Scientists have known for a while that gut bacteria can play a profound role in the weight of mice. Now we have a case report in humans that is not entirely surprising: A woman gained 36 pounds and became obese in the 16 months after a fecal transplant.

The 32-year-old woman had maintained an average weight her entire life, according to a case report in Open Forum Infection Disease. Then she fell sick with Clostridium difficile, a gut infection that is difficult to cure with antibiotics but very easy to treat with a fecal transplant, which replaces the entire gut microbiota with one from someone healthy. The woman's donor was her 16-year-old daughter, who weighed 140 pounds at the time but later went up to 170 pounds.

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After her fecal transplant, the woman started packing on pounds and couldn't lose it, despite "medically supervised liquid protein diet and exercise program." She ended up at 177 pounds and suffered from constipation and bad digestion.

It's impossible to draw conclusions from any single patient, of course, but this case is interesting against the broader context of what we know about the gut microbiota and weight. A decade of studies in mice have found that those implanted with the gut microbiota of obese humans will become obese, too, despite eating the same diet as those given the microbiota of non-obese humans. Gastric bypass surgery in mice also drastically shifts the gut microbiota, and it could be one reason for why the surgery is so effective for losing weight.

And then there are safety questions. The FDA backed off from closely regulating fecal transplants in 2013—which many saw as the right decision—but the process of screening donors is not standardized. There are millions of bacteria involved in a fecal transplant, and we don't really know what each one does.

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The authors of the case report, which include Colleen Kelly at Brown University, one of the pioneering doctors to study using fecal transplants to treat C. difficile, write they now no longer use obese donors. Poop is serious medical business, and sometimes it can go wrong. [Open Forum Infectious Diseases via BBC]

Top image: C difficile bacteria. Credit: CDC