The omega-3 fatty acids that are found in fish are known to benefit the brain but many people are concerned about the risks posed by the mercury found in seafood.

A new study, however, could stamp out this fear as researchers find evidence that mercury in fish does not lead to mental decline. A diet rich in fish may even benefit individuals with a strong genetic risk factor for Alzheimer's.

In a study published in the Journal of the American Medical Association on Feb. 2, researchers performed brain autopsy on about 300 individuals whose diets were assessed prior to their death between the years 2004 and 2014.

The researchers found that the participants who consumed seafood at least once a week had higher levels of mercury in their brain compared with those who ate less. Nonetheless, the researchers did not find a link between increased levels of the neurotoxin in the brain and the hallmarks of Alzheimer's such as amyloid plaques and tangles.

"We saw absolutely no evidence that higher levels of mercury in the brain were associated with any of the neuropathologies associated with dementia," said study author Martha Clare Morris from Rush University in Chicago.

Morris and colleagues found that those who ate seafood at least once per week had less Alzheimer's damage in the brain, albeit this was observed on individuals with a version of the apolipoprotein E (APOE) gene called APOE-4, which places carriers at increased risk of developing Alzheimer's disease.

Nearly 30 percent of the general population carries at least a copy of the APOE-4 variant. About 23 percent of the older participants who took part in the study had at least one copy of APOE-4.

Morris said that they did not find this protective association in participants who did not have the gene variant. Although it is still possible that individuals who do not have APOE-4 could get a smaller amount of protection from Alzheimer's, the researchers said that the study they conducted was not big enough to detect this.

The researchers think that the benefit of seafood consumption in those with APOE-4 has something to do with DHA loss.

DHA is important for brain health and this can be obtained from fish. People with APOE-4 are believed to lose even more DHA in the brain, which is why consuming seafood appear to be more beneficial for them.

"One theory is that seafood consumption may be more beneficial in older age because, as we age, we lose DHA in the brain," Morris said.