In recent years, scientists have become increasingly interested in the way our microbiomethe mix of bacteria found throughout our bodies, but especially in our gutscan affect our weight. Now, a fascinating new case study involving a patient who got a fecal transplant and then became obese has researchers speculating that the donated gut microbes played a role.

As icky as they sound, fecal transplants have proven to be highly effective against the nasty and persistent infection Clostridium difficile (C. diff), which most often occurs in or after a stay at the hospital or another health care setting. Depending on the severity of infection, C. diff can cause life-threatening diarrhea.

A fecal transplant involves delivering bacteria from a donor’s stool to a recipient’s intestines (via colonoscopy or a nasal tube) to restore populations of microbial species wiped out by the infection, which seems to solve the problem.

In the case recently reported in the journal Open Forum Infectious Disease, a 32-year-old Rhode Island woman was cured of her recurrent C. diff—but put on a significant amount of weight in the year after the procedure.

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The donor bacteria was from her teenage daughter, who was slightly overweight at the time. After the transplant, the daughter gained 30 pounds.

At the same time, the mother’s weight rose from 136 pounds to 170 pounds in 16 monthsdespite following a medically supervised liquid diet and exercise program in an effort to fight back against her ballooning waist line. At the three-year mark, the patient weighed 177 pounds, with a BMI of 34.5 (a BMI of 30 or higher is considered obese).

It’s entirely possible that the woman’s weight gain was caused by other factors, from aging and genetics to medications she took before her transplant. However, as the report authors note, she’d never been obese before.

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“We’re questioning whether…some of those ‘good’ bacteria we transferred may have had an impact on her metabolism in a negative way,” said Colleen R. Kelly, MD, of the Warren Alpert Medical School of Brown University, in a statement. To err on the side of caution, she and her co-author suggest doctors avoid using bacteria from overweight donors in the future.

An editorial accompanying their report calls for more research on the delicately complex ecosystem in our guts: “[I]t is hoped that [fecal transplant] studies will lead to identification of defined mixtures of beneficial bacteria that can be cultured, manufactured, and administered to improve human health.”

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