The findings by researchers at the University of Pittsburgh School of Medicine add to growing evidence that lifestyle factors contribute to brain health later in life.
Scientists estimate that more than 80 million people will have dementia by 2040, which could become a substantial burden to families and drive up health care costs, noted senior investigator James T Becker, professor of psychiatry, Pitt School of Medicine.
The anti-oxidant effect of omega-3 fatty acids, which are found in high amounts in fish, seeds and nuts, and certain oils, have been associated with improved health, particularly brain health.
Our study shows that people who ate a diet that included baked or broiled, but not fried, fish have larger brain volumes in regions associated with memory and cognition," Becker said.
"We did not find a relationship between omega-3 levels and these brain changes, which surprised us a little. It led us to conclude that we were tapping into a more general set of lifestyle factors that were affecting brain health of which diet is just one part," said Becker.
Lead investigator Cyrus Raji, who now is in radiology residency training at University of California, Los Angeles, and the research team analysed data from 260 people who provided information on their dietary intake.
They had high-resolution brain MRI scans, and were cognitively normal at two time points during their participation in the Cardiovascular Health Study (CHS), a 10-year multicenter effort that began in 1989 to identify risk factors for heart disease in people over 65.
"The subset of CHS participants answered questionnaires about their eating habits, such as how much fish did they eat and how was it prepared," Raji said.
"Baked or broiled fish contains higher levels of omega-3s than fried fish because the fatty acids are destroyed in the high heat of frying, so we took that into consideration when we examined their brain scans," said Raji.
People who ate baked or broiled fish at least once a week had greater grey matter brain volumes in areas of the brain responsible for memory (4.3 per cent) and cognition (14 per cent) and were more likely to have a college education than those who didn't eat fish regularly, the researchers found.
But no association was found between the brain differences and blood levels of omega-3s.
"This suggests that lifestyle factors, in this case eating fish, rather than biological factors contribute to structural changes in the brain," Becker noted.
The study was published in the American Journal of Preventive Medicine.