Eating fish at least once a week can slow the rate of mental or cognitive decline in elderly people by about 10 per cent, according to a new study.

Martha Morris of Rush University Medical Center, and colleagues collected data on 6,158 people aged 65 and older who lived in Chicago.

As part of the study, the participants were asked questions about what they ate, exercise levels, alcohol consumption, daily activities and medical history. In addition, every three years during the six-year study, they had their mental ability assessed.

Morris found that the more fish people ate, the slower their rate of mental decline.

"The rate of decline was reduced by 10 per cent to 13 per cent per year among persons who consumed one or more fish meals per week compared with those with less than weekly consumption. The rate reduction is the equivalent of being three to four years younger in age," she said.

Morris and colleagues examined whether overall dietary consumption patterns accounted for the association of cognitive decline and fish consumption, but the rate differences did not change after adjusting for consumption of fruit and vegetables.

"Cognitive decline is common among older people and is very much associated with advancing age. Our data offer no insight as to whether this cognitive decline is pathological or the result of a normal aging process. Nonetheless, data from the United States and other countries indicate that it is a widespread and increasing public health problem," Morris added.

Fish is a direct source of omega-3 fatty acids, which have been shown to be essential for neurocognitive development and normal brain functioning. Fish consumption has been associated with lower risk of dementia and stroke and recent studies have suggested that consumption of one omega-3 fatty acid in particular, docosahexaenoic acid (DHA), is important for memory performance in aged animals.

"This study suggests that eating one or more fish meals per week may protect against cognitive decline associated with older age," says Morris. "More precise studies of the different dietary constituents of fish should help to understand the nature of the association."

The findings are published in the online journal Archives of Neurology.