Large daily doses of Vitamin E, long touted as a virtual wonder drug that could protect against cancer, heart disease, dementia and other ailments, actually increase the risk for prostate cancer among middle-aged men, according to a large federal study released Tuesday.

The analysis of data from more than 35,000 healthy men concluded that those who took Vitamin E every day at the large dose levels commonly sold in drug, grocery and health food stores were 17 percent more likely to develop prostate cancer.

“You really have to question now how taking Vitamin E will help someone,” said Eric A. Klein, a Cleveland Clinic prostate cancer expert who led what had been hoped to be a cancer-prevention study. “Not only is it unlikely to help them, it apparently could hurt them.”

The findings, published in the Journal of the American Medical Association, are the latest in a series of carefully designed experiments that have found that for most healthy people who eat a balanced diet, vitamins and other dietary supplements are unnecessary and, at high doses, possibly sometimes dangerous.

“Just because it’s ‘only a vitamin’ or ‘it’s natural,’ we assume it must be safe. But over and over again, we see that’s not necessarily the case,” said Howard Parnes of the National Cancer Institute, which funded the prostate cancer study. “Not only isn’t it the fountain of youth that some people said, it can be harmful.”

The National Institutes of Health launched the $122 million project to study prostate cancer in 2001 after laboratory studies and some clinical data indicated that the anti­oxidant Vitamin E and selenium might protect against prostate cancer, the second most common cancer and cancer killer in men.

The study followed more than 35,533 men 50 or older at 427 sites in the United States, Canada and Puerto Rico. The men were divided into four groups who took daily doses of 400 international units of Vitamin E and 200 micrograms of selenium; Vitamin E and a placebo that looked like selenium; selenium and a placebo that looked like Vitamin E; or two placebos. The recommended daily intake of Vitamin E is about 22 international units.

An independent panel monitoring the experiment halted the study in 2008 when it became clear there was no benefit and indications emerged that the high-dose supplements might be increasing the risk for prostate cancer and diabetes.

The new analysis, based on additional data collected from the same men since 2008, found that the diabetes risk disappeared but that the prostate cancer risk reached statistical significance. There were 620 cases of prostate cancer among the men taking Vitamin E alone, compared with 555 among those taking selenium and Vitamin E, 575 among those taking selenium, and 529 among men taking a placebo. Based on the findings, the researchers calculated that for every 1,000 men taking Vitamin E alone, about 76 developed prostate cancer compared with 65 taking the placebo.

“The public and consumer tend to believe vitamins are innocuous substances and you can take them with impunity,” Klein said. “Clearly that is not the case.”

The new analysis came less than a week after an influential federal panel concluded that the risks of routine PSA screening for prostate cancer outweighed the benefits, touching off an intense debate over how best to protect men from the disease.

Researchers are unclear how Vitamin E would increase the risk for prostate cancer but are exploring several theories, in part by analyzing blood and other samples collected from the study participants.

“There is speculation that maybe at very high doses antioxidants become pro-oxidants,” the NCI’s Parnes said. “Another idea is that when you have very high doses of one nutrient, there can be negative effects on others. Everything is a balance in biology. But we really don’t know.”

About half of U.S. adults regularly take some kind of supplement, according to the latest federal data. Overall, Americans spend more than $28 billion a year on vitamins, minerals and other substances that companies claim can reduce the risk for cancer, heart attack, stroke, diabetes and Alzheimer’s disease, among others, including about $340 million alone in 2010 for Vitamin E, according to the Nutrition Business Journal.

One supplement, beta carotene, might help slow a common form of blindness known as macular degeneration, but even it was found to increase the risk for lung cancer among smokers.

Several other studies are underway to examine possible benefits of other vitamins, including one testing Vitamin D and fish oil to reduce the risk for heart disease and cancer.

Some scientists and the dietary supplement industry questioned the Vitamin E findings, saying the new study and previous ones were flawed for trying to evaluate individual vitamins alone, noting the combination of Vitamin E and selenium did not prove risky.

“This reinforces the theory that vitamins work synergistically and that drug-like trials of nutrients, when used in isolation from other nutrients, may not be the most appropriate way to study them,” said Duffy MacKay, vice president, scientific and regulatory affairs, at the Council for Responsible Nutrition, an industry group.

Parnes, Klein and others stressed that Vitamin E does not appear to pose a risk at lower doses and that there is plenty of evidence that consuming a diet rich in fruits and vegetables, which contain large amounts of vitamins and other nutrients that have antioxidant and other properties, is healthy.

“Bottom line, eat your vegetables and be active,” Benjamin Caballero, director of the Center for Human Nutrition at Johns Hopkins University, said in an e-mail. “You cannot get those health benefits from any pill created by man.”