Walnut intake, cognitive outcomes and risk factors: a systematic review and meta-analysis

Abstract

Background: Walnuts contain nutrients that are associated with improved cognitive health. To our knowledge, no review has systematically examined the effects of walnuts on cognitive function and risk for cognitive decline. Objective: To conduct a systematic review and meta-analysis evaluating the effects of walnut intake on cognition-related outcomes and risk-factors for cognitive decline in adults. Methods: Medline V R , Commonwealth Agricultural Bureau, and Cochrane Central Register of Controlled Trials were searched for randomized controlled trials (RCTs) and observational studies published until April 2020 on walnut intake, cognition (e.g. cognitive function, stroke, and mood), and selected risk factors for cognitive decline (e.g. glucose homeostasis and inflammation). Risk-of-bias and strength-of-evidence assessments were conducted using standard validated tools. Random-effects meta-analyses were conducted when !3 studies reported quantitative data for each outcome. Results: 32 RCT and 7 observational study publications were included. Meta-analysis of cognition-related outcomes could not be conducted due to heterogeneity of tests. None of the 5 cognition RCTs found significant effects of walnuts on overall cognition, although 3 studies found improvements on subdomains and/or subgroups. All 7 observational studies found significant associations and a dose-response relationship between walnut intake and cognition-related outcomes. Meta-analyses of 27 RCTs reporting glucose homeostasis and inflammation outcomes, selected risk factors for cognitive decline, did not show significant effects of walnut intake. Conclusions: Due to the non-uniformity of tests for cognition-related outcomes, definitive conclusions regarding the effect of walnut consumption on cognition could not be reached. Additionally, evidence does not show associations between walnut intake and glucose homeostasis or inflammation, cognitive decline risk-factors. High-quality studies with standardized measures are needed to clarify the role of walnuts in cognitive health. KEY MESSAGESThis is a systematic review and meta-analysis of 5 randomized clinical trials and 7 observational study articles of the impact of walnut intake on cognition decline and 27 randomized clinical trials of the effect of walnut intake on risk factors for cognitive decline including glucose homeostasis and inflammation. The non-uniformity of tests performed to measure cognitive function in the various studies did not allow for a meta-analysis of these studies. A definitive conclusion could therefore not be reached regarding the effect of walnut intake on cognitive decline. The evidence available does not show an association between walnut intake and glucose homeostasis or inflammation.

Walnut Intake Interventions Targeting Biomarkers of Metabolic Syndrome and Inflammation in Middle-Aged and Older Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

“…Neither SBP nor DBP was significantly modified by walnut-enhanced diets, confirming the results of previous analyses [ 53 , 55 , 60 ]. Furthermore, our study did not show statistically significant changes in terms of glycemic markers, which also corroborated prior studies [ 22 , 51 , 53 ]. After following walnut-enriched diets, the anthropometric parameters did not show significant differences compared to the control.…”

“…These findings concur with recently published data showing that the hs-CRP level was not influenced by walnut consumption [ 53 ]. Moreover, our findings agree with a recently published meta-analysis of both interventional and observational studies, which established that walnut intake had no statistical significance on glucose homeostasis and inflammation [ 22 ]. In contrast, observational studies found that nut consumption was inversely associated with inflammatory markers [ 63 ].…”

“…Contrary to the above results, a recent meta-analysis found no associations between walnut consumption and glucose homeostasis as well as inflammation [ 22 ]. Moreover, increasing dietary ALA intake did not affect inflammatory markers [ 23 ].…”

“…Considering the evidence from recent meta-analyses, nut consumption [ 48 , 49 , 50 ] and walnut-enriched diets [ 22 , 51 ] are negatively associated with specific biomarkers of MetS and inflammation in different age groups.…”

Recent Studies on Protective Effects of Walnuts against Neuroinflammation

“…A population-based prospective cohort study related to all types of nuts suggested that a higher long-term intake of walnuts was associated with the better cognitive performance of older women (mean age: 74 years) [ 16 ]. Furthermore, Cahoon et al conducted a systematic review and meta-analysis suggesting that walnut intake may have a beneficial effect on cognition-related outcomes, although with a low level of confidence [ 17 ]. The large 2-year Walnuts and Healthy Aging (WAHA) randomized controlled trial of walnuts for treatment of age-related cognitive decline found no major improvement in the whole cohort but cognitive benefit in the more at-risk participants [ 18 ].…”