It's not exactly dinner time conversation, but it's probably not dinner time, unless you watch SciShow while you're eating dinner. In which case, I applaud you. But let's get it out there, we all burp, we all fart. Sometimes we do these things loudly and often times they smell. But why do we do these things and why do some foods and drinks exacerbate the problem?

(00:21) Formally referred to as belching and flatulence, these two often lampooned bodily functions actually share a common cause: swallowed air.

Every time you swallow you take in some air, but you can take in even more air than normal if you eat or drink too quickly, chew gum, smoke, drink carbonated beverages, or even wear loose fitting dentures. Now, the majority of swallowed air comes back up in the form of burps. As the air builds up in the upper portion of the stomach, it causes stretching that eventually triggers the lower esophageal sphincter to relax. The result is air escaping up the esophagus, into the mouth, where the sound and smell of the burp depends on how much Coke you just chugged.

Babies, in particular, are big burp-ers, especially for their small size. Mostly because they tend to gulp in too much air while nursing. And because young digestive systems haven't developed to the point where babies can easily burp on their own, it's up to mom and dad to pat their backs, help get those gas bubbles out. But for babies and adults, what about the air that isn't burped out?

Well, it passes from the stomach into the small intestine, and later the large intestine. Along the way, as it passes through the gastrointestinal tract that air mixes with the product of some good old bacteria fermentation that's going on in your guts. That fermentation is the result of carbohydrates, like sugars and starches, that can't be digested by enzymes in the small intestines. Some foods that include these hard to digest starches include: cabbage, cauliflower, beans, and bran which explains why you may feel excessively gassy after that big bowl of chili.

When those undigested carbs reach the colon and the lower intestines, bacteria take over. The by-product is a combination of gases that include: carbon dioxide, hydrogen, methane, even hydrogen sulfide, the stinky one. That mix of gases, plus the air that you've swallowed is what makes its way through the large intestine, to the anus, where it is expelled.

Of course, everyone has different amounts of various species of bacteria and yeasts working away in their guts, but I'm sure you're dying to know, so I'll just tell you. The average human toot contains roughly 59% nitrogen, 21% hydrogen, 9% CO₂, 7% methane, and 4% oxygen. It's that last 1% in the form of hydrogen sulfide and other sulfur compounds that is the most potent in terms of producing those notorious, unpleasant, rotten egg smells.

While some of these chemicals are produced by bacteria in your gut, eating sulfur-rich foods, like eggs and onions and beans... That doesn't help on the smelly front.

So, be sure to trundle out these factoids at your next dinner party and thank you for watching this episode of SciShow. And an extra sweet smelling thank you to all of you Subbable subscribers! For a second, I thought it said sweat smelling. You don't smell like sweat!
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