Strict vegetarian diet improves the risk factors associated with metabolic diseases by modulating gut microbiota and reducing intestinal inflammation

<jats:title>Summary</jats:title> <jats:p> Low‐grade inflammation of the intestine results in metabolic dysfunction, in which dysbiosis of the gut microbiota is intimately involved. Dietary fibre induces prebiotic effects that may restore imbalances in the gut microbiota; however, no clinical trials have been reported in patients with metabolic diseases. Here, six obese subjects with type 2 diabetes and/or hypertension were assigned to a strict vegetarian diet ( <jats:styled-content style="fixed-case">SVD</jats:styled-content> ) for 1 month, and blood biomarkers of glucose and lipid metabolisms, faecal microbiota using 454‐pyrosequencing of 16S ribosomal <jats:styled-content style="fixed-case">RNA</jats:styled-content> genes, faecal lipocalin‐2 and short‐chain fatty acids were monitored. An <jats:styled-content style="fixed-case">SVD</jats:styled-content> reduced body weight and the concentrations of triglycerides, total cholesterol, low‐density lipoprotein cholesterol and haemoglobin A1c, and improved fasting glucose and postprandial glucose levels. An <jats:styled-content style="fixed-case">SVD</jats:styled-content> reduced the <jats:italic> <jats:styled-content style="fixed-case">F</jats:styled-content> irmicutes </jats:italic> ‐to‐ <jats:italic> <jats:styled-content style="fixed-case">B</jats:styled-content> acteroidetes </jats:italic> ratio in the gut microbiota, but did not alter enterotypes. An <jats:styled-content style="fixed-case">SVD</jats:styled-content> led to a decrease in the pathobionts such as the <jats:italic>Enterobacteriaceae</jats:italic> and an increase in commensal microbes such as <jats:italic> <jats:styled-content style="fixed-case">B</jats:styled-content> acteroides fragilis </jats:italic> and <jats:italic> <jats:styled-content style="fixed-case">C</jats:styled-content> lostridium </jats:italic> species belonging to clusters <jats:styled-content style="fixed-case">XIVa</jats:styled-content> and <jats:styled-content style="fixed-case">IV</jats:styled-content> , resulting in reduced intestinal lipocalin‐2 and short‐chain fatty acids levels. This study underscores the benefits of dietary fibre for improving the risk factors of metabolic diseases and shows that increased fibre intake reduces gut inflammation by changing the gut microbiota. </jats:p>