CHICAGO – Eating fish once a week slows the memory loss associated with aging by 10 percent a year, according to a Rush University Medical Center study of 6,158 elderly Chicagoans.

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

Careful analyses of diet, lifestyle and cognitive function showed that after six years those who consumed fish weekly were about 3 to 4 years younger mentally than those who seldom ate fish, she said.

Biologically, the findings make sense because fish contain omega 3 fatty acid, which is an important constituent of brain cell membranes, said Alice Lichtenstein, director of the cardiovascular nutrition laboratory at the USDA Nutrition Center on Aging at Tufts University.

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

“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,” Morris said.

Morris said the study was not designed to look at any possible effects of mercury contamination of fish, which has been linked to cognitive impairment. But it’s probably a good idea to be aware of reports on which fish may contain the highest mercury levels, she added.

Fish that tend to have higher mercury content include swordfish and tuna.

The study group is made up of 62 percent blacks and 38 percent whites, and the lowered risk of memory loss was observed in both races, Morris said. An earlier study of the same population group showed that fish consumption lowered the risk of Alzheimer’s disease.

“We know that 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. That’s very important for neuron functioning and how they communicate among themselves.”

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