A single molecule in your intestinal wall, activated by the waste products from gut bacteria, plays a large role in controlling whether you are lean or fatty. When activated, the molecule slows the movement of food through the intestine, allowing you to absorb more nutrients and thus gain weight.
Bacterial byproducts are a source of nutrients, but now it appears that they can also be chemical signals used to regulate body functions.
Humans have a large and varied population of beneficial bacteria that live in their intestines. The bacteria break up large molecules that the host cannot digest, and the host in turn absorbs many of the resulting small molecules for energy and nutrients.
Researchers focused on two species of bacteria that break up dietary fibers from food into small molecules called short-chain fatty acids. They found that short-chain fatty acids can bind to and activate a receptor molecule in the gut wall called Gpr41.
When researchers disrupted communication between the bacteria and the receptor in mice, they found that their intestines passed food more quickly, and the mice weighed less and had a leaner build, even though they ate no less than other mice.