© The Authors 2020. There is increasing evidence linking the gut microbiota to various aspects of human health. Nuts are a food rich in prebiotic fibre and polyphenols, food components which have been shown to have beneficial effects on the gut microbiota. This systematic review aimed to synthesise the evidence regarding the effect of nut consumption on the human gut microbiota. A systematic search of the databases MEDLINE, PubMed, Cochrane CENTRAL, and CINAHL was performed until November 28, 2019. Eligible studies were those that investigated the effects of nut consumption in humans (aged over three years old), utilising next-generation sequencing technology. Primary outcome measures were between-group differences in α- and β-diversity metrics and gut microbial composition. A total of eight studies were included in the review. Included studies assessed the effects of either almonds, walnuts, hazelnuts or pistachios on the gut microbiota. Overall, nut consumption had a modest impact on gut microbiota diversity, with two studies reporting a significant shift in a α-diversity and four reporting a significant shift in β-diversity. Walnuts, in particular, appeared to more frequently explain shifts in β-diversity, which may be a result of their unique nutritional composition. Some shifts in bacterial composition (including an increase in genera capable of producing short-chain fatty acids: Clostridium, Roseburia, Lachnospira and Dialister) were reported following the consumption of nuts. Nut intake may yield a modulatory effect on the gut microbiota, however results were inconsistent across studies, which may be explained by variations in trial design, methodological limitations and inter-individual microbiota.
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