Abstract
Nuts contain fibre, unsaturated fatty acids and polyphenols that may impact the composition of the gut microbiota and overall gut health. This study aimed to assess the impact of nuts on gut microbiota, gut function and gut symptoms via a systematic review and meta-analysis of randomised controlled trials (RCTs) in healthy adults. Eligible RCTs were identified by systematic searches of five electronic databases, hand searching of conference abstracts, clinical trials databases, back-searching reference lists and contact with key stakeholders. Eligible studies were RCTs administering tree nuts or peanuts in comparison to control, measuring any outcome related to faecal microbiota, function or symptoms. Two reviewers independently screened papers, performed data extraction and risk of bias assessment. Outcome data were synthesised as weighted mean difference (WMD) or standardised mean difference (SMD) using a random effects model. This review was registered on PROSPERO (CRD42019138169). Eight studies reporting nine RCTs were included, investigating almonds (n = 5), walnuts (n = 3) and pistachios (n = 1). Nut consumption significantly increased Clostridium (SMD: 0.40; 95% CI, 0.10, 0.71; p = 0.01), Dialister (SMD: 0.44; 95% CI, 0.13, 0.75; p = 0.005), Lachnospira (SMD: 0.33; 95% CI, 0.02, 0.64; p = 0.03) and Roseburia (SMD: 0.36; 95% CI, 0.10, 0.62; p = 0.006), and significantly decreased Parabacteroides (SMD: −0.31; 95% CI, −0.62, −0.00; p = 0.05). There was no effect of nuts on bacterial phyla, diversity or stool output. Further parallel design RCTs, powered to detect changes in faecal microbiota and incorporating functional and clinical outcomes, are needed.
Highlights
Nuts have well-documented benefits for human health, with recent systematic reviews highlighting their benefits for cardiovascular health [1,2] and glycaemic control [3]
From birth and throughout the life cycle, the gut microbiota perform roles vital to host health, ranging from education of the immune system, energy harvest from foods that are otherwise indigestible to humans and the production of short-chain fatty acids (SCFAs), the main energy source of intestinal epithelial cells [4]
Alterations in the composition of the gut microbiota are a feature of functional bowel disorders such as irritable bowel syndrome (IBS) [5] and constipation [6]
Summary
Nuts have well-documented benefits for human health, with recent systematic reviews highlighting their benefits for cardiovascular health [1,2] and glycaemic control [3]. Nuts represent a valuable dietary intervention for targeting cardiometabolic health in the general population. Less is known about the impact of nuts on gastrointestinal health and the gut microbiota. Gastrointestinal health is strongly influenced by the composition of the gut microbiota. From birth and throughout the life cycle, the gut microbiota perform roles vital to host health, ranging from education of the immune system, energy harvest from foods that are otherwise indigestible to humans and the production of short-chain fatty acids (SCFAs), the main energy source of intestinal epithelial cells [4]. Alterations in the composition of the gut microbiota are a feature of functional bowel disorders such as irritable bowel syndrome (IBS) [5] and constipation [6]. Dietary interventions that target the gut microbiota have valuable implications for the maintenance of gastrointestinal health in the general population
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