growing number of medical experts are concerned that Americans are overdoing their vitamin consumption. As many as 70 percent of the population is taking supplements, mostly vitamins, convinced that the pills will make them healthier.

But researchers say that vitamin supplements cannot correct for a poor diet, that multivitamins have not been shown to prevent any disease and that it is easy to reach high enough doses of certain vitamins and minerals to actually increase the risk of disease.

No longer, the experts say, are they concerned about vitamin deficits. Those are almost unheard of today, even with the population eating less than ideal diets and skimping on fruits and vegetables. Instead, the concern is with the dangers of vitamin excess.

"There has been a transition from focusing on minimum needs to the reality that today our problem is excess — excess calories and, yes, excesses of vitamins and minerals as well," said Dr. Benjamin Caballero, a member of the Food and Nutrition Board at the National Academy of Sciences and the director of the Center for Human Nutrition at Johns Hopkins University.

Dr. Caballero said that for some supplements, including vitamin A, the difference between the recommended dose and a dose that could lead to bad outcomes like osteoporosis was not large. Popular multivitamins, he added, often contain what could be risky doses.

"Certainly," he said, "by consuming supplements, people can reach that level."

Doctors who once told patients that multivitamins were, at worst, a waste of money now say they are questioning that idea.

"All of a sudden, scientists are rearing back and saying, `Wait a minute, do we really know that we need this and do we really know that we need that?' " said Dr. Ruth Kava, nutrition director at the American Council on Science and Health, a consumer foundation in Manhattan that is in part financed by industry.

With vitamin A in particular, it is easy to step over the edge into a danger zone, said Dr. Joan McGowan, chief of the musculoskeletal diseases branch at the National Institute of Arthritis and Musculoskeletal and Skin Diseases.

"You can be eating Total cereal, drinking fortified milk, taking a multivitamin," Dr. McGowan said. "You can get into a situation where you're getting more than you need. Until recently, there was little concern about vitamin A and bone health."

Now, she added, "we may have to rethink the issues."

Similar questions are being raised about other vitamins and minerals, notably iron and vitamins E and C.

Researchers say the questions involve multivitamins taken by healthy people, not specific vitamins or minerals taken by groups with specific needs. Some elderly people, for example, may be deficient in vitamin B12 because they lose their ability to absorb it from foods. People who spend little time outdoors may require vitamin D, which the skin makes when it is exposed to sunlight. Even when older people are in the sun, aging skin loses much of its ability to synthesize the vitamin.

Pregnant women who do not receive enough folic acid, a vitamin in fruits and vegetables that is added to enriched flour, are at increased risk of having babies with neural tube defects. Because the vitamin is needed at the very start of pregnancy, some advocate folic acid supplements for all who might become pregnant, just to be sure they are protected.

For most people, however, the issue is not deficits. Instead, nutrition researchers ask: Do people eating relatively healthy diets with fresh fruits and vegetables and not too many calories or fats benefit from multivitamins or other supplements? Do those whose diets are abysmal, heavy on fast foods and lacking in fruits and vegetables, make up for some deficits if they take multivitamin pills?

Dr. Annette Dickinson, president of the Council for Responsible Nutrition, a group that represents the supplement industry, says 70 percent of Americans sometimes take supplements — usually multivitamins or individual vitamins and minerals — and 40 percent take them regularly.

"Our position," she said, "is that most people, literally most people, would benefit from taking a multivitamin every day. It's insuring adequate and even generous intake of all the nutrients."

The most popular individual supplements are vitamins C and E, said Dr. Robert M. Russell, the director the Human Nutrition Research Center of Agriculture Department at Tufts University, who is head of the Food and Nutrition Board. Scientists once thought those vitamins could help prevent ailments like cancer and heart disease, but rigorous studies found no such effects.

Vitamin E supplements can increase the risk of heart attacks and strokes, and studies of vitamin C supplements consistently failed to show that it had any beneficial effects.

"The two vitamins that are the most not needed are the ones most often taken," Dr. Russell said.

Excess vitamin C is excreted in the urine, but excesses of some other vitamins are stored in fat, where they can build up. Of particular concern, researchers say, is vitamin A. It is found in liver, and small amounts are added to milk. But for most people who are reaching worrisome levels, the main source is supplements, multivitamins, nutrition bars, health drinks and cereals.

Several recent large studies indicate that people with high levels of vitamin A in their blood have a greater risk for osteoporosis. People can easily reach a potentially dangerous level, about five times the recommended dose, by taking vitamins and supplements, nutrition researchers say. Some popular multivitamins run 1,500 micrograms a pill, twice the recommended daily amount and a level that, in one recent study, doubled the risk of bone fractures. Some supplements provide as much as 4,500 micrograms a day, well above the level that the National Academy of Sciences calls an upper limit for safety.

"If you have a good source of vitamin A in your food and you take a supplement with another 100 percent, you can easily reach a level that can accumulate" to one associated with increased risk of osteoporosis, Dr. Caballero said.

Dr. Dickinson said that multivitamin manufacturers were decreasing the vitamin A in their products, but that it might take a year for the reformulated products to appear.

Others warn about overdosing on other vitamins and minerals.

Dr. Richard J. Wood, director of the mineral bioavailability laboratory at Tufts, worries about iron overload, which can increase the risk of heart disease. In a large federal research effort, the Framingham study, Dr. Wood found that 12 percent of the elderly participants had worrisome levels. "Hardly anyone had iron deficiency anemia," he said. "But 16 percent were taking iron-containing supplements."

While readily noting that the proof of a benefit is not in, some researchers said they took multivitamins. They agree with Dr. Joann E. Manson, chief of preventive medicine at Brigham and Women's Hospital in Boston, who takes a multivitamin and recommends it to patients whose diets seem imbalanced.

"I think it's a good form of insurance," Dr. Manson said. "I don't think there's a significant downside. We don't have the evidence yet that it is beneficial."

Dr. Robert H. Fletcher, a professor of ambulatory medicine at Harvard Medical School, also takes multivitamins. For him, the deciding factor was whether he ingested enough folic acid. Studies have suggested that high levels of folic acid can protect against heart disease by lowering levels of another substance, homocysteine. High levels of homocysteine are associated with increased risks of heart disease, but there is no study showing definitively that reducing homocysteine levels protects against heart disease.

So far, the folic acid studies are suggestive, not definitive. But Dr. Fletcher said, "If I were a betting man, I'd bet on it."

But a European study, reported recently at a meeting of the American College of Cardiology, found that folic acid supplements actually made matters worse for heart disease patients. The study, the Folate After Coronary Intervention Trial, involved 626 patients who were having stents inserted into blocked arteries to keep them open. Half were randomly assigned to take folic acid, and the rest took a placebo. Six months later, the arteries of those taking folic acid were significantly narrower than the arteries of those taking a placebo, exactly the opposite of what the investigators had expected.

A previous study, however, had found that folate helped such patients. Dr. Eric Topol, an interventional cardiologist at the Cleveland Clinic, said he thought the truth was that it was neither helpful nor harmful for most people. "Over all, the likely explanation is that there is a neutral effect, and these relatively small trials found opposite findings due to the play of chance," he said.

Dr. Topol said B vitamins, like folic acid, "can't be recommended" at this point, except for people with extremely low levels of homocysteine, and even then their value has not been rigorously demonstrated.

Karen Miller-Kovach, chief scientist for Weight Watchers International, has a compromise. She takes a child's multivitamin, with its much lower levels of vitamins and minerals.

"It is virtually impossible to find an adult multivitamin and mineral supplement that is only 100 percent of the R.D.A.," Ms. Miller-Kovach said. "All are 150 percent or so. I worry about getting too much and I worry about imbalances. They put in more of the things that are inexpensive, like B vitamins and things with consumer appeal like vitamin C. The formulas are based on market forces, not nutritional needs."

Others decided against taking the pills.

Dr. Kava, of the American Council on Science and Health, said she abstained. "People ask me what vitamins I take," she said. "I say I don't take any. They look at me askance. They can't believe I'm a nutritionist."

Dr. Caballero also does not take vitamins. "There is no disease I know of that is prevented by multivitamins," he said.

In fact, Dr. Caballero said, typical pills, which contain a variety of minerals as well as vitamins, have ingredients that actually cancel out one another. "Minerals antagonize each other for absorption," he said. "Zinc competes with iron which competes with calcium."

Dr. Caballero also notes that large, rigorous studies that were supposed to show that individual vitamins prevented disease ended up showing the opposite. Those who took the vitamins actually had more of the disease it was meant to prevent.

Two large randomized trials of vitamin A and beta carotene that researchers hoped would show a protective value against cancer found no benefit, and one found that participants who took the supplements had more cancer.

A large study of vitamin E and heart disease found that it did not prevent heart attacks and that people taking it had more strokes.

Another study, of women with heart disease, found that antioxidant vitamins might actually increase the rate of atherosclerosis.

Dr. Caballero said people were deluding themselves if they thought multivitamins could make up for poor diets.

"If you eat junk food every day, vitamins are the least of your problems," he said. "You cannot replace a healthy diet. We don't know what ingredient in a healthy diet is responsible for which condition. We do know that people who consume five servings or more of fruits and vegetables have less disease. But we don't know which ingredient. We tried beta carotene, vitamin E and antioxidants, and they didn't work.

"People are looking for the magic bullet. It does not exist."