Think of your body as keeping a daily gas budget. Some gas comes in, some is made inside, and some leaves. Not all gas is passed. A lot of it is absorbed into the blood or burped up before it ever leaves through the bottom. Burping and passing gas are natural and common, and everybody does both.
Gas in the digestive tract has two main sources: air that is swallowed, and gas made when bacteria in the large intestine break down food. Think of the first as a delivery and the second as something made at home. Gas leaves the body in two ways: out of the mouth as a burp, or out of the bottom as a fart.
Start with the delivery. You swallow a little air every time you eat and drink. Eating or drinking fast makes you swallow extra air, and so does talking while you eat. Chewing gum and sucking on hard candy make you swallow more often than normal, and part of what you swallow is air. Most swallowed air comes back up as burps.
Some of the air takes a different road. A little swallowed air can skip the burp and move on into the intestines, but most of that small amount is soaked up into the blood instead of coming out. People burp up to about 30 times a day, and burps come most often soon after eating. Fizzy drinks have gas bubbles in them, and those bubbles can come back up as burps.
Now the homemade gas. Tiny helpful germs called bacteria live in your large intestine. Bacteria ferment food: they break down food that was not digested earlier and give off gas as a byproduct. Most of the gas that comes out of the bottom is made this way, not swallowed. The bacteria always make some gas. When there is more undigested carbohydrate for them to work on, there tends to be more gas.
What reaches the bacteria? Food carbohydrates come as sugars, starches and fiber. The stomach and small intestine cannot fully digest some of them, so they travel on to the large intestine. That is why foods with fiber, like beans and cabbage, make more gas. The longer food stays in the large intestine, the more time the bacteria have to work, so more gas can build up.
Now for the numbers. Studies of how much gas people pass give this: on average people pass about 600 to 700 milliliters of gas a day. That sits inside the usual range of half a liter to one and a half liters. Most people pass gas up to about 20 times a day, and more often around mealtimes and just after eating.
What is in that gas? Gas is a mix of gases: carbon dioxide, nitrogen, oxygen and hydrogen, and sometimes methane. About three quarters of fart gas (74 percent) is hydrogen, methane and carbon dioxide made by bacteria in the large intestine as they ferment food. Most of these gases have no smell at all.
So what makes the smell? You may have heard that gas is mostly stink. In fact, a small part of fart gas is hydrogen sulfide. It smells like rotten eggs and is made by the microbes in the large intestine. How strong a fart smells goes along with how much hydrogen sulfide is in it, a gas that contains sulfur. Some foods, such as cabbage and meat, can make gas smell stronger.
Last, who has the biggest budget? It depends on the person. What counts as a normal amount of farting is different for every person. Two things change how much gas a person has: how much air they swallow, and which foods they eat. Some people have extra gas after milk, cheese or ice cream, because their bodies have trouble digesting lactose, the sugar in milk.
Words to know
- absorbed: soaked in
- ferment: break down food
- carbohydrate: sugar or starch
- hydrogen: a light gas
- lactose: milk sugar
