In a recent study published in the Nutrition Reviews Journal, researchers systematically reviewed the evidence on the effects of dietary patterns, nutritional supplements, and foods on cognitive decline in people with mild cognitive impairment (MCI).
Study: Dietary pattern, food, and nutritional supplement effects on cognitive outcomes in mild cognitive impairment: a systematic review of previous reviews. Image Credit: BONDARTPHOTOGRAPHY/Shutterstock.com
Cognition decreases naturally with age, but MCI may be diagnosed when significant memory loss occurs more than normal aging.
Increasing evidence indicates the protective contribution of nutritional interventions for healthy brain aging and preventing cognitive decline in older individuals. Thus, targeting dietary approaches may provide a cost-effective and viable means to prevent/slow neurodegeneration.
About the study
In the present study, researchers examined the available evidence from relevant reviews on the effects of dietary patterns, supplements, and foods on cognitive function in MCI subjects.
Systematic reviews/meta-analyses of randomized controlled trials or cohort studies were eligible for inclusion. Reviews were excluded if included populations had other conditions associated with cognitive impairment.
A literature search was performed in several databases for articles published after 1990, using broad keywords and medical subject headings for intervention, outcomes, and the type of study.
Titles and abstracts were screened, and full texts of relevant studies were reviewed. Data on setting and context, objectives, quality assessment tools, and analysis to synthesize evidence, among others, were extracted from studies.
The overlap of primary studies was assessed/managed to minimize duplication bias and over-estimation of outcome effects. All non-overlapping systematic reviews were included; the most comprehensive review was selected if the overlap was identified.
Methodological quality was assessed using a measurement tool to assess systematic reviews (AMSTAR) 2. Data were qualitatively synthesized to summarize evidence based on the nutritional intervention/exposure type.
The literature search retrieved 6,677 hits; 46 systematic reviews met the inclusion criteria, and 20 were included in the analysis.
These reviews reported on 43 primary studies with subjects diagnosed with MCI at baseline and were published between 2008 and 2021. The primary studies, published from 2005 to 2020, in the reviews had sample sizes ranging between four and 1,875 individuals.
The reviews reported the effectiveness of 18 nutritional interventions on cognitive measures for MCI patients. Interventions were a low-carbohydrate diet, a Mediterranean diet, food-derived supplements, and individual foods, nutrients, and phytonutrients.
Cognitive outcomes were the rate of MCI-to-dementia progression and changes in global cognition and cognitive sub-domains, such as memory, attention, language, executive function, processing speed, and visuospatial skills.
Tools/tests were Alzheimer’s disease (AD) assessment scale-cognitive scale (ADAS-Cog) scores, mini-mental state examination (MMSE) scores, specific cognitive-domain tests, and different neuropsychological test batteries.
Eight reviews reported adverse effects, three reported mild/moderate gastrointestinal disturbances with the intake of B vitamins, medium-chain triglyceride (MCT), and omega-3 polyunsaturated fatty acid (PUFA), and one reported no serious adverse events. Thirteen reviews were of critically low quality, four were of low quality, two were of moderate quality, and only one was a high-quality review.
Seven studies addressed Mediterranean diet adherence on AD risk in MCI individuals. Individuals in the mid-tertile of Mediterranean diet adherence had a 45% lower AD risk, while those in the highest tertile had a 48% reduced risk relative to those in the lowest tertile.
Three studies examined the effect of a low carbohydrate diet on cognition, which observed a significant positive result on verbal memory performance.
Three reviews included primary studies on MCTs. Of these, one addressed the effect of MCT supplementation on cognition and observed a positive impact on language after a six-month intervention of 30g/day of MCT.
Twelve reviews examined B vitamin interventions, of which six were deemed the most comprehensive. Five primary studies across four reviews revealed improved cognitive sub-domains with B vitamin supplementation. One review, which performed a subgroup meta-analysis of five studies using different folic acid formulations, revealed improved global cognition but had substantial heterogeneity.
Nonetheless, one review found no effect of supplementing B vitamins. One study found a significant effect of vitamin D3 supplementation on intelligence quotient (IQ) tests. Two reviews addressed the efficacy of Brazilian nuts as a selenium source on cognition. Of these, one found a protective effect of nut intake on verbal fluency.
Two reviews demonstrated that Souvenaid consumption for two years improved clinical dementia rating-sum of boxes. However, the intervention did not affect global cognition, dementia incidence, memory, or executive function. Four reviews examined the effect of probiotics on cognition. Pooled analyses of the four studies showed an overall positive impact on cognitive function.
The authors found limited evidence to suggest that nutritional interventions improve/maintain cognitive function in MCI subjects.
Notably, included reviews were generally of low quality, particularly in critical domains. Overall, the findings suggest that the Mediterranean diet, omega-3 fatty acids, B vitamins, and Souvenaid had protective effects on cognition in MCI patients.
More research is needed for tailored nutritional guidelines for MCI subjects.