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.
The latest data, based on a longer-term follow-up of the men in the trial, found that users of vitamin E had a 17% 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 doses being studied in the Select trial were 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.
In the Iowa study, use of multivitamins, vitamin B6, folic acid, iron, magnesium, zinc and copper were all associated with increased risk of death.