Older men who take vitamin E and selenium supplements have the same risk of dementia as people who don't use these products, according to a new study that quashes hopes these antioxidants might prevent cognitive decline.

Previous research has linked antioxidants to the prevention of cellular damage that can occur with aging as well as in cancer and other diseases. Antioxidants may achieve this by halting or slowing oxidative stress, which has also been linked to the progression of dementia.

"Antioxidants, either through food or supplements, are believed to reduce oxidative stress throughout the body," said senior study author Frederick Schmitt of the University of Kentucky in Lexington.

"It could be that antioxidant supplements are less effective than those consumed through food," Schmitt said by email. "The take–home message is that the evidence for antioxidant supplements is limited."

The researchers examined data on 7,540 older men who took part in a larger trial of the effects of selenium and vitamin E on cancer risk. Participants were randomly assigned to one of four groups that received either vitamin E or selenium supplements, both supplements or placebo pills.

About half the men were followed for five years and half for an additional six years. The study found no differences in dementia risk between any of the groups, the researchers reported March 20th in JAMA Neurology.

At the start of the study, the men were 68 years old on average and had no history of cognitive or neurological problems. During the study, 325 of them developed dementia, or roughly 4.4 percent of the men in each treatment group.

One limitation of the study is that many participants dropped out early. During the study, other research emerged linking vitamin E to an increased risk of prostate cancer and linking selenium to higher odds of diabetes; these findings may have prompted at least some men to leave the antioxidant study, the authors note.

Based on the results, however, people without dementia should not be taking antioxidant supplements just to prevent cognitive decline, the researchers conclude.

It's possible that the study participants got enough antioxidants from their diets that the supplements didn't appear beneficial, Schmitt said. The dose of supplements or the formulation might have also contributed to the lack of benefit found in the study.

A Mediterranean diet rich in fruits, vegetables, legumes, whole grains, fish and healthy fats may help ward off dementia even if supplements do not, and exercise may also help prevent cognitive decline, Schmitt said.

Foods rich in antioxidants include a variety of berries like blueberries, cranberries, goji berries and elderberries as well as dark chocolate, pecans, artichokes and kidney beans.

The antioxidant vitamin E can be found in nuts, seeds, vegetables, and fish oil but the body may need supplements to get enough of this nutrient. Brazil nuts, tuna and certain other fish, as well as red meat and poultry can contain selenium, but supplements may also be needed to boost supplies of this nutrient.

"If you aren't taking antioxidant supplements, there is scant evidence that they will be of significant help in preventing dementia," said Dr. Steven DeKosky of the McKnight Brain Institute at the University of Florida in Gainesville.

"However, healthy diets that contain vegetables with antioxidants would be good to choose as it does have the natural compounds needed," DeKosky, author of an accompanying editorial, said by email.