart disease. But Stampfer points to a separate randomized, clinical trial that concluded vitamin E may cut the risk of prostate cancer.
"For healthy people, having more antioxidant capacity I think could be beneficial, and so it seems like a good gamble," Stampfer said.
The idea that nutrition researchers can't say for certain which vitamins and 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 insurance policy. The pay-off may never come, but it gives some peace of mind.
Researchers at the conference recommended funding for more studies to nail down some of the promising yet unproven correlations between vitamins and prevention of chronic disease.
Ultimately, researchers such as Irwin Rosenberg of Tufts University hope to see the multivitamin approach give way to a more tailored one where people take the precise cluster of just the vitamins they need.