Anyone who has ever ridden in an elevator, carpooled, or stayed over at a new partner’s house for the night has tried to hold in a fart. This is a fact. Sometimes, it works: Other times, the pressure builds too much and your butt lets forth with a noise like angry zoo animals forcibly deflating a zeppelin. So what exactly is happening down there?
According to gastroenterologist Dr. Satish S.C. Rao, your body is going to fight you all the way to expel that excess gas, and it’s a fight your gut is going to win. “The gas in your gut is a mix of nitrogen, hydrogen, oxygen, carbon dioxide, methane and trace gasses like hydrogen sulphide and some other volatile gasses,” he says. “Some of these, like hydrogen, methane and oxygen, can be absorbed by the gut, but nitrogen cannot be absorbed, and the body will find a way to get rid of it. Sooner or later, it’s going to come back at you—it’ll get stronger and stronger until the bowel vigorously contracts to overcome the anal sphincter resistance.”
The really bad news? By holding in the fart, all you’ve really done is make it more powerful. As the volume of gas increases, the pressure builds, and what eventually escapes is the original fart’s louder, smellier big brother. “If there is a significant volume of gas, the rectum will distend quickly in response,” says Rao. “The anal sphincter will have very little holding ability—it’ll have some, but really not a lot. Eventually, it will make its way out.”
But wait, it gets worse! Not all gas is expelled through the anus: Some will come out of your mouth. While Rao is quick to point out that there are many more likely explanations for bad breath—including issues with dental hygiene; fermentation caused by bacteria in the mouth; problems with the sinus; and chronic infections in the lungs—there are two ways in which your gas can affect your breath. The first of these is when gas is absorbed through the lining of the gut into the bloodstream. From there, it makes its way to the lungs, and is then exhaled.
The other way is far more gross, as it involves gas literally bubbling up from your intestines and coming out as a burp (unlike your regular burps, which are made up of significantly less smelly gas from the stomach). “If you’ve eaten a carbohydrate-rich meal, this will be fermented in the small intestine [the section immediately connected to the stomach, which leads to the large intestine],” says Rao. “You’ll belch this gas out.” That’s because, at this point in the digestive process, the gas is still much closer to your mouth than your anus—where gas from the large intestine is expelled—and so it takes the path of least resistance. “When you do this, this fermented gas is going to smell like fermented gas,” says Rao. In other words: Fart-burps. We’re sorry to have ruined your day.