Gastric bypass (GBP) surgery has become a popular procedure for the treatment of morbid obesity. While many assume it is the reduction of the functional volume of the stomach that helps people feel fuller while eating less that is the solely responsible for the patient’s weight loss, it has been known for several years that there are other contributing factors at work. New research provides further evidence of this and could allow patients to see some of the weight loss benefits of GBP surgery without undergoing the surgery itself.

It is estimated there are around 100 trillion microorganisms in the human gastrointestinal tract. While people can live without them, research suggests the human/gut microorganism relationship is a mutually beneficial one, with the microorganisms doing everything from preventing the growth of harmful bacteria to producing vitamins for their host. Research has also shown differences in the microbial population – or microbiota – found in the gut of obese individuals (both mice and humans) and those in the gut of lean individuals.

Previous research has also shown that GBP surgery leads to changes in the gastrointestinal microbiota in humans and animals by resetting the balance between two types of bacteria. While some thought these changes might be a result of subsequent weight loss, researchers from Harvard University and Massachusetts General Hospital (MGH) have shown that the GBP surgery itself is directly responsible.

In experiments on mice, the researchers found that transferring samples of the altered microbiota from mice that had undergone GBP into mice raised in sterile conditions that hadn’t received surgery resulted in rapid weight loss in the germ-free mice.

“Simply by colonizing mice with the altered microbial community, the mice were able to maintain a lower body fat, and lose weight – about 20 percent as much as they would if they underwent surgery,” said Peter Turnbaugh, a Bauer Fellow at Harvard’s Center for Systems Biology.

This was despite the mice subjects being fatted up on a high-fat, high-sugar diet beforehand. “In some ways we were biasing the results against weight loss,” Turnbaugh said. “The question is whether we might have seen a stronger effect if they were on a different diet.”

While more study is needed to understand the underlying mechanism responsible for the weight loss, the research suggests it could one day be possible to provide some of the weight loss benefits of GBP surgery without actually performing the surgery itself.

"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," explains Lee Kaplan, MD, PhD, director of the Obesity, Metabolism and Nutrition Institute at MGH and an associate professor of Medicine at Harvard Medical School.

"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," adds Kaplan

Kaplan and Turnbaugh are senior authors of the study that appears in the journal Science Translational Medicine.