Eating seafood may lead to higher levels of mercury in the brain, but a study out Tuesday found that increased mercury does not appear to raise the risk of dementia.

The study in the Journal of the American Medical Association (JAMA) was based on the autopsies of 286 people who died at an average age of 90.

Participants in the study began reporting their food intake via questionnaire on average nearly five years before their death.

Those who ate more seafood also had higher levels of mercury in the brain.

But the autopsies showed no increasing signs of brain disease in those who had more mercury in the brain.

Mercury is a neurotoxin that makes its way into the air and water -- and then accumulates in fish -- when trash is incinerated and coal is burned for electricity.

Researchers said the toxic effects of mercury on cognition and brain development may be reduced by selenium, an essential nutrient present in seafood.

Among a certain group of people -- those with a gene variant known as apolipoprotein E (ApoE4) which is linked to a higher risk of developing Alzheimer's -- researchers found fewer signs of brain disease in those who ate fish once per week, or more.

Seafood consumption in the high-risk group was "significantly correlated with less Alzheimer disease pathology," particularly in the amount of beta amyloid protein plaques and tau protein tangles, said the study, led by Martha Clare Morris of Rush University Medical Center in Chicago.

This protective effect was not seen in people without the high-risk gene variant.

About two percent of the population inherits two copies of this gene variant, placing them at a severe risk of Alzheimer's. About 15 percent of the population inherits one copy of ApoE4.

According to the study, those who took fish oil supplements showed no statistically significant change -- neither improvements nor declines -- in brain health.

Researchers said their experiment is the first to examine the relationship between brain health and levels of mercury.

Most of the participants in the study were white, and 67 percent were women, so the finding may not apply to younger people or those of all ethnic groups.

The study was funded by grants from the National Institutes of Health.

According to an accompanying editorial by Edeltraut Kroger and Robert Laforce Jr. of Universite Laval in Quebec, the study offers more evidence that eating seafood is good for the brain.

The research "provides reassurance that seafood contamination with mercury is not related to increased brain pathology," they wrote.

"Eating fatty fish may continue to be considered potentially beneficial against cognitive decline in at least a proportion of older adults," they added.

"Such a simple strategy is encouraging in the light of the lack of evidence on protection against many neurodegenerative diseases such as Alzheimer disease and Parkinson disease, another cause of dementia."