Eating Fish Can Help Prevent Alzheimer's (2)

Summarized by Robert W. Griffith, MD
December 18, 2006

Summary

Older people who eat an average of 3 servings of fish a week consume about 0.18 grams/day of docosahexaenoic acid and halve their risk of developing dementia.

Introduction

The Mediterranean Diet has been shown to reduce the risk of developing Alzheimer's disease. The scientific community is trying to identify the component or components that confer this benefit. We recently summarized a study that showed that it seems to be a separate action from the known beneficial effects of the diet on cardiovascular conditions (see first link below). Another study published in the Archives of Neurology has taken a different approach; it examines the role of a specific omega-3 fatty acid in subjects who have a diet-related reduced risk of Alzheimer's.

The Mediterranean Diet includes plenty of fish or other sources of omega-3 fatty acids. Researchers used data from the Framingham Heart Study to determine the possible relationship between dietary and plasma content of 8 omega-3 fatty acids and the risk of Alzheimer's disease. One of these, docosahexaenoic acid (DHA), demonstrated such a relationship. The results of this study are published in the Archives of Neurology; here's a summary.

What was done

In the Framingham heart Study, volunteers are examined every two years. In 1986-1988 there were 1,921 participants free of dementia, of whom 899 provided the plasma samples that allowed them to form the subjects of this study. These subjects were followed for an average of 9.1 years.

A 126-item food frequency questionnaire was mailed out to determine the dietary intake of DHA and fish. Completed questionnaires were returned by 4,888 subjects.

The plasma samples were analyzed for apolipoprotein E genotype, total homocysteine level, and fatty acids - in particular the level of DHA. The participants were classified into 4 groups (quartiles) according to their plasma DHA levels.

The subjects were administered the Mini-Mental State Exam (MMSE) at each biannual examination. If there was a fall in score of 3 or more points from the previous exam, the subjects underwent full neurological and psychological work-up; the diagnosis of dementia followed careful assessment by a team of experts. Cases were classified into Alzheimer and non-Alzheimer dementia.

What was found

The average age of the 899 participants was 76, and 63% of them were women. Ninety-nine developed dementia during the follow-up period; 71 of these had Alzheimer's.

After adjusting for differences in age, gender, apolipoprotein E genotype, plasma homocysteine, and educational levels, it was found that subjects in the highest quartile of plasma DHA concentration, compared with those in the 3 lower quartiles, had half the risk of developing dementia; and they had only 2/3 the risk of developing Alzheimer's. (The relative risks were 0.53 and 0.61, respectively; the reduced risk for all-cause dementia was statistically significant.)

Similar analyses were done for the plasma levels of alpha-linolenic acid, eicosapentaenoic acid, linoleic acid, arachidonic acid, oleic acid, palmitic acid, and stearic acid. None showed a significant relationship with the risk of dementia.

The dietary analyses showed that average DHA and fish intake were significantly linked to plasma DHA levels in the different quartiles. Thus the highest plasma DHA quartile (and thus the lowest dementia subjects) ate the most fish and had the highest DHA intake. The main omega-3 fatty acids contained in fish are DHA and eicosapentaenoic acid, but, as stated, there was no evidence that plasma eicosapentaenoic acid levels were linked to reduced risk of dementia.

What these findings mean

This study shows quite convincingly that plasma docosahexaenoic acid (DHA) predicts the occurrence of dementia, including Alzheimer's disease, independent of age, sex, apolipoprotein E genotype, homocysteine, and education level; people who ate larger amounts of fish - an average of 3 servings a week - consumed about 0.18 grams/day of DHA and halved their risk of developing dementia over a 9-year period.

The role of fish (an integral part of the Mediterranean Diet) in prevention of dementia has been known for some time. What has been missing is the identity of the main component for this beneficial effect. Based on this study, it seems safe to conclude that the 'active agent' is, in fact, DHA. The level of this omega-3 fatty acid in the blood is derived from the conversion of alpha-linolenic acid to DHA in the liver, and from dietary DHA. The other omega-3 fatty acid contained in fish - eicosapentaenoic acid - plays no obvious role in preventing dementia. It's fair to recommend that people try to eat fish (preferably fatty fish) at least twice a week; the benefits are likely to be considerably greater than the possible risk of excess mercury intake.

Source

• Plasma phosphatidylcholine docosahexaenoic acid content and risk of dementia and Alzheimer disease. EJ. Schaefer , V. Bongard, Aa. Beiser,   et al., Arch Neurol , 2006, vol. 63, pp. 1545--1550

Related Links
Eating Fish Can Help Prevent Alzheimer's (1)
The Latest View on Omega-3 Fatty Acids
Omega-3 Intake and the Brain

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