TUESDAY, Aug. 5 (HealthDay News) -- Fish may be brain food after all -- not making you smarter, as your grandmother said, but by lowering the risk of cognitive decline and stroke as you get older, according to a new study.

The benefit appears to come from fish rich in omega-3 fatty acids. In fact, eating fish such as tuna three times a week can reduce the risk of dementia or stroke by 26 percent, the study found.

"Older adults who consumed tuna or other baked or broiled fish had a lower risk of having abnormalities on brain MRIs," said study co-author Dr. David Siscovick, a professor of medicine and epidemiology at the University of Washington Cardiovascular Health Research Unit.

For the study, Siscovick and his colleagues looked for small vessel disease, which is known to lead to cognitive impairment, dementia and stroke. "This study looked at whether or not different types of fish intake were related to structural changes in the brain," he said.

The researchers collected data on 3,660 men and women 65 and older. They underwent brain scans to detect what doctors call silent brain infarcts, which are small lesions in the brain that are associated with the loss of thinking skills, dementia or stroke. These lesions are found in about 20 percent of otherwise healthy older people, and they're only detectable on brain scans, the researchers said.

Five years later, the researchers repeated the scans on 2,313 of the original study participants. In addition, the participants answered questions about the amount of fish in their diet.

Siscovick's team found that people who ate tuna that was broiled or baked, or other fish high in omega-3 fatty acids, three or more times a week, had almost a 26 percent lower risk of silent brain infarcts compared with people who didn't eat fish regularly. In fact, eating just one serving of fish a week reduced the risk by 13 percent.

What's more, people who ate fish regularly had fewer abnormal changes in their brain's white matter, the researchers found. Abnormal changes in white matter and small brain infarcts are signs of small vessel disease, Siscovick said.

But how the fish was cooked appeared to be important, according to the findings, which were published in the Aug. 5 issue of the journal Neurology.