Two new studies add to the growing body of evidence that taking extra doses of vitamins can do more harm than good.

A study of vitamin E and selenium use among 35,000 men found that the vitamin users had a slightly higher risk of developing prostate cancer, according to a report published Tuesday in The Journal of the American Medical Association. A separate study of 38,000 women in Iowa found a higher risk of dying during a 19-year period among older women who used multivitamins and other supplements compared with women who did not, according to a new report in The Archives of Internal Medicine.

The findings are the latest in a series of disappointing research results showing that high doses of vitamins are not helpful in warding off disease.

“You go back 15 or 20 years, and there were thoughts that antioxidants of all sorts might be useful,” said Dr. Eric Klein, a Cleveland Clinic physician and national study coordinator for the prostate cancer and vitamin E study. “There really is not any compelling evidence that taking these dietary supplements above and beyond a normal dietary intake is helpful in any way, and this is evidence that it could be harmful.”

The Selenium and Vitamin E Cancer Prevention Trial, known as the Select trial, was studying whether selenium and vitamin E, either alone or in combination, could lower a man’s risk for prostate cancer. It was stopped early in 2008 after a review of the data showed no benefit, although there was a suggestion of increased risk of prostate cancer and diabetes that wasn’t statistically significant. The latest data, based on longer-term follow-up of the men in the trial, found that users of vitamin E had a 17 percent higher risk of prostate cancer compared with men who didn’t take the vitamin, a level that was statistically significant. There was no increased risk of diabetes.

The dose being studied in the Select trial was 200 micrograms of selenium and 400 international units of vitamin E. By comparison, most multivitamins contain about 50 micrograms of selenium and 30 to 200 international units of vitamin E.

Among the women in the Iowa study, about 63 percent used supplements at the start of the study, but that number had grown to 85 percent by 2004. Use of multivitamins, vitamin B6, folic acid, iron, magnesium, zinc and copper were all associated with increased risk of death. The findings translate to a 2.4 percent increase in absolute risk for multivitamin users, a 4 percent increase associated with vitamin B6, a 5.9 percent increase for folic acid, and increases of 3 to 4 percent in risk for those taking supplements of iron, folic acid, magnesium and zinc.

“Based on existing evidence, we see little justification for the general and widespread use of dietary supplements,” the authors wrote.

Everyone needs vitamins, which are essential nutrients that the body can’t produce on its own. But in the past few years, several high-quality studies have failed to show that high doses of vitamins, at least in pill form, help prevent chronic disease or prolong life.

A January 2009 editorial in The Journal of the National Cancer Institute noted that most studies of vitamins had shown no cancer benefits, but some had shown unexpected harms. Two studies of beta carotene found higher lung cancer rates, and another study suggested a higher risk of precancerous polyps among users of folic acid compared with those in a placebo group.

In 2007, The Journal of the American Medical Association reviewed mortality rates in randomized trials of antioxidant supplements. In 47 trials involving 181,000 participants, the rate of dying was 5 percent higher among the antioxidant users. The main culprits were vitamin A, beta carotene and vitamin E; vitamin C and selenium seemed to have no meaningful effect.