‘The freeze-dried poop method’ might not sound like a weight-loss strategy that would catch on, but—as some researchers are now testing—it may be an effective way to slim down.

In a randomized, controlled clinical trial starting this year, researchers will test out such a fecal formula for the treatment of obesity. They’ll also try to glean critical details about the human microbiome and its role in our health and metabolism. The trial, led by Elaine Yu, an assistant professor and clinical researcher at Massachusetts General Hospital, will involve taking fecal samples from lean, healthy donors then freeze-drying the stool, putting a gram or two into capsules, and giving them to 20 obese patients.

Such poop-packed pills, which are designed to replace a person’s intestinal microbes with those from a donor via their feces, have proven effective at treating tenacious gut infections. This has led researchers to ponder whether the transplants could remedy other health problems, including obesity and metabolic disorders. A few animal studies and some anecdotal data in humans suggests the answer is yes—and Yu hopes to get a final answer with the upcoming trial.

At the moment, “we have no idea what the result will be,” Yu told Ars. But she and colleagues are cautiously optimistic.

A few years ago, researchers took the gut microbes from a set of twins—one lean, one obese—and transplanted them into two sets of microbe-free mice. Even though all the mice were on the same diet, the rodents that received the obese twin’s microbes became chubby. The mice that got the lean twin’s mix stayed slim, suggesting that the microbes were calling the shots when it came to the animals’ weight.

In line with those results, another study on lean and obese twins’ microbes suggested that obesity is linked to having altered mixes and lower diversity of gut microbes.

There was also a concerning case last year in which microbes seemed to hijack a woman’s weight after a fecal matter transplant (FMT). She underwent the FMT to treat a recurring intestinal infection from Clostridium difficile. The fecal donor was the woman’s daughter, who was overweight but otherwise healthy. After the transplant, the woman was cured of her infection, but inexplicably gained weight.

There’s a lot that scientists don’t know about the microbes in our bodies and what they do, Yu said, “which is highly unsatisfying.” And it makes trials of microbe swapping risky. For that reason, Yu and colleagues are using extreme caution in selecting healthy fecal donors—screening for infections, metabolic disorders, and other health complications. They’re beginning to screen donors now and start transplants later this year.

Patients will receive weekly poop doses for six weeks, then researchers will track their weight and health at three, six, 12 months, and possibly beyond depending on how the trial goes. During that time, researchers will ask the transplant recipients to maintain their normal eating and health habits.

In the future, Yu hopes that microbe treatments could go hand-in-hand with dietary interventions to treat obesity and metabolic disorders. She cautions it’s too early to make such predictions for now.

Scientists are just in the early stages of microbiome work, she said. With this microbe-swapping trial, Yu and her colleagues will for the first time try to definitely prove that microbes can cause weight changes. If that’s really the case, she said, then “we can do all of the detail-oriented work to try to figure out which microbial communities are being affected to give more information for targeted treatments in the future.”