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Mice studies

Gut microbiota contributes to weight loss after bypass

The weight-loss effect of bypass beyond restriction and malabsorption could be explained by changing gut microbiota. Photo: Wikimedia / Microrao
Gastric bypass mice have adjusted gut microbiota composition, which helps them lose weight
Bacteria from Roux-en-Y mice help non-bypass mice lose weight when transplanted into gut
Finding could potentially lead to new treatment modalities

Changing gut microbiota is a significant contributor to weight loss following gastric bypass, according to new research published in Science Translational Medicine.

The study showed that mice who received a gastric bypass saw a “rapid and sustained” shift in the makeup of their gut bacteria. When bacteria from mice who received Roux-en-Y gastric bypass was transferred to obese mice who had not had an operation, they lost weight and fat density.

Dr Lee Kaplan, director of the Obesity, Metabolism and Nutrition Institute at Massachusetts General Hospital and lead author on the paper, suggested to the Scientist that the effect could explain up to 20% of the power of the gastric bypass 

Kaplan said in a statement that he believed the research could lead to new treatment modalities for the obese. "Our study suggests that the specific effects of gastric bypass on the microbiota contribute to its ability to cause weight loss,” he said. “Finding ways to manipulate microbial populations to mimic those effects could become a valuable new tool to address obesity."

The study compared faecal samples from three groups. The first consisted of mice who underwent Roux-en-Y gastric bypass; the second had sham surgery; and the third group had sham surgery coupled with a calorie-controlled diet, designed to induce weight loss.

Three weeks after surgery, both the gastric bypass mice and the calorie-restricted mice had lost around 30% of their body weight. However, while the gut microbiota had barely changed in both sham surgery groups, the Roux-en-Y mice were found to have significantly higher abundance of bacteria more commonly seen in lean organisms.

While it is already known that gastric bypass surgery alters the gut microbiota of patients, the authors say that this study is the first to show that the change is due to the surgery itself, rather than the resultant weight loss, and is also the first to provide empirical evidence that this change is a causal factor helping to reduce their weight and adiposity.

To establish this, the researchers took gut microbiota from each of the three groups and implanted them in the guts of non-obese, microbe-free mice. Two weeks later, the mice who had received gut microbiota from the Roux-en-Y mice had lost a significant amount of weight, while the other two groups had not lost any weight at all.

While the paper does not establish the exact mechanism by which the bacteria aid in weight loss, the authors hypothesise that a change in the quantity of short-chain fatty acids produced by the microbes in the gut could lead to metabolic changes in the patient.

"We need to learn a good deal more about the mechanism by which a microbial population changed by gastric bypass exerts its effects – including whether it contributes to the improvement in diabetes and other metabolic disorders we see in patients having the procedure – and then we need to learn if we can produce these effects, either the microbial changes or the associated metabolic changes, without surgery," said Kaplan. "The ability to achieve even some of these effects without surgery would give us an entirely new way to treat the critical problem of obesity, one that could help patients unable or unwilling to have surgery."

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