Gastric bypass may depend on gut microbes

Gastric bypass, a drastic surgical option for weight loss, reroutes and even sometimes removes parts of your stomach to shrink its size to encourage weight loss. Previously, this surgery was thought to limit the amount of food the patient can eat; however, researchers now believe it might activate changes in the types of microbes living in your gut. If this new study proves true, weight loss might depend on altering the microbial communities within your gut instead of potentially dangerous surgeries.

Researchers believe this study provides the best proof thus far that bariatric surgery actually alters the gut bacteria instead of simply limiting caloric intake.

In addition to weight loss, bariatric surgery typically cures type 2 diabetes. Currently, researchers are trying to understand how gastric bypass alters the metabolism and digestive system functions so effectively.

One theory on the effectiveness of bariatric surgery is that it triggers its effects through the influence on the microbiome.

Previous studies indicated that gastric bypass significantly alters the microbial communities found in the gut in both rodents and humans.

Rodents that underwent the most common type of gastric bypass, Roux-en-Y gastric bypass, had samples taken from their intestines. These samples were then transplanted to the intestines of germ-free mice. Nearly five percent of the germ-free mice body weight was lost after two weeks. However, researchers only observed short-term variations within the mice in these studies.

The Microbiome and Gastric Bypass Surgery

Researchers are currently searching for long-term changes and results. Samples were taken from the feces of 14 women who had undergone Roux-en-Y surgery or vertical banded gastroplasty, a less common gastric bypass surgery, at least nine years ago and then sequenced to profile the microbes residing in their guts. The women, despite undergoing surgery and losing significant amounts of weight, were still considered obese with an average body mass index (BMI) of nearly 32. Seven more samples were then taken from severely obese women who had undergone surgery.

The variety and abundance of the microbial communities varied significantly between the gastric bypass surgery patients and those who had not undergone surgery. However, researchers noted that the guts of the two surgery groups were home to many of the same microbes. The scientists of this study believe that the two procedures cause long-lasting changes to the gut microbiome.

Some notable changes occurred in the surgery group. Some microbes were found in reduced numbers while the abundance of others increased. One of these microbes included Escherichia coli.

After sequencing the surgery patients’ fecal samples, their bacteria were then transplanted into germ-free mice, which are typically lean. The rodents receiving the transplants from the Roux-en-Y patients gained 43% less fat than the rodents that received transplants from the control patients. The mice transplanted with microbes from the vertical banded gastroplasty patients gained 26% less fat than the rodents from the control groups.

In addition, the study also indicates that the source of the transplant significantly affected how it digested food. Mice receiving transplants from the Roux-en-Y group burned less carbs and more fat than the control group did, which might explain the variation in fat accumulation.

This study reveals that the changes triggered by gastric bypass to the microbiome are stable.

In the future, scientists expect that altering a patient’s gut microbiome may prove just as beneficial as gastric bypass without the risks. A more thorough understanding of the microbes that live in our gut is required before such therapies can be used successfully though.


Tremaroli, V., Karlsson, F., Werling, M. et al. Roux-en-Y Gastric Bypass and Vertical Banded Gastroplasty Induce Long-Term Changes on the Human Gut Microbiome Contributing to Fat Mass Regulation. Cell Metabolism. 2015. 22(2):228-38. DOI:

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