Individuals who have higher levels of a fatty acid known as docosahexaenoic acid (DHA) in their blood may have a significantly lower risk of developing dementia and Alzheimer's disease, according to a report in the November issue of Archives of Neurology, one of the JAMA/Archives journals.

Age, family history and genetic factors have all been found to increase the risk of dementia and Alzheimer's disease, a neurodegenerative disorder that causes 70 percent of cases of dementia in the elderly, according to background information in the article. Recent studies have found that high levels of homocysteine, an amino acid that is derived from proteins in the diet and that can accumulate in the blood and contribute to heart disease, increase the risk for Alzheimer's disease and dementia. In addition, DHA, an omega-3 polyunsaturated fatty acid found in fish, appears to affect dementia risk and to be important for the proper functioning of the central nervous system.

Ernst J. Schaefer, M.D., Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, and colleagues studied the association between DHA levels and dementia in the blood of 899 men and women who were part of the population-based Framingham Heart Study. The participants of an average age of 76 years provided blood samples and underwent neuropsychological testing, and were followed for an average of nine years. A subgroup of 488 also filled out a questionnaire assessing their diet, including information about fish consumption. None of the participants had dementia at the beginning of the study; and they were given a mental examination every two years to screen for its development.

Through the nine-year study period, 99 out of 899 participants developed dementia, including 71 with Alzheimer's disease. After controlling for other known risk factors for dementia, including age and homocysteine levels, and dividing the study population into fourths (quartiles) based on levels of DHA, the researchers found that men and women in the quartile with the highest DHA levels had a 47 percent lower risk of developing dementia and 39 percent lower risk of developing Alzheimer's disease than the other three quartiles with lower DHA levels. Among the participants who completed the dietary questionnaire, those in the top quartile of blood DHA levels reported that they ate an average of .18 grams of DHA a day and an average of three fish servings a week. Participants in the other quartiles ate substantially less fish.

DHA levels in the blood vary by the degree to which the liver converts alpha-linolenic acid, an essential fatty acid, to DHA and also by the amount of DHA in the diet. "In our study, the correlation between [blood] DHA content and fish intake was significant, indicating that fish intake is an important source of dietary DHA," the authors write.

"In the future, it will also be important to determine whether combined dietary supplementation with DHA can decrease further mental deterioration in patients with established dementia," they conclude.

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(Arch Neurol. 2006;63:1545-1550. Available pre-embargo to the media at www.jamamedia.org.)

Editor's Note: Please see the article for additional information, including other authors, author contributions and affiliations, financial disclosures, funding and support, etc.

Editorial: Link Between Fatty Acids and Dementia Makes Biological Sense

There is a strong basis in biology for the neuroprotective effects of DHA, writes Martha Clare Morris, Sc.D., Rush University Medical Center, Chicago, in an accompanying editorial.

Lipids, a collective term for fats and oils, make up about 50 to 60 percent of the brain's dry weight, and DHA is the most abundant fatty acid found in the cell membranes of the brain's gray matter, she writes. Studies done in the 1980s and 1990s found that DHA is important to a variety of brain cell components and functions. "Indeed, the level of DHA in the brain has been shown to be very important for learning ability and memory in early life in studies of rodents, baboons and humans," Dr. Morris writes. "It is only recently that the omega-3 fatty acids have been investigated for their importance to the aging brain. The DHA composition of the brain decreases with age as a result of increased oxidative [result of oxygen exposure] damage to the lipid membranes."

Research indicates that consuming more DHA in the diet later in life increases DHA levels in the aging brain. However, more studies are needed to determine whether omega-3 supplements could prevent dementia, she concludes.

(Arch Neurol. 2006;63:1527-1528. Available pre-embargo to the media at www.jamamedia.org.)