Vegan Diet Alters Microbiome and Insulin Sensitivity, Drops Weight

Becky McCall

September 17, 2019

BARCELONA — A low-fat vegan diet induces changes in gut microbiota that are related to altered body composition and insulin sensitivity, and result in weight loss, according to results of a randomized controlled trial in overweight/obese adults.

Over the 16-week intervention, body weight was significantly reduced in individuals on the vegan diet compared with those who stayed on their everyday (nonvegan) diet, with a loss of –5.8 kg (P < .001), which was largely because of a drop in fat mass, with a treatment effect of –3.9 kg (P < .001). Visceral fat was also significantly reduced with the vegan eating plan.

No calorie restriction was imposed in either diet.

Hana Kahleova, MD, PhD, director of clinical research at the Physicians Committee for Responsible Medicine (PCRM), Washington, DC — the main remit of which is to promote plant-based diets — led the work and will present the findings here at the European Association for the Study of Diabetes (EASD) 2019 Annual Meeting.

"This is a pretty good result of about one pound or half a kilogram [weight loss] on average per week in the vegan group," said Kahleova.

Kahleova explained that previous work has shown individuals can lose twice as much weight on a vegan diet as a nonvegan diet with the same calorie intake. "In conducting our study, we wanted to find out why this is so," she noted in an interview with Medscape Medical News.

The results hint at changes to the gut microbiome producing beneficial effects, she said.

"Eating a plant-based diet with ample fiber changes the gut microbiome composition for the better by feeding the right kind of bacteria...notably short-chain fatty acid producing Faecalibacterium prausnitzii, [which] deliver many metabolic benefits including weight loss, increased insulin sensitivity, and fat loss, including visceral fat loss," she noted.  

However, Emma Elvin, senior clinical advisor at Diabetes UK, cautioned that 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 specifically vegan compared to it being reduced calorie before recommending the widespread adoption of a vegan approach."

"It's true that many of the foods in a balanced vegan diet are good for us, but that doesn't mean all vegan diets are healthy," she added.

"That said, evidence to date has shown certain foods in plant-based diets — such as fruits, vegetables, and wholegrains — have been associated with reducing the risk of type 2 diabetes."

Gut Microbiome Changes of Vegan Versus Everyday Diet

For the study, 148 overweight/obese adults with no history of diabetes were randomized to follow a low-fat vegan diet (n = 73) or no dietary changes (n = 75). The average age in the vegan and control groups was 53 years and 57 years, respectively; 60% and 67% were women, respectively; and body mass index was around 33 kg/m3 in both groups.    

The vegan diet contained no animal products and comprised legumes, nuts, vegetables, fruit, and wholegrains. Calorie intake was unrestricted in both groups.

The objective of the study was to test the effect of the plant-based vegan diet on gut microbiota composition, body weight, body composition, and insulin sensitivity over 16 weeks.

"We were interested in metabolic outcomes," Kahleova explained.

"We've known for a while that plant-based diets are very effective for weight management, because unlike many diets, the vegan one is sustainable over the long term and the benefits go beyond the immediate intervention. By comparison, the keto diet, for example, is good while you're on it but people come off it and regain above their original weight."

However, she added that, to date, there was little understanding of the mechanisms driving the benefits of a vegan diet, which this study and others aim to shed light on.

F. prausnitzii  Feed on Fiber and Induce Fat Loss, Including Visceral Fat

When the gut microbiome composition of participants was assessed, Kahleova and colleagues found the whole family of bacteroidetes was increased in those on the vegan diet.

And one species was found to be especially important — Faecalibacterium prausnitzii. "The relative abundance of F. prausnitzii increased in the vegan group and was associated with a treatment effect of +4.8%," Kahleova reported.

"Lower counts of these bacteria have been described in patients with diabetes, and this has been associated with insulin resistance and inflammation," she added.

F. prausnitzii produces short-chain fatty acids, and these have many metabolic benefits including prevention of cardiovascular disease, insulin sensitizing effects, and positive effects on the immune system.

These short-chain fatty acids are produced by these bacteria, which feed on fiber abundant in plant-based food but not in animal products. "The more plants eaten, the more these bacteria are fed and the greater the amount of short-chain fatty acids produced," explained Kahleova.

"The increase in F. prausnitzii seen in our study correlated with weight loss and an increase in insulin sensitivity," she said.

And crucially, two thirds of the weight loss was "explained by fat loss," she noted, adding that "visceral fat was also lost readily on the vegan diet."

Vegan vs Vegetarian Diet and the Challenges of Being Vegan

Prior studies have shown that vegans have a lower risk of diabetes than even vegetarians, Kahleova said. A vegetarian diet is more liberal and can have high levels of saturated fat, she noted.

But she acknowledged that changing to a vegan diet is not without its challenges.

"It can be difficult at the start, for example, in finding and making recipes, as well as eating out. Also, we often eat the same as, or need to cook for, our families," she said.

She also cautioned guarding against a drop in vitamin B12 levels that can be associated with a vegan diet. 

Patients with diabetes taking metformin (which also lowers vitamin B12) and especially older people (taking metformin or not), may be vitamin B12 deficient, and these people may need to take supplements, she advised.

Kahleova said among her plans for future research is a cross-over comparison trial of a low-fat vegan diet with the American Diabetes Association (ADA) recommended portion-controlled, carbohydrate restricted diet for patients with diabetes. The composition of the gut microbiome will be measured.

"We are also in the midst of another study comparing a low-fat vegan diet with a Mediterranean diet rich in olive oil and nuts, which will address the fat quality and quantity component of each diet. And in the future, we might compare diets of low-fat vegan with high-fat vegan."

The study was funded by the Physicians Committee for Responsible Medicine. Kahleova has reported no relevant financial relationships.

EASD 2019 Annual Meeting. Presented September 19, 2019. Abstract 700.

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