Cognitive Health and Impairment Systematic Literature Review
Cognitive Health and Impairment
Reference: Petersson, S.D. and E. Philippou, Mediterranean Diet, Cognitive Function, and Dementia: A Systematic Review of the Evidence. Adv Nutr, 2016. 7(5): p. 889-904.
One-sentence summary: A Mediterranean diet reduced the risk of cognitive impairment and dementia, and was associated with improved cognitive functioning.
Study type: A systematic literature review of 5 RCTs and 27 observational studies.
Diet: Adherence to the Mediterranean diet or a Mediterranean diet intervention. Adherence to a Mediterranean diet was defined through scores that estimated the conformity of the dietary pattern of the studies’ population with the Mediterranean dietary pattern.
Outcomes measured: Cognitive function or cognitive decline (e.g. dementia).
Population: Most of the studies (n = 26) included participants aged >60 years, and nineteen of the 32 studies included only women. Studies were from the United States (16 studies), Europe (11 studies), Australia (4 studies) and China (1 study).
- Dementia: 6 of 9 studies (2 of 2 cross-sectional and 4 of 7 longitudinal studies found a protective effect, and the other 3 studies did not find a significant association.
- Cognitive impairment: 7 of 10 studies (3 of 4 cross-sectional, 3 of 5 longitudinal and 1 of 1 RCT) found the Mediterranean diet to be protective, and the other 3 studies did not find a significant association.
- Cognitive function (e.g. global cognition and verbal ability): 18 of 23 studies (2 of 3 cross-sectional, 12 of 15 longitudinal and 4 of 5 RCTs) found a significant association for at least one measure, whilst the remaining 5 studies found no significant associations.
Quality assessment of studies was undertaken using custom checklists derived from existing widely used questionnaires that included items related to population, recruitment, assessment of exposure and outcome, confounding, and statistical analyses. The overall quality of the studies was rated as average.
- Different cognitive function tests were used between studies, making the findings difficult to compare.
- Most studies were observational and did not adjust for important confounders.
- Given that many participants showed signs of memory impairment, the retrospective studies and the use of a Food Frequency questionnaire could give a distorted account of foods eaten.
- It is unclear whether the Mediterranean diet is protective as a whole or through the action of its individual components.
- The same adherence score to a Mediterranean diet can mean a high and low consumption of different foods.
- Nine studies included participants <65 years of age, which could make cognitive decline harder to detect.
- The type of tool used to measure adherence to a Mediterranean diet varied between studies, and 7 studies modified the Mediterranean diet score, which diverges from the traditional Mediterranean diet.
The bottom line:
A Mediterranean diet may improve cognitive performance and be protective for cognitive impairment and dementia; although the majority of the evidence is observational without adjusting for confounders. More RCTs and large epidemiological studies adjusted for confounders are needed in order to strengthen the empirical evidence for the role of the Mediterranean Diet in cognitive function, as well as the roles of the individual dietary components.
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