Eating Fish Can Help Prevent Alzheimer's (1)

Summarized by Robert W. Griffith, MD
December 4, 2006

Summary

A Mediterranean-type diet reduces the risk of developing Alzheimer's disease by about 50%. This is a separate effect from the known benefits of the diet on cardiovascular conditions.

Introduction

The Mediterranean Diet (MD) has been credited with lower risks for several forms of cancer, obesity, high cholesterol levels, high blood pressure, high blood sugar, coronary heart disease, and overall mortality. Most recently the diet has been linked with a lower risk of developing Alzheimer's disease. All this is quite a burden for a diet to bear, and obviously calls for efforts to discover the 'secret ingredients' that bring the most benefits. Two recent studies published in the Archives of Neurology have addressed possible ways in which the MD can help prevent Alzheimer's. We summarize the first one here; the second will be the subject of a subsequent article on this site.

People living in New York City were studied by researchers from the Taub Institute for Research on Alzheimer's Disease and the Aging Brain. Dietary patterns were analyzed and associations sought with Alzheimer's, once the possible role of vascular conditions (such as stroke, diabetes, high blood pressure and so on) had been discounted. The intent was to determine if vascular disease(s) represented a link between diet and Alzheimer's. This might allow interruption of the process by non-dietary means; e.g. antihypertensive medication.

What was done

Two groups of volunteers living in Manhattan were recruited in 1992 and 1999. At baseline a full medical work-up was done, along with a Clinical Dementia Rating (CDR) test, to assess the presence of dementia. Apolipoprotein E genotype was determined for each volunteer.

A 61-item food frequency questionnaire was administered by a trained interviewer. A MD score was constructed using the daily gram intake of beneficial foods in 7 categories: dairy, meat, fruits, vegetables, legumes, cereals, and fish; fat and alcohol intake were built into the final MD score. The score ranged theoretically from 0 to 9, with a higher score indicating higher adherence to a Mediterranean-type diet. The subjects were classified into three groups (tertiles), according to their MD scores.

Information was gathered on race, level of education, total calorie intake, smoking, body mass index (BMI), and comorbidities (stroke, diabetes, high blood pressure, heart disease, high lipid levels).

What was found

The original sample size was over 4100 people, but roughly half were eliminated from the analysis: those who had no dementia at baseline but developed it during the study, uncertainty about the CDR score, missing dietary information, and non-calculation of the MD score. This left 1984 volunteers for analysis. Their average age was 76, and 68% of them were women.

There were 194 subjects with Alzheimer's in the final group analyzed. Compared with non-demented volunteers, they were older, less well educated, and had lower BMIs. There were fewer men and more Hispanics in the subjects with Alzheimer's, and they had a higher incidence of stroke.

As expected, higher adherence to the MD was associated with a reduced risk for Alzheimer's disease. The average MD score for non-demented subjects was 4.4, and for Alzheimer's subjects it was 3.8; this difference was statistically significant i.e. it could not have occurred by chance alone. Those subjects in the highest tertile (the one-third of volunteers with a high MD score) had 32% of the chance of getting Alzheimer's as those in the lowest (worst) MD tertile; the middle tertile group were 47% as likely to develop Alzheimer's.

The above risks (odds ratios) were calculated after adjustment for age, sex, race, education, apolipoprotein E genotype, calorie intake, smoking and BMI. In another calculation, adjustments were made for co-existing vascular conditions: history of stroke, diabetes, high blood pressure, heart disease, and lipid abnormalities. When these adjustments were made, the risk levels were virtually unchanged: 31% for the high tertile, and 48% for the middle tertile volunteers.

What these findings mean

The main finding - a clear relationship between a high MD intake and a decreased risk of Alzheimer's - was a confirmation of previous studies. More importantly, the analyses in which adjustments were made for vascular conditions showed no differences from those for the whole population. This clearly demonstrates that vascular disease is not the chief factor in the causation of Alzheimer's disease, as some have thought.

It's possible that the measurement of vascular conditions was not exact enough to have picked up an important role in the development of Alzheimer's. For instance, measurement of atherosclerotic plaque in the carotid arteries or coronary artery calcification might have provided different results; however, this seems unlikely.

One must look for other reasons for the beneficial effects of a Mediterranean diet. These can be sought either by analyses of specific components of the diet, or by looking at 'markers' in the body for the action of different components (e.g. a specific omega-3 fatty acid in the plasma). This latter approach will be the subject of the next article in this series. In the meantime, eat more fish!

Source

• Scarmeas N, Stern Y, Mayeux R, Luchsinger JA. Mediterranean diet, Alzheimer disease, and vascular mediation. Arch Neurol 2006; http://archneur.ama-assn.org

Related Links
Drink Fruit or Vegetable Juices and Avoid Alzheimer's
Physical Function and Alzheimer's - Cause or Effect?
Deal with Risk Factors for Alzheimer's, Now

Please take a moment to give us your comments. For questions about Health matters you may check our "Questions & Answers" Portal and Service.