Diet's Relationship to Brain Health Provides Much Food for Thought

New research evidence supports a beneficial effect of a Mediterranean type of diet on preventing cognitive decline and Alzheimer's disease in older adults, according to two studies published in the August 12 Journal of the American Medical Association.

Dubbed “the Mediterranean diet,” this way of eating by people from different cultures near and around the Mediterranean Sea is characterized by a high consumption of fruits, vegetables, legumes, nuts, and fish; a low intake of meats and poultry; the use of olive oil as the main source of fat; and a low-to-moderate intake of wine. The diet has long been linked to a variety of long-term health benefits, such as reduced risks of cardiovascular diseases and cancer, in epidemiological studies.

The two latest studies replicated previous observations from a study published in the June 2006 Annals of Neurology by Nikolaos Scarmeas, M.D., an assistant professor of neurology at Columbia University College of Physicians and Surgeons, and colleagues. Taken together, the studies suggested that sticking to these dietary habits for years was associated with a lower incidence of cognitive impairment and Alzheimer's disease in older adults.

One of the current studies, which was conducted in New York between 1992 and 2006, was a follow-up study by Scarmeas's group. During an average of 5.4 years, 282 of 1,880 community-dwelling adults in the study cohort were diagnosed as having newly developed Alzheimer's disease. The mean age of the entire cohort was 77 years at baseline. All participants were asked about their lifestyle habits and tested for neurological functions every 1.5 years or so. Their adherence to the Mediterranean way of eating and level of physical activity were categorized into high, middle, and low levels.

After controlling for age, level of education, and other confounding factors, the authors found that people with high adherence to a Mediterranean type of diet had significantly less risk of developing Alzheimer's than people with low adherence to such diets. Analyzed separately, a high level of physical activity was associated with significantly reduced incidence of Alzheimer's disease compared with a low level of physical activity. The risk difference of disease incidence between middle and low levels of either indicator did not reach statistical significance.

The other study was carried out by Catherine Féart, Ph.D., from Université Victor Segalen Bordeaux 2, France, and colleagues, who followed a cohort of 1,410 older adults in Bordeaux since 2001-2002. The participants had a mean age of 76 at the start of the study, and the median follow-up period was about four years. A higher level of adherence to traditional Mediterranean types of food sources, scored on a scale of 0 to 9, was associated with significantly slower cognitive decline from baseline on the Mini-Mental State Examination, but was not significantly associated with changes in performance on three other cognitive tests for verbal skills and short-term visual and verbal memories.

Different from the New York study, the French study did not find a significant association between adherence to Mediterranean-style diet and the incidence of dementia, but the authors acknowledged that the sample size had not been powered to detect such an association.

Before concluding that the way of eating can prevent Alzheimer's disease, David Knopman, M.D., a professor of neurology at the Mayo Clinic, cautioned in an accompanying editorial that this evidence is “moderately compelling” and not specific enough to explain the mechanisms of the diet's protective effects. He pointed out that the cerebrovascular changes associated with Alzheimer's and other types of dementia have their roots in midlife, which may be modified by the Mediterranean diet as it modifies other cardio- and cerebrovascular disorders. To prevent late-life cognitive impairment, people in their midlife should adopt “as many healthy behaviors as possible, including diet,” he recommended.

An abstract of “Adherence to a Mediterranean Diet, Cognitive Decline, and Risk of Dementia” is posted at<jama.ama-assn.org/cgi/content/abstract/302/6/638>. An abstract of “Physical Activity, Diet, and Risk of Alzheimer Disease” is posted at<jama.ama-assn.org/cgi/content/abstract/302/6/627>.▪