Commentary: Mediterranean Diet and Cognitive Function: Are We Approaching Clarity in This Area?
Whitmer, Rachel A.; Kushi, Lawrence H.
From the Kaiser Permanente Division of Research, Oakland, CA.
Editors’ note: Related articles appear on pages 479, 490, and 503.
Correspondence: Rachel A. Whitmer, Kaiser Permanente Division of Research, 2000 Broadway, Oakland, CA 94612. E-mail: firstname.lastname@example.org.
Ancel Keys and his colleagues first brought attention to the potential health benefits of the Mediterranean diet through their Seven Countries study.1 Data from Greece, Serbia, and Italy clearly showed lower coronary heart disease rates among Mediterranean populations (as well as among the Japanese), compared with those from northern Europe or the United States. Interest in the role of the Mediterranean diet in chronic disease prevention was further spurred by the expansion of epidemiologic studies to consider not just individual nutrients but also dietary patterns.2 The Mediterranean diet has captured the interest of scientists and lay people alike.1,3 Consumption of olive oil has nearly doubled worldwide in the two decades since 1990; in the United States, consumption increased from 85,000 tons in 1991 to 294,000 tons in 2011—about two pounds per capita per year.4
The benefit of a Mediterranean diet on primary prevention of coronary artery disease was underscored recently in a randomized controlled trial in Spain.5 This study complements an earlier trial demonstrating a similar impact on secondary prevention.6 Because many of the proposed biological pathways relating the Mediterranean diet to prevention of atherosclerotic diseases apply also to cognitive impairment and dementia (blood flow restricted by atherosclerosis, effects of antioxidants, fatty acid composition), possible benefits for cognition have become another area of growing interest.
The systematic review by Lourida et al,7 published in the current issue of EPIDEMIOLOGY, identified 12 papers from prospective or cross-sectional studies on Mediterranean diet and its association with cognitive function, mild cognitive impairment, or Alzheimer’s disease. Longitudinal studies of cognitive function generally showed that greater adherence to a Mediterranean diet was associated with better function. This was not universal; for example, in the PATH Through Life Study, greater adherence to a Mediterranean diet pattern was associated with less cognitive decline at 4 years8 but greater cognitive decline at 8 years.9
Studies of the Mediterranean diet and Alzheimer’s disease also demonstrated a benefit. However, four of these papers were from the Washington Heights-Inwood Columbia Aging Project (WHICAP), all indicating decreased risk of Alzheimer’s disease with Mediterranean-style dietary patterns.10–13 Lourida et al report only two other smaller studies that examined dementia as the outcome,14,15 with limited power to detect associations.
Lourida et al also identified three longitudinal studies that examined mild cognitive impairment as an outcome—the PATH Through Life Study, suggesting increased risk of impairment with greater Mediterranean diet adherence,7 the Mayo Clinic Study on Aging, suggesting decreased risk,15 and the WHICAP, with decreased risk.12 It may be fair to say that the perception that Mediterranean diet patterns are beneficial for cognitive function in the elderly rests principally with the WHICAP findings.
In this context of this limited literature, studies such as that of Samieri et al16 (also appearing in this issue) are welcome additions. Samieri et al report data from a large sample—6174 participants in the Women’s Health Study. Unfortunately, the findings are mixed. Samieri et al report essentially no association of Mediterranean diet scores with cognitive function itself (the average of up to three assessments of several instruments) or with changes in cognitive function. Although there were a few associations with individual components of the Mediterranean diet score, these results do not support the more general idea that the Mediterranean dietary pattern is associated with better cognitive function.
However, Samieri et al’s report from the Women’s Health Study is not the only recent addition to this literature. Two other prospective cohort studies have just been published; results of all three studies are summarized in the Table. One of these, also led by Samieri et al,17 was based on data from the ongoing Nurses’ Health Study. With over 14,000 women, this is the largest cohort study to date to examine these associations. Using up to four repeated telephone-based cognitive assessments over a 6-year period, Samieri et al17 examined both the mean cognitive function scores averaged over the four assessments, and the change in these scores over time. Greater adherence to Mediterranean dietary patterns was associated with higher mean cognitive function scores, but there were no associations with change in scores. This may partly be a function of measurement error, in which a single set of assessments may provide an inexact estimate of relative cognitive function, whereas the average of up to four such assessments may better categorize individuals. Examining change across those assessments does not diminish measurement error and may compound it. The other prospective study reporting Mediterranean dietary patterns and cognitive function was the SU.VI.MAX Study of 3083 men and women.18 There was only the suggestion of an association between lower adherence to Mediterranean diet and lower cognitive function scores. These three prospective studies add substantially to this literature. Although none contradicts the possibility of a potential benefit of Mediterranean dietary patterns on cognitive function, only the Nurses’ Health Study clearly supports it. In addition, recent cross-sectional studies have reported associations of Mediterranean dietary patterns with cognitive function.19–21 These studies add to the growing literature indicating that those who consume diets that align with Mediterranean dietary patterns also perform better on cognitive function tests.
TABLE. Recent Prospe...Image Tools
Despite the generally null findings of Samieri et al,16 there is increasing clarity that Mediterranean dietary patterns may indeed confer protection against cognitive decline. It is unclear whether it is the overall dietary pattern that is important, or whether select aspects of the diet, or other lifestyle factors that may “travel” with Mediterranean diets, or perhaps the whole package is most important. These studies do suggest, however, that the pleasures of Mediterranean cuisine can have salutary health effects—and that, with delayed cognitive decline and decreased risk of Alzheimer’s disease, we may be able to savor and enjoy these pleasures well into our older ages.
ABOUT THE AUTHORS
RACHEL A. WHITMER is an epidemiologist at the Kaiser Permanente Division of Research (DOR) with a focus on risk and protective factors for cognitive aging and dementia. Her work has focused on cardiometabolic influences over the lifecourse as well as comparative effectiveness studies of Alzheimer’s disease. LAWRENCE H. KUSHI is a nutritional epidemiologist at the DOR with primary research interests in cancer. He has longstanding interests in cultural traditions as models for healthful eating, such as those of East Asia and the Mediterranean region.
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