A higher level of DHA in the blood is associated with a 47% reduction in the risk of developing all-cause dementia, a study in the November Archives of Neurology finds.

Study participants with the highest level of DHA in their blood also had a 39% reduced risk of developing Alzheimer's disease, Ernst Schaefer, MD, Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging, Tufts University, Boston, et al., write.

These individuals had a mean intake of .18g of docosahexaenoic acid (DHA) per day and ate three servings of fish a week, according to a food frequency questionnaire completed by more than half of the 899 study participants.

The prospective follow-up study involved surviving participants from the longitudinal population-based Framingham Heart Study; 36.5% of the subjects were men, and the median age was 76.

To determine the level of DHA in participants' blood, researchers measured individual's plasma phosphatidylcholine (PC) DHA levels at baseline.

Plasma PC DHA content "is determined by the degree of conversion of alpha-linolenic acid to DHA within the liver and by the consumption of foods rich in DHA," Schaefer et al. say.

Individuals in the top quartile of plasma PC DHA levels had a mean plasma PC DHA level equal to 4.2% of total fatty acids. The mean for the total population was 3.6%.

During the follow-up, which averaged a little over nine years, 99 subjects developed dementia, including 71 cases of Alzheimer's disease.

After adjusting for age and sex, the researchers found subjects in the highest quartile had a relative risk of 0.53 for dementia compared with those in the lower three quartiles.

"These data indicate a substantial reduction in risk for all-cause dementia in the highest quartile group," Schaefer et al. say.

After adjusting for body mass index, hypertension, diabetes mellitus, smoking status, alcohol intake and history of stroke, the RR for all cause dementia was 0.54, while the RR for Alzheimer's disease was 0.62, the researchers conclude.

The researchers "found no relationship of dementia with plasma PC eicosapentaenoic acid level," they note.

Schaefer et al. say limitations of the study are "plasma PC DHA levels were measured on only one occasion;" "dietary data were available for only a subset of nonrandomly selected subjects"; and "there were only 99 new incident cases of dementia and even fewer new cases of Alzheimer's disease."

"It will be important to determine whether combined dietary supplementation with DHA can decrease further mental deterioration in patients with established dementia," the researchers say.

The study is "an important contribution to the young field of study on diet and neurological diseases," Martha Clare Morris, ScD, Rush University Medical Center, Chicago, Ill., writes in an accompanying editorial.

An article on the relationship between omega-3s and cognitive decline was also published in the October Archives of Neurology. The study showed omega-3 fatty acid supplementation may slow cognitive decline in some patients with very mild Alzheimer's disease (¹ 'The Tan Sheet' Oct. 16, 2006, In Brief).

Morris also writes in the editorial, "to date there are no published human studies on the brain effects of omega-3 fatty acid supplementation, primarily because of very low use of these supplements in the general population."

"Epidemiological data to determine an effective dose level of DHA for a clinical trial" are limited, Morris continues. However, "based on the available epidemiological evidence, the amount of DHA required for protective benefit is likely small."

NIH intends to study DHA's effects on Alzheimer's disease in an upcoming multi-million dollar trial. Martek Bioscience will supply their branded life'sDHA for the study (² 'The Tan Sheet' Oct. 23, 2006, In Brief).

- Rebekah Moan

Higher DHA Intake Associated With Reduced Risk Of Dementia – JAMA Study

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