After weight-loss surgery, new gut bacteria keep obesity away


By Sharon Begley, Reuters

NEW YORK — The logic behind weight-loss surgery seems simple: rearrange the digestive tract so the stomach can hold less food and the food bypasses part of the small intestine, allowing fewer of a meal’s calories to be absorbed. Bye-bye, obesity. A study of lab mice, published on Wednesday, begs to differ. It concludes that one of the most common and effective forms of bariatric surgery, called Roux-en-Y gastric bypass, melts away pounds not — or not only — by re-routing the digestive tract, as long thought, but by changing the bacteria in the gut. Or, in non-scientific terms, the surgery somehow replaces fattening microbes with slimming ones. If that occurs in people, too, then the same bacteria-changing legerdemain achieved by gastric bypass might be accomplished without putting obese patients under the knife in an expensive and risky operation. “These elegant experiments show that you can mimic the action of surgery with something less invasive,” said Dr. Francesco Rubino of Catholic University in Rome and a pioneer in gastric-bypass surgery. “For instance, you might transfer bacteria or even manipulate the diet” to encourage slimming bacteria and squelch fattening kinds, said Rubino, who was not involved in the study. Fattening Bugs, Slimming Bugs For many obese patients, particularly those with type 2 diabetes, gastric bypass has succeeded where nothing else has. Severely obese patients routinely lose 65 to 75 percent of their excess weight and fat after the operation, studies show, and leave their diabetes behind. Oddly, however, the diabetes remission often occurs before significant weight loss. That has made bypass surgeons and weight-loss experts suspect that Roux-en-Y changes not only anatomy but also metabolism or the endocrine system. In other words, the surgery does something besides re-plumb the gut. That “something,” according to previous studies, includes altering the mix of trillions of microbes in the digestive tract. Not only are the “gut microbiota” different in lean people and obese people, but the mix of microbes changes after an obese patient undergoes gastric bypass and becomes more like the microbiota in lean people. Researchers did not know, however, whether the microbial change was the cause or the effect of post-bypass weight loss. That is what the new study, by researchers at Massachusetts General Hospital and Harvard University, set out to answer.

They first performed Roux-en-Y on obese mice. As expected, the animals quickly slimmed down, losing 29 percent of their weight and keeping it off, the researchers report in the journal Science Translational Medicine.