Fish Linked to Less Alzheimer’s Disease in Those Most at Risk
No brain impairment seen from mercury in seafood.
If concerns about mercury in seafood have kept you from the possible brain benefits of consuming more fish, an unusual new study has good news. Researchers did find that older adults who ate more seafood had higher brain levels of mercury - but that toxin was not associated with any signs of dementia. On the other hand, people at greatest genetic risk for Alzheimer's who consumed the most seafood showed less evidence of the disease’s damage in the brain.
"That’s great news," says Tammy Scott, PhD, an assistant professor at the Friedman School and scientist in Tufts' HNRCA Neuroscience and Aging Laboratory. "It would still be prudent, however, for people to try to minimize their mercury intake while still consuming the recommended amounts of seafood and omega-3 fatty acids. The USDA dietary guidelines recommend around eight ounces of seafood per week for an average consumption of 250 milligrams per day of the longer chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)."
BRAIN EXAMS: The latest study, published in JAMA, stands out because scientists were able to actually examine autopsied brain tissue from 286 participants who had reported their seafood intake an average of 4.5 years before their deaths. The participants in Rush University’s Memory and Aging Project were initially free of dementia; average age at death was almost 90.
"Everybody's saying seafood has so many health benefits, but everybody's afraid of the mercury," commented lead researcher Martha Clare Morris, ScD. "We saw absolutely no evidence that higher levels of mercury in the brain were associated with any of the neuropathologies associated with dementia."
Pregnant and breastfeeding women, those considering getting pregnant and young children are advised to limit their intake of fish high in mercury, specifically tilefish from the Gulf of Mexico, shark, swordfish and king mackerel. Consumption of white (albacore) tuna should be limited to six ounces a week for these groups. But a 2014 advisory from the US Food and Drug Administration and Environmental Protection Agency also urged these at-risk groups to eat more fish that is lower in mercury in order to gain important developmental and health benefits.
For other adults, experts agree that the benefits of seafood consumption outweigh any risks from mercury. The American Heart Association, for example, advises consuming at least two seafood meals a week.
EARLIER EVIDENCE: The protective benefits of seafood consumption in the latest Rush study were limited to participants with the ApoE4 genetic variant linked to increased risk of developing Alzheimer's disease. But Morris and colleagues speculated that this increased risk made it easier to detect a strong difference between the brains of those who did and did not frequently eat seafood. People without the genetic variant might have had too little Alzheimer’s proteins in their brains to detect.
Previous studies have linked greater fish consumption to lower risk of dementia and Alzheimer’s regardless of genetics. In a nine-year study of nearly 500 elderly adults, Tufts researchers reported that those eating the most fish—an average of nearly three servings a week—were at significantly lower risk of dementia and Alzheimer's disease.
In an earlier study by Morris and colleagues, participants who consumed fish once a week or more had 60% less risk of Alzheimer’s disease compared with those who rarely or never ate fish. Total intake of DHA (but not EPA, the other primary omega-3 fish oil) was also associated with lower risk of Alzheimer's.
OMEGA-3 QUESTIONS: It’s still not clear, however, whether the apparent brain benefit of fish consumption is due to its omega-3 content, Morris added, or whether it "is more complicated to understand."
Results from clinical trials testing supplements of fish oil for cognitive protection have been mixed. In 2015, a five-year clinical trial with more than 3,000 participants reported no benefits from omega-3 supplements against cognitive decline. But Tufts' Scott cautions that this was not necessarily the final word on fish-oil pills, because of the design of the study and the population tested. (See the December 2015 newsletter.)
The bottom line? "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."
TO LEARN MORE: JAMA, Feb 2, 2016 -