Vice talks to a proctologist:

Farts are made by two things. They are made by one, the amount of air you swallow–so people who drink a lot of soda, chew a lot of gum, suck on candies, they get a lot of air into their colon, and that air comes out in farts. The second component is gas production by the colon. The colon’s job is to break down the nutrients in food products, like proteins and fats and sugars, and in the process of breaking them down they produce either sulfur or methane, neither of which smell great.

If, let’s say, the colon has stuff in it like grapes and beans, and if it’s just sitting there for a few days it’s just going to ferment more and more until it becomes very smelly, versus if what you eat goes through quickly–like if you had the same beans, but it came out eight hours later, you’ll tend not to have as much gas from those beans. So it has to do with what your intestinal transit is. For most people, it takes 32 hours from the time they eat something to the time they shit something. That’s the average, so that means there are people who move their bowels every three or four days, and they have more time for the beans to ferment in the colon, thereby producing larger amounts of gas and more frequent, smellier spasms of gas.

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