Fecal transplants are being used to treat obesity by transplanting the gut microbiome of thin people into obese individuals, leading to weight loss.

“And indeed, that has been shown to be the case also in fecal transplants for certain psychiatric illnesses. ... One of the more powerful and salient examples is for obesity. There's some people for which even if they ingest very low numbers of calories, even if they go on a liquid protein diet, simply can't lose weight. These are somewhat rare disorders but these are people that would either get gastric bypass surgery. Some people are now getting these fecal transplants from people that have healthy weight and they take the stool from them, they put it into lower digestive tract, and they can see substantial improvement in weight loss in people that were otherwise unable to do that.”

“haven't they done some fecal transplants from lean to obese that have at least partially addressed the phenotype correct”