A 16-week vegan diet can boost the gut microbes that are related to improvements in body weight, body composition and blood glucose control, suggests a short-term study in the US.

Researchers noted that gut microbiota play an important role in weight regulation, the development of metabolic syndrome, and type 2 diabetes.

“Eating more fibre is the number one dietary recommendation for a healthy gut microbiome” Study authors

They aimed to test the effect of a 16-week plant-based diet on gut microbiota composition, body weight, body composition, and insulin resistance in overweight adults with no history of diabetes.

The study included 147 participants – 86% were women and 14% men – with a mean age of 55, who were randomised to follow a low-fat vegan diet or to make no changes to their diet for 16 weeks.

Gut microbiota composition was assessed, using uBiome kits, at the start and end of the trial period.

Meanwhile, dual energy X-ray absorptiometry was used to measure body composition. A standard method called the PREDIM index was used to assess insulin sensitivity.

Following the study, body weight was found to have reduced significantly in the vegan group – m by an average of -5.8 kg – particularly due to a reduction in fat mass and in visceral fat.

Insulin sensitivity also increased significantly in the vegan group, according to researchers from the Physicians Committee for Responsible Medicine (PCRM), based in Washington DC.

They said the relative abundance of Faecalibacterium prausnitzii increased in the vegan group by 4.8%, which was associated with decreases in body weight, fat mass and visceral fat.

“More research is needed to understand how plant-based diets affect gut microbiota” Emma Elvin

Faecalibacterium prausnitzii is one of the short-chain fatty acids producing bacteria, which degrade plant complex sugars and starch, noted the study authors.

The relative abundance of Bacteoides fragilis also increased in the vegan group by 19.5%, which was associated with decreases in body weight, fat mass and visceral fat, and a rise in insulin sensitivity.

The new research was presented today at this year’s Annual Meeting of the European Association for the Study of Diabetes (EASD) in Barcelona.

The authors said: “We have demonstrated that a plant-based diet elicited changes in gut microbiome that were associated with weight loss, reduction in fat mass and visceral fat volume, and increase in insulin sensitivity."

They stated that the “main shift” in the gut microbiome composition was due to an increased relative content of short-chain fatty acid producing bacteria that feed on fibre.

As a result, high dietary fibre content “seems to be essential” for the changes observed in the study, they said.

“Eating more fibre is the number one dietary recommendation for a healthy gut microbiome,” they said.

They added: “We plan to compare the effects of a vegan and a standard portion-controlled diet on gut microbiome in people with type 2 diabetes, in order to separate out the positive effects of the reduced calories in the diet from those caused by the vegan composition of the diet.”

Emma Elvin, a senior clinical advisor at Diabetes UK, said: “This study suggests connections between a vegan diet, gut microbiota, and weight, body composition and insulin sensitivity.

“However more research is needed to understand how plant-based diets affect gut microbiota and – crucially – what distinct effects can be attributed to the diet being vegan specifically, compared to it being reduced calorie, before recommending the widespread adoption of a vegan approach.

“Evidence to date has shown that certain foods in plant-based diets – such as fruits, vegetables and wholegrains – have been associated with reducing risk of type 2 diabetes,” she added.