A succession of large-scale human studies, including two published earlier this month in leading medical journals, suggests that multivitamins and many other dietary supplements often do not have health benefits and in some cases may even cause harm.
The data have prompted some nutrition researchers to say taking vitamins is a waste of money for those without a specific nutrient deficiency or chronic illness. Such findings have also fueled a debate about whether the field should continue conducting expensive human trials to figure out whether particular supplements affect health.
"The better the quality of the research, the less benefit [supplements] showed," said Marion Nestle, professor of nutrition, food studies and public health at New York University. "It's fair to say from the research that supplements don't make healthy people healthier."
For instance, vitamins B-6 and B-12 are often touted as being good for the heart, but several studies have failed to find that they lower risk of cardiovascular disease, according the Office of Dietary Supplements, part of the National Institutes of Health. Vitamin C has not been shown in many studies to lower a person's risk of getting a cold. Calcium, while important to bone health, does not lower risk of heart disease or cancer and may increase risk of kidney stones.
"We have an enormous body of data telling us that plant-rich diets are very healthy," said Josephine Briggs, head of the National Center for Complementary and Alternative Medicine, another NIH center. "As soon as we take these various antioxidants [and other nutrients] out and put them in a pill, we're not consistently getting a benefit."
Researchers and nutritionists are still recommending dietary supplements for the malnourished or people with certain nutrient deficiencies or medical conditions. For instance folic acid, the supplement form of folate reduces the likelihood of a common birth defect if taken by pregnant women.
Studying the effects of vitamins and supplements in the real world is difficult, since people eat foods with multiple nutrients that can interact with supplements and skew results. And observational trials can only show an association, not cause and effect.
That is one reason the Council on Responsible Nutrition, which represents the supplement industry, says it is too early to say supplements do not have health benefits. Duffy MacKay, the group's vice president of regulatory and scientific affairs, says lengthier studies may be required to show the benefits of some supplements.
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