Americans may be taking too many vitamin supplements in the hope of staying healthy and living longer, a panel of U.S. experts said on Wednesday.

Some people clearly need supplements, the panel said - for instance, older women benefit from taking calcium and vitamin D to strengthen their bones, and younger women need to take folic acid to reduce birth defects in any children they may have.

But no studies show strongly that people prosper from popping multivitamin and mineral supplements and some suggest that eating healthily and exercising provide more benefit, the panel said.

“Half of American adults are taking multivitamins and minerals and the bottom line is that we don’t know for sure that they’re benefiting from them,” said Dr. J. Michael McGinnis, a senior scholar with the Institute of Medicine, who chaired the panel.

“In fact, we’re concerned that some people may be getting too much of certain nutrients,” added McGinnis, whose organization advises the federal government on health issues.

The 13-member panel included experts in nutrition, biostatistics, biochemistry, toxicology, geriatric medicine, family medicine, pediatrics, cancer prevention, consumer protection and other fields.

They said much more study was needed on what vitamins Americans lacked in their diets and whether taking supplements provided actual benefit.

Many people may assume that because vitamins and minerals are vital for health, that more is better. But some are toxic at high levels, including vitamin A and iron, and others are simply excreted in the urine.

Beta-carotene was shown in a surprise study to raise the risk of lung cancer in smokers.

The panel said anti-oxidant vitamins and zinc might help nonsmoking adults with early stage, age-related macular degeneration, which can cause blindness.

But many foods are fortified with vitamins and minerals, so the value of supplements is often questioned.

There are no good studies showing people who take multivitamin or mineral supplements can prevent chronic diseases such as cancer and heart disease.

Many of the studies that suggest a protective effect also show that people who take vitamins tend to take care of their health in other ways, such as eating plenty of fruits and vegetables, exercising and not smoking, so it is difficult to determine whether the vitamins were responsible for health benefits.