Gas can leave your body in two ways: out of the mouth as a burp, or out of the bottom as a fart. Everybody has gas, and everybody farts and burps. Belching releases gas from the upper digestive tract, the esophagus and stomach. Passing gas, which is called flatulence, happens in the lower digestive tract. Both are normal parts of how a body works.

Start at the top. You swallow a little air every time you eat and drink. You may have heard that burps come from the food in your stomach. In fact, most belching comes from swallowed air, and that air most often never reaches the stomach and instead builds up in the esophagus, the tube that leads to the stomach. Some people burp a lot because they swallow too much air and release it before it reaches the stomach.

Both exits are run by muscles. When you burp, two sphincters, which are ring-shaped muscles, take turns. Measurements show that the lower one, near the stomach, loosens first, and then the upper one, near the throat, relaxes and lets the gas out. A fart is a muscle teamwork event too. Pressure tests show waves of squeezing in the colon, rising pressure in the rectum, and the anal sphincter relaxing early, which lets gas out and can be done on purpose or without trying.

Gas does not leave the gut in a smooth stream, and in experiments where gas was fed into the gut, gas collected from the rectum came out in pulses, which suggests the gut's own muscle patterns control the flow. Eating speeds gas along, and experiments found that liquid or solid meals make gas move through the gut faster, matching what people notice: more burps and farts after meals.

You may have heard that gas means something is wrong. Healthy people make a lot of gas every day, and without a way out, gas trapped in the digestive system causes bloating and swelling of the belly. In fact, passing gas is rarely a sign of a medical issue.

Most gas that is passed comes from harmless bacteria in the large intestine, tiny helpful germs. They break down food your body could not digest, which is called fermenting, and they give off gas as a byproduct. The longer food stays in the large intestine, the more time bacteria have to ferment it, so more gas can build up. Hydrogen is a big part of the story. Gut bacteria that ferment food can make a lot of hydrogen, and one review reports that up to 1 liter of hydrogen gas can be produced in the gut in 24 hours. Some gut microbes use hydrogen up. These hydrogen-using bacteria stop it from building up in the colon, where it would slow the fermenting done by other microbes.

Which foods feed the bacteria? Some carbohydrates, which are sugars, starches and fiber, are not digested or absorbed in the small intestine. They move on to the large intestine, where bacteria break them down and make gas. When there is more undigested carbohydrate for the bacteria to work on, there tends to be more gas. Most foods with carbohydrates can cause gas, but fats and proteins cause little gas. Foods with fiber, like beans and cabbage, make more gas because the body cannot fully digest fiber.

Starches hold a surprise. Most starches, including potatoes, corn, noodles and wheat, produce gas as they are broken down in the large intestine. Rice is the one starch that does not. Almost anyone who eats large amounts of vegetables or fruits gets some extra gas.

Milk sugar is called lactose, and fruit sugar is called fructose. Both can cause gas in some people. Some people have extra gas after milk, cheese or ice cream, because their bodies have trouble digesting the sugar in milk.

What about the smell? Most gas has no smell at all. A small part of fart gas is hydrogen sulfide, and it smells like rotten eggs. How strong a fart smells goes along with how much hydrogen sulfide is in it.

Where does the rotten-egg gas come from? It starts with food and microbes. Sulfate can come from several foods, and gut bacteria called sulfate reducers use hydrogen to turn that sulfate into hydrogen sulfide. Besides the rotten-egg gas, farts can contain other gases that have sulfur in them.

Which foods are behind the smell? Foods high in sulfur, like proteins and cabbage-family vegetables such as cauliflower, cabbage and broccoli, are usually behind foul-smelling gas. Asparagus, beans, cabbage, coffee, dairy products, eggs, onions and prunes are foods linked to smelly gas.

Words to know

• esophagus: food tube
• sphincter: ring muscle
• bloating: belly swelling
• fructose: fruit sugar
• hydrogen: a light gas
• fermenting: breaking down food