ScienceDaily (Jan. 3, 2007) — People who ate the most fish on a weekly basis -- putting them in the top quarter of a study population -- were nearly 50 percent less likely to develop the mental deterioration known as dementia over time than participants in any of the other three quarters.

The observational study was led by Ernst J. Schaefer, an Agricultural Research Service-funded scientist. ARS is the U.S. Department of Agriculture's chief scientific research agency. Schaefer is a physician specializing in nutrition and health with the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston, Mass.

He and co-authors were looking for a relationship between blood levels of the fatty acid DHA and the risk of developing dementia. DHA is short for docosahexaenoic acid, a so-called "heart-healthy" omega-3 fatty acid. Several different studies have linked either low DHA, or low fish intake levels, with the incidence of dementia.

The study was published in the November 13 issue of the Archives of Neurology. Schaefer and colleagues analyzed available dietary questionnaires and blood levels of DHA of nearly 900 men and women, aged 55 to 88, who participated in the longitudinal Framingham (Mass.) Heart Study.

At the beginning of a nine-year period, all of the participants were found to be free of dementia. Using proportional regression analysis, the researchers determined the relative impact not only of blood levels of DHA, but also of potential "confounding" variables such as age, gender, homocysteine and apolipoprotein-E levels, genotype and education.

They found that the participants who reported consuming an average of about three servings of oily fish a week--equivalent to blood levels of DHA at 180 milligrams daily--were associated with a significantly reduced risk of developing dementia of all types, including Alzheimer's disease. No other fatty acid blood level was independently linked to the risk of dementia.

The study suggests that relatively higher fish consumption over time correlates with a lower incidence of dementia in the over-55 set.

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