(Reuters Health) - For older people, the brain-protecting benefits of eating fish outweigh any potential harms from mercury, according to a small U.S. brain study.

Researchers found that older adults who ate the most seafood did have higher brain levels of mercury, but didn’t seem to suffer any ill effects from it. And if they also had a gene variant that raises risk for Alzheimer’s disease, high fish intake seemed to lower their risk of developing the disease.

“We had a unique opportunity to look at seafood consumption and relate it to brain health,” in a group of elderly people before and after death, said lead author Martha Clare Morris of Rush University Medical Center in Chicago.

“Seafood is touted for its many health benefits,” Morris told Reuters Health. “A large number of studies have shown that it slows cognitive decline with aging and reduces risk of Alzheimer’s disease and dementia.”

But some older people worry that consuming fish with mercury will actually damage their brains, she said, so this study should allay those concerns.

The researchers used detailed data on a group of older people living in Chicago retirement homes or subsidized housing and participating in a memory and aging research project. During the study, which ran from 2004 to 2013, participants described their fish and seafood consumption among other foods, and after many of them died, their brains were autopsied for research purposes.

On average, participants died at around age 90, and they had answered the dietary questionnaires about four and a half years before death. Of the 544 participants who died by 2013, about half had brain autopsies and the new analysis is based on those results.

Researchers found that as the number of seafood meals a person ate each week rose, so did the level of mercury detected in their brain on autopsy. But those who said they ate seafood one or more times a week also had less Alzheimer’s-related brain pathology, such as plaques or neurofibrillary tangles, compared to those who ate little or no seafood.

Among people with the “e4” version of a gene known as apolipoprotein E (APOE), which is associated with heightened risk of developing Alzheimer’s, the disease was less common for seafood eaters than for those who ate little or none. The results were similar when researchers looked at just omega-3 fatty acid levels in the diet, most of which come from certain types of oily fish.

However, taking fish oil supplements was not linked to any brain structure changes related to dementia, according to the results published in JAMA.

“The (beneficial) fish consumption levels were on average more than two seafood meals per week,” which is not infrequent, but would be less than in some other areas like Alaska, Morris said.

She was not able to identify which specific types of seafood were most closely linked to reduced brain risk, she said.

At these moderate levels, though, the mercury associated with fish did not seem to do any harm.

“There is mercury toxicity that does affect the brain, but we do not have data on very high levels of seafood consumption,” Morris said.

“There has been evidence that the mercury level in fish may limit its benefit for the development of the unborn child in pregnant women,” said Edeltraut Kroger of Universite Laval in Quebec City, Canada, who coauthored an editorial alongside the new results. “Pregnant women or women considering pregnancy should limit their intake of fish which has higher levels of mercury.”

The omega-3 fatty acids found in seafood, particularly fatty fish, are important for brain health, Morris said.

“Fish is the most important food source for the intake of omega-3 unsaturated fatty acids,” Kroger agreed.

“These play an important role in brain cells and may protect against cardiovascular disease,” Kroger said by email.

We need not be afraid of mercury in fish with regard to Alzheimer’s disease, he said.

“Right now for Alzheimer’s disease there are no effective treatments and no cure,” Morris said. “Eating seafood may be one way to reduce your risk.”

SOURCE: bit.ly/1JVPeoX and bit.ly/1PSGaTX JAMA, online February 2, 2016.