FRIDAY, Nov. 17 (HealthDay News) -- Adding further weight to the theory that fish may be brain food, new research found that people with diets rich in fish have a significantly lower risk of dementia and Alzheimer's disease.
The key appears to be docosahexaenoic acid (DHA), an omega-3 polyunsaturated fatty acid that appears to affect dementia risk and to be important for the proper functioning of the central nervous system.
"If you have a high level of DHA, a fatty acid found in fish, it reduced your risk of dementia by about half," said study lead researcher Dr. Ernst J. Schaefer, senior scientist and director of the Lipid Metabolism Laboratory at the Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University in Boston.
It's known that omega-3 fatty acids protect the heart and the circulatory system. "Just as fish is good for your heart, it's probably good for your brain as well," Schaefer said.
Fatty fish like mackerel, lake trout, herring, sardines, albacore tuna and salmon are high in DHA.
The study findings are published in the November issue of the Archives of Neurology.
In the study, Schaefer and his colleagues collected data on DHA levels and dementia in 899 men and women who were part of the Framingham Heart Study. Over nine years of follow-up, 99 people developed dementia, including 71 with Alzheimer's disease.
The researchers found that people with the highest blood levels of DHA had a 47 percent lower risk of developing dementia and a 39 percent lower risk of developing Alzheimer's, compared with those with lower DHA levels.
Levels of DHA in the blood vary by how much the liver converts alpha-linolenic acid, an essential fatty acid, to DHA and also by the amount of DHA in the diet, the researchers noted.
People with the highest blood levels of DHA said they ate an average of two to three servings of fish a week. People with lower DHA levels ate substantially less fish, the researchers reported.
Schaefer thinks the same benefit can be realized by taking fish-oil supplements. "Everything that we know suggests that supplements would be as effective as eating fish," he said. "Since low fish intake appears to be a risk factor for developing dementia, either eat more fish or use one or two fish oil capsules a day."
However, Schaefer added that a randomized clinical trial is still needed to see if DHA really protects the brain from dementia.
Martha Clare Morris is an epidemiologist at Rush University Medical Center in Chicago and author of an accompanying editorial in the journal. "This is the first study to link blood levels of DHA to protection against Alzheimer's disease," she said, adding that recent animal studies have shown that DHA reduces amyloid plaques -- a hallmark of Alzheimer's -- in the brain and also improves memory.
"There is a lot of animal and biochemical evidence to support what this new study shows," Morris said.
But, she said, she's not sure there is enough data to suggest the value of fish oil supplements. "It looks like the protective benefits from omega-3 fatty acids are at a very low level. There is very little evidence that you get better protection from higher intake," she said. "Whether fish oil supplements are protective is yet to be seen."
Another expert thinks clinical trials are needed to see if DHA really protects against Alzheimer's.
"This shows in a prospective study that DHA is the only plasma lipid to cut the risk for developing dementia a decade or more later," said Greg M. Cole, a neuroscientist at the Greater Los Angeles VA Healthcare System and associate director of the Alzheimer's Disease Research Center at UCLA's David Geffen School of Medicine.
This apparent protection is associated with eating fish, Cole said. "Other studies have pointed to fish intake as protective but have been far less clear that the omega-3 fatty acids in fish were the factor associated with risk reduction," he said. "This matters because if it is the fat, you could take fish oil supplements and avoid mercury contamination issues."
More information
The Alzheimer's Association can tell you more about Alzheimer's disease.
