Eating fish at least once a week is good for the brain, slowing age-related mental decline by the equivalent of three to four years, a CNN study suggests.

Previous studies found that people who ate fish lowered their risk of Alzheimer's disease and stroke. Fish such as salmon and tuna that are rich in omega-3 fatty acids also have been shown to prevent heart disease.

"We found that people who ate one fish meal a week had a 10 percent slower annual decline in thinking," says co-author Martha Clare Morris, an epidemiologist at Rush University Medical Center. "Those who ate two fish meals a week showed a 13 percent slower annual decline."

The study was posted on the Web site of the Archives of Neurology and will appear in the journal's December issue.