Diet, Exercise Habits, and Risk of Alzheimer Disease—Reply

To the Editor: In their cohort study, Dr Scarmeas and colleagues found an association of self-reported adherence to a Mediterranean-type diet and high levels of physical activity with a reduced risk of developing Alzheimer disease in a population of elderly residents of New York. However, we have several concerns about the study. First, self-selection represents a potentially important source of bias, since individuals who adopt healthy lifestyle habits may be different in various ways from those who do not engage in such habits. Second, self-report measures of diet and exercise can be inaccurate. The accuracy of the Food Frequency Questionnaire is controversial, especially in individuals who may already have memory impairment, and objective measures of physical activity, such as accelerometers, are likely more accurate. Third, the study assessed only one of many possible diets. The DASH (Dietary Approaches to Stop Hypertension) diet is similar but emphasizes greater consumption of low-fat dairy products and carbohydrates and is lower in fat and cholesterol. Moreover, its efficacy in reducing blood pressure has been demonstrated in all segments of the population, including African Americans, a group disproportionately affected by vascular disease and Alzheimer disease. We agree with the authors that randomized controlled trials (RCTs) are needed to establish a causal relationship between dietary and exercise habits and risk of Alzheimer disease. While numerous observational studies have shown that physically active individuals perform better on neurocognitive tests compared with their less active counterparts, results of RCTs of exercise have been inconsistent. There also have been many observational studies of diet and neurocognition, but most RCTs have focused on dietary supplementation, with generally negative or inconsistent results. Although the observations of Scarmeas et al are encouraging and potentially important, establishing the value of dietary and activity recommendations should await results from future RCTs.