How a Gut Odor Can Reach the Breath
The smell of flatulence comes from tiny amounts of odor-producing compounds made as gut bacteria break down food. Many of the gases that make up intestinal gas are odorless; sulfur-containing compounds are among the substances that can give it a pungent smell.
Some small, volatile compounds can pass through the intestinal wall into the bloodstream. Blood carries them around the body, including to the lungs, where a portion can move into the air in the lungs and leave with an exhale. That is why certain gut-derived odors can show up in breath rather than only in gas released from the rectum.
Holding It In Does Not Reroute It Directly
When someone postpones passing gas, the gas remains in the digestive tract for a time; it does not travel straight from the bowel to the mouth. Some components may be absorbed, while the rest can remain in the gut and eventually be released later. The breath effect depends on which compounds are present and how much reaches the lungs, so it is not a reliable or inevitable result of holding in gas.
The memorable twist is that an odor can leave the body by a different route from the one it started on. A trace that began with bacterial activity in the gut may briefly be carried out in an exhale.
