To the Editor: In their cohort study, Dr Scarmeas and colleagues1 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,2 especially in individuals who may already have memory impairment, and objective measures of physical activity, such as accelerometers, are likely more accurate.