A large clinical study by researchers at the National Institutes of Health has found that omega-3 supplements do not slow down the cognitive decline, a belief long upheld by the medical community, The National Institute of Health (NIH) reports.

"Contrary to popular belief, we didn't see any benefit of omega-3 supplements for stopping cognitive decline," Dr. Emily Chew, deputy director of the Division of Epidemiology and Clinical Applications and deputy clinical director at the National Eye Institute, part of the NIH, said. 

As part of the study, researchers studied about 4,000 patients for a 5-year period for the years 2006 to 2012 as part of the Age-Related Eye Disease Study 2. The average age of people in the study was 72, with females constituting 58 percent of the group.

In addition to eye exams, the participants were given cognitive tests every two years during the study to test delayed recall, attention and memory, and processing speed.

The participants were divided into four groups. One group was given omega-3, the other was given lutein and zeaxanthin, one was given omega-3, as well as lutein and zeaxanthin and the other received a placebo.

All the participants experienced some level of cognitive decline, which showed that none of the supplements made a difference.

"It may be, for example, that the timing of nutrients, or consuming them in a certain dietary pattern, has an impact," said Dr. Lenore Launer, a researcher at the National Institute on Aging. "More research would be needed to see if dietary patterns or taking the supplements earlier in the development of diseases like Alzheimer's would make a difference."

The study was published in the Journal of the American Medical Association.

An earlier study conducted at the University of Pennsylvania also found no evidence of the health benefits of taking fish oil supplements.