Over half of U.S. adults use multivitamins, mostly the pretty healthy people who also eat nutrient-fortified foods.

Yet there's little evidence that most of the pills do any good — and concern that some people may even get a riskyvitamin overload, advisers to the government said Wednesday.

Worried about bottles that promise 53 times the recommended daily consumption of certain nutrients, specialists convened by the National Institutes of Health called Wednesday for strengthened federal oversight of the $23 billion dietary supplement industry — especially efforts to pin down side effects.

For the average healthy American, there's simply not enough evidence to tell if taking vitamins is a good or bad idea, said Dr. J. Michael McGinnis of the Institute of Medicine, who led the NIH panel's review.

"We don't know a great deal," he said, calling for more rigorous research.

Moreover, McGinnis added, "The product with which we're dealing is virtually unregulated," meaning there are even questions about how the bottles' labels convey what's really inside.

Vitamins and minerals, often packaged together, are the most-used dietary supplements, and widely assumed to be safe. After all, vitamins naturally occur in some of the healthiest foods, and vitamin deficiencies have been known to be dangerous since scurvy's link to a lack of fruits and vegetables was discovered centuries ago.

Ironically, the NIH panel concluded, the people most likely to have nutrient deficiencies are the least likely to use multivitamins.

Yet among the generally healthy and affluent, use of vitamin supplements — along with fortification of foods with extra vitamins — has skyrocketed in recent years as scientists speculated that high doses of certain nutrients might prevent cancer or other diseases.

That's where safety questions arise, because too much of certain nutrients can be bad.

There are only a few proven disease-preventing supplements, the NIH panel concluded:

--Women of childbearing age should take folic acid supplements to prevent spina bifida and related birth defects.

--Calcium and vitamin D together protect the bones of postmenopausal women.

--Antioxidants and zinc may slow the worsening of the blinding disease called age-related macular degeneration.

On the other hand, smokers should avoid taking beta-carotene supplements, because the pills can increase their risk of lung cancer, the report stresses.

For other vitamins, concern arises mainly with super doses that exceed the government's "recommended daily amount," or RDA. Between 1 percent and 11 percent of supplement users may be exceeding the upper limits set for certain nutrients, if they add together their doses from pills and their diets, said Cornell University nutritionist Patsy Brannon.

Leading her list: Too much niacin can damage the liver. Among other examples, too much vitamin A can cause birth defects, and too much vitamin E can cause bleeding problems.

Aghast at the super-doses on some bottles, panelist William Vaughan of Consumer's Union asked, "Why would I take 53 times what people tell me is the RDA?"

If you choose to take vitamins, use those labeled with 100 percent of the RDA or "daily value," advised Brannon. Together with a good diet, that would provide most people plenty without getting near the upper limit.

Some vitamins also can interact dangerously with medications, and doctors should ask their patients what they take, the panel said.

Congress limited the Food and Drug Administration's oversight of vitamins and other dietary supplements in 1994. Unlike most medications, most supplements sold today never had to be proven safe, much less proven to bring any health benefit.

The NIH panel marks the fourth scientific report in recent years urging more FDA authority over supplements, urging the agency to, among other things, mandate that manufacturers report customer side effects just like medication makers do.

Legislation that would do that has languished in Congress since 2004; the industry's Council for Responsible Nutrition said Wednesday it supported that call.

But "for millions of Americans who struggle with diet and nutrition, a daily multivitamin provides a safe, affordable, and reliable means of filling nutrition gaps and promoting overall good health," added council president Steven Mister.