Medical Mystery: Woman Becomes Obese After Fecal Transplant

— -- A woman became obese after a fecal transplant -- hinting at the complexity of how obesity works in the body, experts say.

The unnamed woman weighed 136 pounds -- but gained 34 pounds over the next 16 months -- going from a healthy body mass index to an obese one, according to a case study published in an Oxford Journal called Open Forum for Infectious Diseases.

"The patient actually said this: 'From the moment I had the fecal transplant, I felt like a switch flipped in my body,'" said. Dr. Colleen Kelly, a gastroenterology at the Warren Alpert School of Brown University. "She felt like prior to the fecal transplant, she had never had to worry about weight."

The woman, who never struggled with weight before the transplant, underwent the transplant in 2011 to cure C. difficile, a serious bacterial infection that can lead to diarrhea and potentially fatal inflammation of the colon, Kelly wrote in the case study she co-authored. The transplant donor was the patient's 16-year-old daughter, who was normal weight at the time but later weighed 170 pounds, according to the study.

Kelly said obesity is a complex problem, and many factors could have contributed to her patient's weight gain, including genetics and a return to normal health after being sick with C. difficile, which causes patients to lose weight because they are so sick.

She said she usually sees patients gain five to 10 pounds following a fecal transplant to treat C. difficile, and this case was different.

"It was really dramatic," Kelly said. "Despite her best efforts she not been able to take off that weight."

As a result, Kelly said she doesn't use obese donors anymore or donors with large waists in fecal transplants.

Sean Davies, an obesity researcher at the Vanderbilt School of Medicine in Nashville, Tennessee, said it was an interesting case study but he would be "leery" about making to much of it.

Still, he said, this case study is "in-line" mouse studies comparing fecal transplant outcomes from normal weight or obese donors.

"Taking the fecal content from obese mice and transferring it to germ-free mice causes greater weight gain than transplanting the bacteria from lean animals," he said. "But in this single case study, there's a lot that could have happened besides just transplant bacteria."