A 16-week vegan diet was found to boost the gut microbes involved in improving body weight, body composition and blood sugar control. This is according to a study by Dr. Hana Kahleova and colleagues, presented at the Annual Meeting of the European Association for the Study of Diabetes in Barcelona, Spain. The authors note that the results are likely due to an increased intake of fiber, although other experts have flagged the study for its “preliminary and uncontrolled” nature.

“We know that plant-based diets work well for weight management. However, we still don’t fully understand the mechanisms. That’s why we decided to perform this study and look at gut microbiome. This study showed the missing link between gut microbiome and metabolic health. It highlights the importance of fiber. The US population is only consuming about a half of the recommended daily intake of fiber. Increasing the intake would result in major improvements in the population’s metabolic health,” Dr. Kahleova tells.

The study involved 147 overweight adults with no history of diabetes being split into a group who followed a low-fat vegan diet and a group who followed made no dietary changes over the course of 16 weeks. The vegan group was found to have lost an average of 5.8kg, with 3.9kg of this being a reduction of fat mass. Insulin sensitivity was also reduced in the vegan group.

Additionally, the relative abundance of Faecalibacterium prausnitzii increased in the vegan group. The short-chain fatty acid produces bacteria that degrade plant complex sugars and starch to produce health-promoting butyrate or other short-chain fatty acids. These have been found to have a beneficial effect on body weight, body composition and insulin sensitivity.

In this study, changes in F. prausnitzii were associated with decreases in body weight, fat mass and visceral fat. The relative abundance of Bacteoides fragilis also increased in the vegan group, which was associated with decreases in body weight, fat mass and visceral fat, and increases in insulin sensitivity.

“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 fiber. Therefore, high dietary fiber content seems to be essential for the changes observed in our study. We plan to compare the effects of a vegan and a standard portion-controlled diet on the 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,” the authors continue.

At baseline and 16 weeks, gut microbiota composition was assessed, using uBiome kits. Dual energy X-ray absorptiometry was used to measure body composition. A standard method called the PREDIM index was used to assess insulin sensitivity.

“This general effect of fiber on gut microbes has been observed and studied for decades. What we certainly cannot infer without further research is that the changes in gut bacteria caused the weight losses or the improvements in metabolic health. Without other information, simple correlations cannot prove causality,” Dr. Ian Johnson, Nutrition researcher and Emeritus Fellow, Quadram Institute Bioscience, comments on the study.

Earlier this year, another study found that vegan meals increased plasma concentrations of the peptide amylin in people, further showing the connection between plant-based foods and the gut.

It is not possible to attribute weight loss and insulin sensitivity to changes in the gut microbial flora as these are already well-known to change as a result of a vegan diet, explains Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London. “However, it does illustrate that a calorie-restricted diet high in carbohydrates (presumably unrefined) is of benefit for people with diabetes, which is opposite to the misplaced public perception that high carbohydrate diets increase risk of diabetes,” he continues.

The researchers are continuing to collect data from more study participants, which they plan to present next year.