A gut bacteria transplant may not help you lose weight

Changing your gut microbes may not help you lose belly fat.

In a preliminary study, obese people got either capsules containing gut microbes from a lean person or placebo pills. Microbes from the lean donor took hold in the guts of the obese recipients. But early results suggest that the bacteria didn’t change the volunteers’ weight or levels of a hormone that helps signal fullness, gastroenterologist Jessica Allegretti reported May 8 at a news conference. 

People with obesity often have different types of gut microbes than lean people do. And previous studies with lab animals and anecdotal evidence from people have suggested that transfers of intestinal bacteria and other microbes — collectively known as the gut microbiome — from a donor to a recipient may lead to weight loss or gain, depending on whether the donor was lean or obese. So the researchers reasoned that giving obese people gut microbes from a lean person, known as a fecal or intestinal microbiome transplant, might help overweight people control appetite and shed pounds.

In the study, 11 obese people swallowed 30 capsules containing gut microbes from the same lean donor. Another 11 obese people got identical-looking capsules that contained no gut bacteria. At four weeks and again at eight weeks after the initial dose, the volunteers downed 12 additional capsules. Twelve weeks after the experiment’s start, the researchers measured changes in the people’s gut microbe mix, their weight, levels of a protein called GLP-1 and bile acid production.

When people eat fiber, gut microbes process that fiber into short chain fatty acids. Those molecules cause the small intestine to make GLP-1, which in turn tells the brain that the person has had enough to eat. Other weight loss studies have indicated that GLP-1 levels change as people lose weight.

Allegretti and colleagues thought that changing the gut microbe mix might boost GLP-1 production. Instead, they found no change in the satiety protein between the transplant and placebo groups. There was also no indication that the bacteria led to weight loss.

But people who got the lean microbes increased production of certain bile acids, which help break down fats. Providing new gut microbes that make bile-acid-processing enzymes may alter the metabolism of fat in obese patients, says Allegretti, who directs the fecal microbiome transplant program at Brigham and Women’s Hospital in Boston. 

The study was small, and mainly designed to determine whether such microbiome transplants are safe and if a lean donor’s microbes could settle in in an obese person’s guts. “We did not expect to answer the question. ‘Does [fecal microbiome transplant] work to treat obesity?’ from this first step,” Allegretti says. But “I’ve seen enough from this data that makes me want to continue exploring it further.” She will report more results from the study May 20 in San Diego at the Digestive Disease Week 2019 medical conference.

In the United States, the Food and Drug Administration allows intestinal microbiome transplants for treating people with infections with Clostridium difficile for whom other treatments have failed. C. diff is a severe bacterial infection that takes hold when people’s normal gut microbes have been ravaged by antibiotics. The bacteria can cause symptoms ranging from diarrhea to life-threatening inflammation of the intestines and other organs. In those instances, intestinal microbe transplants can save lives.

So far, gut microbe transplants to treat diabetes have made some obese people with the disease more sensitive to insulin. But that hasn’t really helped them lose weight, says Alexander Khoruts, a gastroenterologist at the University of Minnesota in Minneapolis. Khoruts was not involved in new study, but has experience with transplanting gut microbes from one person to another. Most studies in humans, including an unpublished trial Khoruts and his colleagues conducted, have found only minuscule changes in waist circumference, weight or body mass index in obese people who have gotten gut microbes from someone lean.

Still, the new work is the first to compare gut microbe transplants to placebo, says Vincent Young, an infectious disease physician at the University of Michigan Medical School in Ann Arbor. That alone is “potential progress,” he says.  

But it’s still too early to determine what role the microbiome plays in obesity. Researchers aren’t yet sure what a healthy microbiome is, for example. Nor do they know or whether the microbes in the colon (those are the ones transferred in this and other fecal microbiome transplants) or those in the small intestine play a bigger role in determining body weight.

For now, swapping gut microbes doesn’t seem to be an easy cure for obesity, Allegretti says. The team may need to alter the dose of lean microbes or tweak the treatments in other ways to maximize the results.