Background
The goal of this systematic review was to determine whether dietary patterns are associated with risk of cognitive impairment, dementia and Alzheimer’s disease. Dietary patterns were defined as the quantities, proportions, variety or combination of different foods, drinks and nutrients in diets, and the frequency with which they are habitually consumed.
Conclusion statement
Limited evidence suggests that a dietary pattern containing an array of vegetables, fruits, nuts, legumes and seafood consumed during adulthood is associated with lower risk of age-related cognitive impairment, dementia and Alzheimer’s disease. Although the number of studies available on dietary patterns and neurodegenerative disease risk is expanding, this body of evidence, which is made up of high-quality observational studies, has appeared only in recent years and is rapidly developing. It employs a wide range of methodology in study design, definition and measurement ascertainment of cognitive outcomes and dietary pattern assessment.
Limited
Methods
Literature searches were conducted using PubMed, EMBASE, Navigator (BIOSIS, CAB Abstracts and Food Science and Technology Abstracts) and Cochrane databases to identify studies that evaluated the association between dietary patterns and risk of cognitive impairment, dementia and Alzheimer’s disease. Studies that met the following criteria were included in the review: randomized controlled trials (RCTs), non-randomized controlled trials or prospective cohort studies; human subjects aged two years and older who were healthy or at elevated chronic disease risk; subjects from countries with high or very high human development (2012 Human Development Index); and published in English in peer-reviewed journals. The date range was January 1980 to January 2014. The intervention or exposure was adherence to a dietary pattern (e.g., a priori patterns, data-driven patterns, reduced rank regression (RRR) or patterns derived from other methods, and a description of the dietary pattern(s) (i.e., foods and beverages) consumed by subjects was provided. The outcome was incidence of cognitive impairment, dementia and Alzheimer’s disease.
Data from each included article were extracted and risk of bias was assessed. The evidence was qualitatively synthesized, a conclusion statement was developed, and the strength of the evidence (grade) was assessed using pre-established criteria including evaluation of the quality and risk of bias, quantity, consistency, magnitude of effect and generalizability of available evidence.
Findings
- This systematic review includes 30 articles (two articles analyzed data taken from RCTs Systematic Reviews of the Dietary Patterns, Food and Nutrients Subcommittee, 2015 DGAC 126 Archived from www.NEL.gov on March 21, 2017 and 28 articles used data from prospective cohort studies published since 1980 (with all but two published since 2008) that examined the relationship between dietary patterns and age-related cognitive impairment, dementia and Alzheimer’s disease
- Twenty of the articles included in this review assessed the relationship between dietary patterns and cognitive impairment, 10 articles examined cognitive impairment or dementia and eight articles focused on Alzheimer’s disease
- The articles used several different methods to assess dietary patterns. Two articles analyzed data from RCTs that tested or described dietary patterns, 23 articles used indices and scores to assess dietary patterns quality or adherence, three articles used data-driven methods and three used RRR.
- Most (18 of 28) articles found an association between dietary patterns and age-related cognitive impairment, dementia and Alzheimer’s disease. Despite some heterogeneity in this body of evidence, some common elements of dietary patterns were associated with measures of cognitive impairment, dementia and Alzheimer’s disease:
- Patterns higher in vegetables, fruits, nuts, legumes and seafood were generally associated with reduced risk of age-related cognitive impairment, dementia and Alzheimer’s disease
- Patterns higher in red and processed meats were generally associated with greater age-related cognitive impairment. Relatively few studies reported on refined sugar and added salt, and patterns including these nutrients tended to report greater cognitive impairment.
Limitations
The ability to draw strong conclusions was limited by the following issues:
- Although some studies included participants from a range of race/ethnic and socioeconomic groups, the results are most applicable to the general healthy, aging population
- Dietary patterns were derived using dietary intake measured at baseline only, and therefore may not reflect patterns consumed throughout relevant periods of life before enrollment in the study or changes in intake that may have occurred over the duration of the study
- Several studies measured cognitive function only at a single time point (follow up), and therefore could not assess change in cognitive function over time
- Though these studies controlled for a number of confounders, not all apparently relevant potential confounders were adjusted for (e.g., existing or family history of cognitive decline, dementia or Alzheimer’s disease; baseline health status; changes in dietary intake over time) and, as with all association studies, residual confounding is possible.