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.
"We didn't find any association with eating fried fish and having lower rates of these abnormalities," Siscovick said, adding that fish rich in omega-3 fatty acids aren't typically fried.
Besides tuna, fish rich in omega-3 fatty acids include salmon, mackerel, herring, sardines and anchovies.
"The message to the public is to eat more fish that has higher levels of omega-3 fatty acids, rather than eating more fried fish," Siscovick said.
Greg M. Cole, associate director of the Alzheimer's Disease Research Center at the UCLA David Geffen School of Medicine, said that fish consumption -- along with other components of a healthy diet -- may explain the study's findings.
The same people with higher fish intake also had other dietary differences, such as higher intake of fruits and vegetables, said Cole, who's also a neuroscientist at the Greater Los Angeles VA Healthcare System. "This may be related to the more than one factor, analogous to the Mediterranean diet that is also rich in fish and fruits and vegetables and has been frequently associated with less cardiovascular risk," he said.
Eating fatty fish has also been associated with reduced risk of Alzheimer's disease, Cole added.
For more on the benefits of eating fish rich in omega-3 fatty acids, visit the American Heart Association.
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