September 11, 2019

Your gut bacteria may be tied to obesity. (But fixing the problem won't be easy.)

Daily Briefing

    Editor's note: This popular story from the Daily Briefing's archives was republished on June 19, 2020.

    Researchers are increasingly interested in the connection between the gut microbiome and obesity, but a new study from researchers at Massachusetts General Hospital suggests that transferring microbiota from a lean donor to an obese recipient might not improve the recipients' metabolic health, contrary to other research, Anahad O'Connor writes for the New York Times.

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    What we know about fecal matter transplants and weight

    According to O'Connor, scientists have long known that there are "striking" differences between the microbiome of lean people and people who are obese. Previous research has revealed a link between microbiota, the trillions of microbes that live in the gut and weight gain and metabolic disease. For example, obesity, insulin resistance, and fatty liver disease have been associated with less microbial diversity and higher levels of a group of organisms called Firmicutes.

    Now, researchers are looking into whether they can change a person's metabolism using fecal microbiota transplants (FMT), in which gut bacteria from a lean donor is transferred—usually in the form of a capsule containing a stool sample—into the gut of an obese patient.

    Jessica Allegrett, director of the Fecal Microbiota Transplant Program at Brigham and Women's Hospital, said, "Obesity is a very complex disorder. Perhaps the microbiome is a contributing part of it, and maybe for everyone it's slightly different. But even for patients where the microbiome is playing a big part, I think this would be something that is part of a larger weight loss program."

    In at least one case described in a medical journal, a lean woman gained 34 pounds after receiving an FMT from her daughter, who was overweight. The doctors were unsure, however, whether the transplant was behind her weight gain, O'Connor writes.

    And some experts remain skeptical of the approach, O'Connor writes.  

    Purna Kashyap, head of the Gut Microbiome Laboratory at the Mayo Clinic, said, "The logic behind it falls apart." He added, "It's saying, because I don't know what's going on, let me just treat it with everything that I have and hope for the best."

    New research on FMTs

    For the new study from Mass General, Elaine Yu, an endocrinologist at the hospital, and her colleagues recruited 24 obese men and women with insulin resistance and four lean donors.

    Yu noted that she was initially skeptical that people would be interested in receiving the transplants, given the requirement that they ingest a capsule containing a stool sample. But interest was high, with volunteers coming from as far away as Hawaii and Alaska, O'Connor reports.

    The donors were screened for a variety of medical conditions to ensure they would not transmit any infections to their recipients. The researchers selected donors who had a history of being lean, ideally people who said they could eat anything and stay skinny, O'Connor writes.

    Half of the obese patients took frozen capsules containing stool from the donors each week while the others took a placebo. After 12 weeks, the researchers found that the FMTs were safe, and that all the participants had acquired the microbiota resembling that of their donors.

    However, the researchers found no improvement in the metabolic health of the obese participants who took the FMTs.

    Yu said that future research on the subject could help discover paths to new drugs or probiotics that can alter the microbiome, but cautioned that FMTs will never be a panacea for obesity.

    "It would be great if there was a treatment that could come out of this research," she said. "But I don't think we're going to find some magic potion that will be able to cure obesity in the absence of any other intervention" (O'Connor, New York Times, 9/10).

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