Medical Mystery: Woman Becomes Obese After Fecal Transplant

She gained more than 30 pounds afterwards.

"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."

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.

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."