Antioxidant supplements vitamin E and selenium -- taken alone or in combination -- did not prevent dementia in older men, a US study said Monday.

Antioxidants as potential treatment for cognitive impairment or dementia have been of interest for years because oxidative stress, a process that causes cell damage, has been implicated as a dementia pathway, said the study published online in the US journal JAMA Neurology.

To find out, the US Prevention of Alzheimer's Disease by Vitamin E and Selenium clinical trial enrolled over 7,500 people aged 60 and older who were randomized assigned to vitamin E, selenium, both, or placebo, for an average of about five years.

After that, a subset of nearly 3,800 men agreed to be observed for an additional six years while they were not taking the supplements.

In the end, more than 300 of these people were diagnosed with dementia, but the incidence of the disease was not different among the four study groups, according to the results of the study.

"To our knowledge, this is the first large-scale primary prevention trial to investigate the association of antioxidant supplements with reducing dementia incidence," the study wrote.

"The results showed that neither vitamin E nor selenium had a significant preventive effect on incidence."

As a result, these supplements "are not recommended as preventive agents," it concluded.