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 found 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, and bacterial changes may account for 20 percent of that loss, the researchers say.
The findings mean that eventually treatments that adjust the microbe levels, or "microbiota," in the gut may be developed to help people lose weight without surgery, said Dr. Lee Kaplan, director of the obesity, metabolism and nutrition institute at the Massachusetts General Hospital, and an author of a study published Wednesday in Science Translational Medicine.
Not everyone who hopes to lose weight wants or needs surgery to do it, he said. About 80 million people in the United States are obese, but only 200,000 a year have bariatric operations.
"There is a need for other therapies," Kaplan said. "In no way is manipulating the microbiota going to mimic all the myriad effects of gastric bypass. But if this could produce 20 percent of the effects of surgery, it will still be valuable."
In people, microbial cells outnumber human ones, and the new studies reflect a growing awareness of the crucial role played in health by the trillions of bacteria and other microorganisms that live in their own ecosystem in the gut. Perturbations there can have profound and sometimes devastating effects.
One example is infection with a bacterium called C. difficile, which sometimes takes hold in people receiving antibiotics for other illnesses. The drugs can wipe out other organisms that would normally keep C. difficile in check. Severe cases can be life-threatening, and the medical profession is gradually coming to accept the somewhat startling idea that sometimes the best therapy is a fecal transplant – from a healthy person to the one who is sick, to replenish the population of "good germs."
Kaplan's research set out to answer questions raised by earlier studies, which had shown that the microbiota of an obese person changed significantly after gastric bypass surgery, becoming more like that of someone who was thin.
But was the change from the surgery itself, or from the weight loss that followed the operation?
And did the microbial change have any effects of its own?
Because it would be difficult and time-consuming to study those issues in people, the researchers used mice that had been fattened up by a rich diet. One group had gastric bypass operations, and two other groups had "sham" operations. In the sham procedures, the animals' intestines were severed and sewn back together.
The point was to find out whether just being cut open, without having the bypass, would have an effect on weight or intestinal bacteria.
One sham group was kept on the rich food, while the other was put on a weight-loss diet.
In the bypass mice, the microbial populations quickly changed, and the mice lost weight. In the sham group, the microbiota did not change much, not even in those on the weight-loss diet.
Next, the researchers transferred intestinal contents from each of the groups into other mice, which lacked their own intestinal bacteria. The animals that received material from the bypass mice rapidly lost weight; stool from mice that had the sham operations had no effect.
Exactly how the altered intestinal bacteria bring about weight loss is not yet known, the researchers said.
A second study, by researchers at Cedars-Sinai Medical Center in Los Angeles, found that overweight people were more likely to have a certain type of intestinal microbe that may contribute to weight gain by helping to digest certain nutrients and making the calories available. That study was published Tuesday in the Journal of Clinical Endocrinology & Metabolism.