Whole-diet approach, Mediterranean diet, and Alzheimer disease.

V ery recently, Scarmeas and colleagues reported the results of a community-based study involving 2258 nondemented individuals in New York in which adherence to a traditional Mediterranean diet (MeDi) was associated with significant reduction in risk for incident Alzheimer disease. Scarmeas and colleagues used in this report a scale indicating the degree of adherence to the traditional MeDi: a value of 0 or 1 was assigned to each of 9 indicated components with the use of the sex-specific median as the cut-off. However, in the study of Scarmeas and colleagues, the ratio of the median daily intake of monounsaturated fatty acids to saturated fatty acids (one of the hallmarks of the MeDi) for individual food categories by MeDi score tertiles was 1 and overall about 2.5 times lower than the same value calculated from other studies on MeDi. In the last years, the study approach was to associate single micronutrients or macronutrients to age-related cognitive decline, mild cognitive impairment, Alzheimer disease, or vascular dementia. Findings from the Italian Longitudinal Study on Aging demonstrated that in a 8.5-year follow-up, high monounsaturated fatty acid, polyunsaturated fatty acid, and total energy intake was significantly associated with a better cognitive performance in time. Furthermore, in the same sample, high intake of polyunsaturated fatty acids appeared to have a borderline nonsignificant trend for a protective effect against the development of mild cognitive impairment. We read with great interest the recent article by Scarmeas and colleagues, which in a case-control study nested within a community-based cohort in New York (ie, the same sample of the previous report) found that higher adherence to the MeDi was associated with lower risk for prevalent Alzheimer disease; the introduction of vascular variables (stroke, diabetes mellitus, hypertension, heart disease, lipid levels) in the model did not change the magnitude of the association. The wide diffusion of this methodological approach has undeniable advantages, but some concerns exist. In fact, several devices related to the use of dietary composite score should be considered in evaluating the effects of nutrient intakes in different groups (low and high consumption) in unbiased manner. In particular, recent findings from the sample from the Italian Longitudinal Study on Aging suggested that a variation in group heterogeneity, which could be caused by selection of cut-offs, can change in predictable ways the reliability and validity of the scale indicating the degree of adherence to a particular dietary pattern. The effects of change in observed-score variance on reliability can be estimated if it is assumed that error variance remains constant after selection. On the basis of these considerations, it is not difficult to guess that a “wholediet” approach such as what the MeDi score measures could not be readily transferred to other populations, including a study population who was traditionally eating according to MeDi pattern. If the findings of Scarmeas and colleagues were successfully replicated, randomized clinical trials should ideally follow to specifically address the question of whether a dietary pattern may have a role in the prevention of cognitive impairment.