The Evidence Is Thin on Multivitamins

By Sally Squires
Tuesday, January 16, 2007

If you're among the 52 percent of Americans who take multivitamins or minerals, you may be surprised to learn how little experts know about their benefits -- or their potential harm.

In a series of papers in this month's American Journal of Clinical Nutrition, an expert panel convened by the National Institutes of Health notes that there's not enough evidence to recommend for or against these common dietary supplements.

Yet U.S. consumers -- hoping to prevent heart disease, cancer, bone loss and many other conditions -- spend $23 billion a year on multivitamins and minerals, according to the Nutrition Business Journal.

"If you're taking a multivitamin, there's no reason to stop," notes Paul Coates, director of the NIH Office of Dietary Supplements. "But if you're not taking a multivitamin, there's also no reason to start taking one, either."

That's not official public health policy. But, as Coates says, "it does represent the recommendation of the panel looking at the best information available."

And it's just one measure of the many complexities of scientific debate concerning multivitamins and minerals. "Most people assume that the ingredients in multivitamin supplements are safe," the panel concluded in its report. "There is evidence, however, that certain ingredients in multivitamin supplements can produce adverse effects."

Vitamin A is one concern. It helps preserve vision and boost immunity, but high doses can be toxic and have been linked to an increased risk of osteoporosis, the debilitating bone disorder. And many dietary supplements provide at least 100 percent of the recommended dietary allowance -- 900 micrograms for men, 700 for women -- and run the risk of pushing consumers over the 3,000-microgram tolerable limit set by the National Academy of Sciences.

Other studies have found problems with beta carotene, the substance that gives carrots and pumpkins their orange coloring. In the body, beta carotene is converted to vitamin A. There's evidence that eating a diet rich in fruit and vegetables packed with beta carotene can help protect against lung cancer. But when scientists gave beta carotene dietary supplements to smokers, their risk of lung cancer increased -- exactly the opposite of what had been expected.

"This is the real challenge," Coates says. "We look for the benefits of nutrients. Then later we start to think about the possible risks of overexposure. There may be susceptible populations. Do we know who the susceptible populations are? Probably not."

Folate, or folic acid, presents a new dilemma. This B vitamin provides heart benefits for adults. In women, it helps protect the children they bear from spina bifida and other neural tube defects that once affected about 4,000 pregnancies annually in the United States.

To help reduce the occurrence of these defects, the Food and Drug Administration in 1998 began requiring folate fortification of all enriched flour as well as macaroni, noodles, pasta, corn grits, bread, rolls and buns. As expected, rates of neural tube defects have declined significantly.

But there's a downside: Too much folate can mask a type of anemia in the elderly.

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