A woman who had a successful fecal transplant to treat a bacterial infection in her intestines has experienced unexpected and rapid weight gain, causing doctors to consider the role of gut bacteria in metabolism and health.

The case, reported in the journal Open Forum Infectious Diseases, details the woman’s 2011 fecal transplant to treat her C. difficile infection. At the time of the procedure, she was 32 years old and weighed 136 pounds, with a Body Mass Index (BMI) of 26. The donor, according to a news release, was her own teenage daughter, who was found to be overweight, but otherwise healthy.

Within sixteen months, the woman was medically considered obese, weighing 170 pounds with a BMI of 33. Despite a medically supervised liquid protein diet and exercise program, three years after the procedure the unidentified woman weighed 177 pounds, and registered a BMI of 34.5, according to the news release.

“We’re questioning whether there was something in the fecal transplant, whether some of those “good” bacteria we transferred may have had an impact on her metabolism in a negative way,” case author Dr. Colleen Kelley of the Warren Alpert Medical School of Brown University said in the news release.

While there are other possible contributing factors to her weight gain, such as several antibiotics used to treat an infection, a resolution of her C. difficile infection, genetic factors, aging and stress, the woman had never been overweight before the surgery. Previous studies in animals have linked bacteria in the gastrointestinal tract and excess weight. In the studies, transfer of gut bacteria from obese to normal-weight mice was found to lead to a marked increase in fat, according to the news release.

The authors suggest clinicians should avoid selecting donors who are overweight, according to the news release. The authors of the study involving mice had also recommended selecting stool donors who are not overweight for fecal transplant.

Fecal transplants have been shown to be an effective way to treat people with C.dificile infections, but the Food and Drug Administration (FDA)— which regulates the treatment as an experimental drug—  expressed concern last year over its safety.