Eating at least one seafood meal per week was linked to a reduced risk of having Alzheimer's disease in older adults with a gene known as APOE4, a U.S. study said Tuesday.

This association, however, was not found in the brains of people who ate fish weekly but did not carry the gene, said the study, which was published in the Journal of the American Medical Association (JAMA).

Researchers from the Rush University Medical Center also examined the brains for levels of mercury, which can be found in seafood and is known to be harmful to the brain and nervous system.

They found that seafood consumption was associated with increased mercury levels in the brains but not the amount of beta amyloid protein plaques and tau protein tangles, the hallmarks of Alzheimer's disease.

The findings were based on data from the medical center's Memory and Aging Project study, during which older volunteers completed annual dietary questionnaires over a number of years.

At the start of the study, the participants were cognitively normal, but some eventually developed cognitive impairment and dementia.

The brains of 286 deceased study participants, whose average age was 89.9 years, were analyzed for neuropathologies, or detrimental brain changes, of Alzheimer's disease and other dementias.

"This appears to be the first study showing an absence of an increased risk of Alzheimer disease or dementia related to mercury based on the level of brain pathology, suggesting that seafood can be consumed without substantial concern of mercury contamination diminishing its possible cognitive benefit in older adults," Edeltraut Kroger and Robert Laforce of Canada's Universite Laval wrote in an accompanying editorial.

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

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