SCIENCE: Bacteria in the Intestines May Help Tip the Bathroom Scale, Studies Show. “The bacterial makeup of the intestines may help determine whether people gain weight or lose it, according to two new studies, one in humans and one in mice. The research also suggests that a popular weight-loss operation, gastric bypass, which shrinks the stomach and rearranges the intestines, seems to work in part by shifting the balance of bacteria in the digestive tract. People who have the surgery generally lose 65 percent to 75 percent of their excess weight, but scientists have not fully understood why. Now, the researchers are saying that bacterial changes may account for 20 percent of the weight loss.”