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A growing body of evidence suggests that adherence to the Mediterranean diet (MD) may protect against cognitive decline and dementia. Many epidemiologic studies and several randomized controlled trials (RCTs) have found positive effects of the MD on cognitive function, but findings remain inconsistent. The aim of this systematic review was to provide an update on the current knowledge of the effects of the MD on cognitive function, cognitive impairment, Alzheimer disease (AD), and all-type dementia. Five databases were searched—PubMed, Embase, CINAHL, CENTRAL, and PsycINFO (1806 to 25 May 2015)—with the use of prespecified criteria. Human studies that were published in English without any restriction on study type, population assessed, intervention period, follow-up time, or publication date, and that examined the association between adherence to the MD and cognitive function or dementia symptoms (as measured by cognitive function tests), were included. Only primary publication types were included. Thirty-two studies from 25 unique cohorts, including 5 RCTs and 27 observational studies, met the inclusion criteria. The majority of studies showed that the MD was associated with improved cognitive function, a decreased risk of cognitive impairment or decreased risk of dementia, or AD. Three studies found no correlation between the MD and AD, 3 further studies found no association between the MD and cognitive impairment, and 5 studies found no association between the MD and cognitive function. There was large heterogeneity, and studies differed with regard to quality. Based on the findings and the limitations in study design, we conclude that adherence to the MD is associated with better cognitive performance. However, it should be noted that the majority of findings come from epidemiologic studies that provide evidence for a correlation between the MD and cognition but not for a cause-and-effect relation. More controlled trials are required to establish a causational relation.