The findings, he told Reuters Health, don't rule out a potential benefit on brain health in some people -- and it's still possible long-term use of the supplements may be helpful for preserving thinking and memory skills.

All three studies included in the analysis involved cognitively healthy people over age 60 who were randomly assigned to use supplements or margarine with fish oil or a placebo oil without omega-3 fatty acids.

None of those studies showed a comparative benefit for fish oil on measures of cognition, including word and number learning tests.

"The truth is many people are spending a lot of money on supplements without solid evidence they do something," said Dr. Nikolaos Scarmeas, a neurology and aging researcher at Columbia University Medical Center in New York.

Scarmeas, who wasn't involved in the new study, agreed it may take more than a few years for any benefit of the fatty acids to show up -- if one does exist.