When it comes to preventing dementia, eating seafood is a double-edged sword. On the one hand, it’s high in mercury, a neurotoxin. Bad for the brain. On the other hand, it’s high in omega-3 fatty acids, which support nerve functioning. Good for the brain.

 So what happens to people who eat seafood regularly, compared with those who eat little or none? Choose one…

A. They’re more likely to get dementia. All that mercury undoes any good that omega-3s might do.

B. They’re less likely to get dementia. All those omega-3s protect the brain even with the extra mercury.

The correct answer is: B.

The mercury seems to be worth it.

This is something of a breakthrough finding. While earlier population studies had suggested that the cardiovascular and other benefits of eating seafood outweighed the risks of consuming contaminants, doubts remained. In a new study, researchers at Rush University Medical Center in Chicago looked at what you might call hard evidence—autopsies of 286 men and women (average age 90). They had already been studying these people when they were alive, so they knew how much seafood they were eating, and now they could look directly at their body tissues and inside their brains to see if there was accumulation of mercury—and neurological evidence of Alzheimer’s disease.

The surprise answer was that while the seafood eaters did have higher levels of mercury, there was no increased incidence of Alzheimer’s. That’s true even for those who had the highest levels of mercury.

While mercury didn’t harm, however, seafood protected those at the highest risk. These are the estimated about one-quarter of the population who carry a gene variant (apolipoprotein E4) that triples Alzheimer’s risk. Seafood didn’t protect everyone, but in this group, those who ate seafood regularly, compared with those who rarely or never ate it, were 47% less likely to show the brain pathology that defines Alzheimer’s disease.

Bottom line: By all means, choose seafood lowest in mercury—good choices include catfish, clams, flounder, salmon, sardines, scallops, shrimp, squid and light (not albacore) tuna. But don’t let worry about mercury stop you from getting the brain-protective benefits of seafood.

For tips on buying fish that’s really what it claims to be, see Bottom Line’s The Salmon Hoax. Ready to start pan frying these beauties? See Bottom Line’s Guide to Fabulous (and Healthy!) Fish Recipes.