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Gut microbes may be behind weight loss after gastric bypass

Mice slim down after receiving bacteria transplanted from rodents that had the surgery

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Intestinal bacteria may be responsible for at least part of the fat-shedding effects of a popular weight-loss surgery, a new study in mice suggests. Those naturally occurring bacteria not only trim the tummies of mice that have had the surgery but, when transplanted into mice that have not had surgery, cause them to lose weight as well.

Roux-en-Y, the most common technique for gastric bypass, diverts food around most of the stomach and upper small intestine. Some patients go on to lose large amounts of weight, and the surgery may produce other health benefits, such as improving symptoms of type 2 diabetes (SN: 9/10/11, p. 26). In mice, those benefits stem from a bacterial blend fostered by bypass surgery, researchers report March 27 in Science Translational Medicine.

The finding could be a first step toward “bypassing the bypass” as a means of treating obesity and diabetes, says coauthor Lee Kaplan, a gastroenterologist who heads Massachusetts General Hospital’s Weight Center. Possible treatments may include replacing or augmenting an obese person’s intestinal community.

Still, says Guilherme Campos, a surgeon at the University of Wisconsin School of Medicine and Public Health in Madison, the study was done in mice, so the role that gut microbes play in humans’ weight loss is unknown. “Is it the main driver? Likely not, but it is still likely one of the components that assist gastric bypass patients to lose weight in the long run.”

Previous studies of people and rats have found that the natural mix of microbes in the intestines changes after gastric bypass, with some groups growing more prominent and others diminishing in number. No one knew whether the altered microbial composition was merely a side effect of the surgery, or whether shifting bacterial populations could help people lose weight.

To find out, Kaplan and colleagues fattened up mice then performed either bypass or a sham surgery on the animals. Mice in the bypass group lost about 29 percent of their body weight within three weeks of the procedure. But even before the mice dropped weight, those in the bypass group already had an altered mix of intestinal bacteria.

Compared with the sham operation group, the bypass mice had more of certain types of microbes called Gammaproteobacteria, particularly Escherichia species. Some species of Escherichia are pathogens, but others help prevent inflammation and maintain intestinal health. Bypass mice also had more Akkermansia bacteria, which can feed on mucus lining the intestines, particularly when the host is cutting calories. The researchers speculate that the microbes somehow trigger fat-burning changes in the host’s metabolism.

Then the researchers transplanted bacteria from the intestines of bypass mice into mice that had been raised without any bacteria. The formerly germ-free mice slimmed down, trimming about 5 percent of their body weight, even though they started out lean. Germ-free mice that received bacteria from the guts of sham surgery mice actually packed on a bit of fat.

The researchers are investigating what the bacteria do to cause fat reduction.

Bacterial transplants might eventually help some people, but they won’t solve the global obesity epidemic, Campos says. A person’s social environment and exercise habits are far more complex than a mouse’s. “The factors that lead to morbid obesity in humans are completely different than what is seen in a laboratory setting,” he says. Patients who don’t change their lifestyles along with their gut microbes may regain lost weight, he says.

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