U.S. Department of Health and Human Services

Surgically Remodeling the Intestines—and the Bacteria Within—To Treat Obesity

Gaining new insight into Roux‑en‑Y gastric bypass surgery, a treatment for obesity, researchers studying a mouse 
model found that restructuring of the digestive tract leads to weight loss and metabolic beneits in part by altering the communities of bacteria that normally live in the intestines. Gastric bypass, a form of bariatric surgery, is thought to work in a number of ways, but the mechanisms are not well understood. Among various changes observed after gastric bypass are alterations in gut bacteria.

Thus, a team of researchers sought to determine whether the changes in types of bacteria (gut microbes) in the intestines after gastric bypass contribute to weight loss. To do this, they started with groups of obese mice that had been fed a high‑fat diet, performed gastric bypass surgery on some of the mice and sham surgery (as a control) on others, and then analyzed intestinal bacteria. Despite access to unlimited food after surgery, the mice that had gastric 
bypass lost 29 percent of their initial body weight and maintained their lower weight throughout the study. These mice also had reduced visceral body fat, the type of fat most associated with metabolic disease, and other metabolic beneits. Their resident gut microbes changed rapidly after surgery, as different types of bacteria lourished in the newly 
renovated intestinal habitat. By contrast, a group of mice that had only sham surgery, which did not alter the intestinal tract, regained all of their initial body weight after recovering from surgery. These mice also had more body fat, and their gut bacteria differed from those of the gastric bypass group. The researchers then tested whether putting mice on a 
low‑calorie diet would lead to similar changes in gut bacteria. Dieting did not have quite the same effect: the relative abundances of various types of bacteria from the mice on a diet differed from those of the gastric bypass group. Finally, the researchers investigated whether the unique collection of gut microbes resulting from gastric bypass could cause 
weight loss—even without gastric bypass surgery. They extracted bacteria from mice that had undergone gastric bypass and administered the bacteria to other mice that had not been treated surgically. Although the recipient mice did not lose as much weight as the gastric bypass group, the bacterial transfer did lead to some weight loss, along with reduced body fat. This study indicates that changes in gut bacteria contribute to the weight loss and reduced body fat from gastric bypass surgery in mice. Because changes in gut bacteria have also been observed in people after surgery, this study also has implications for understanding how gastric bypass works in humans. Further research into the effects of gut microbes on weight and metabolism after gastric bypass in people may lead to interventions that achieve similar health 
beneits without the cost and risks of surgery.

Liou AP, Paziuk M, Luevano JM Jr, Machineni S, Turnbaugh 
PJ, and Kaplan LM. Conserved shifts in the gut microbiota 
due to gastric bypass reduce host weight and adiposity. Sci 
Transl Med 5: 178ra41, 2013.