“That rate of reduction is equivalent to being three to four years younger in age,” said Martha Clare Morris, lead author of the study. Morris is associated with the Rush University Medical Center in Chicago. Under the study, she and her team scrutinized the cognitive function of 3,718 subjects, aged 65 and above. All the participants were involved in the Chicago Health and Aging Project.

Through tests and memory games, the team analyzed the mental capabilities of the subjects thrice in a period of six years. To understand the link between food and mental capabilities, the subjects were made to fill out questionnaires about their food habits. Those who ate fish regularly were found to be three to four years mentally younger than their non-fish eating counterparts. Around 62 per cent African-Americans and 38 per cent white Americans participated in the study and the results were found to be the same for both races.

“Basically we found that eating fish at least once a week appears to have a small but significant delay in the decline of one's thinking ability with age. People who rarely eat fish have a somewhat faster decline in their thinking ability over time,” Morris said.

However, the findings come as no surprise to most as fish contains omega-3 fatty acids that are linked to the development of the brain.

Alice Lichtenstein of Tufts University's USDA Nutrition Center on Aging, said that omega-3 fatty acids form an important constituent of brain cell membranes. She added that many other studies have found that eating fish regularly or taking fish oil supplements cuts the risk of Alzheimer's disease, which is caused by the degeneration of the brain.

But Morris and her team could not link the findings with omega-3 fatty acids specifically. “This study suggests that eating one or more fish meals per week may protect against cognitive decline associated with older age. More precise studies of the different dietary constituents of fish should help to understand the nature of the association,” she said.

Morris added that another study on the same group has shown that fish cuts the risk of Alzheimer's. “We know that as we grow older the neurons lose omega-3 fatty acid. By eating fish or other sources of omega-3 fatty acid you can replace that which is lost in the neuron membranes. That's very important for neuron functioning and how they communicate among themselves,” she said.

But Greg M Cole of the Alzheimer's Disease Research Center at the UCLA David Geffen School of Medicine was doubtful that this study indicated that omega-3 fatty acids were behind the slowing of mental decline. “This study from Morris looks at a large biracial population in Chicago and also finds reduced cognitive decline associated with increased fish intake, but doesn't find much evidence to relate this to greater omega-3 fatty acid intake. One problem is that the questionnaires on fish intake were not that highly correlated with actual blood levels of omega-3. These new results suggest the jury is still out on whether it is the oil in the fish, specifically the omega-3 fatty acids, that we should try to increase,” he said.

The findings of the study have been published in the Archives of Neurology.