Eating fish once a week slows memory loss associated with aging by 10% a year, according to a major study of elderly people.

For people who eat more than one fish meal a week, the slowdown in memory loss amounted to 13% a year, Rush University Medical Center epidemiologist Martha Clare Morris reported in the online version of the journal Archives of Neurology.

Analysis of the diet, lifestyle and cognitive function of 6,158 elderly people in Chicago showed that, after six years, those who ate fish weekly were about three to four years younger mentally than those who seldom ate fish.

The findings make sense, because fish contains omega-3 fatty acid, which forms part of brain cell membranes, Tufts University USDA Nutrition Center on Aging cardiovascular nutrition laboratory director Alice Lichtenstein said.

Lichtenstein, who was not involved in the Rush study, said the findings were consistent with other studies that showed eating fish or taking fish oil supplements lowered the risk of heart attack, stroke and Alzheimer's disease.

"We found that eating fish at least once a week appears to have a small but significant delay in the decline of thinking ability with age," Morris said. There were too few people taking fish oil supplements to measure their effect on memory.

Morris said the study was not designed to look at any possible effects of mercury contamination in fish, which has been linked to cognitive impairment. However, fish may contain the highest mercury levels, she added. Fish that tend to have higher mercury content include swordfish and tuna.

The study group was made up of 62% African Americans and 38% white, and the lowered risk of memory loss was present in both races, Morris said. An earlier study of the same group showed that fish consumption lowered the risk of Alzheimer's disease.

"As we grow older, the neurons lose omega-3 fatty acid," Morris said.

"By eating fish or other sources of omega-3 fatty acid, you can replace that which is lost in the neuron membranes."

'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'