Intestinal gas comes from two main sources: air you swallow and gas produced by bacteria in your large intestine. Everyone passes gas at least 14 times a day, and the mix of what creates that gas varies surprisingly from person to person. Five odorless gases (nitrogen, oxygen, carbon dioxide, hydrogen, and methane) account for 99% of what you pass. The smell comes from trace sulfur compounds that make up the remaining 1%.
Swallowed Air
Every time you eat, drink, or swallow saliva, a small amount of air travels down into your stomach and intestines. Most of this air is nitrogen and oxygen. Some gets burped back up, but the rest moves through your digestive tract and eventually exits as gas.
Certain habits increase how much air you swallow:
- Eating too fast
- Talking while eating
- Chewing gum or sucking on hard candy
- Drinking through a straw
- Drinking carbonated beverages
- Smoking
Carbonated drinks add an extra layer because the carbon dioxide dissolved in the liquid releases inside your stomach. If you’ve ever noticed more burping or bloating after soda or sparkling water, that’s a direct result of all that extra CO2.
Bacterial Fermentation in the Colon
The bigger source of lower intestinal gas is the trillions of bacteria living in your large intestine. When carbohydrates and fibers pass through your small intestine without being fully absorbed, they reach the colon, where bacteria break them down through fermentation. This process generates hydrogen, carbon dioxide, and in some people, methane. About 29% of people produce methane; the rest don’t, which is one reason gas volume and composition vary so much between individuals.
The variation in gas makeup is dramatic. Nitrogen can range from 11% to 92% of a single episode of flatulence, hydrogen from 0% to 86%, and methane from 0% to 54%. Your personal mix depends on what you ate, which bacteria dominate your gut, and how completely your small intestine absorbed your last meal.
Foods That Produce the Most Gas
Not all carbohydrates ferment equally. The biggest gas producers are a group of short-chain carbohydrates that your small intestine absorbs poorly, leaving them as fuel for colonic bacteria. These include:
- Fructans: found in wheat, onions, and garlic
- Galacto-oligosaccharides: found in beans and legumes
- Lactose: the sugar in milk and dairy, especially problematic if you’re lactose intolerant
- Excess fructose: found in mangoes, figs, honey, and high-fructose corn syrup
- Sugar alcohols: sorbitol, mannitol, xylitol, and maltitol, common in sugar-free gums and candies
These carbohydrates don’t just produce gas. They also pull water into the intestine through osmosis, which is why gas-heavy foods often cause bloating and loose stools at the same time. The combination of extra water and gas stretches the intestinal walls, creating that uncomfortable pressure feeling.
What Makes Gas Smell
The five main gases in flatulence are all odorless. The smell comes entirely from sulfur-containing compounds, primarily hydrogen sulfide, methyl mercaptan, and dimethyl sulfide. These exist in tiny concentrations, measured in parts per billion, but your nose is remarkably sensitive to sulfur. Foods high in sulfur, like eggs, cruciferous vegetables (broccoli, cabbage, cauliflower), and meat, tend to produce the most pungent gas. If your gas is voluminous but doesn’t smell, it’s likely dominated by hydrogen or carbon dioxide from carbohydrate fermentation rather than sulfur-rich protein breakdown.
Where Gas Goes After It Forms
Not all intestinal gas exits as flatulence. A large portion of hydrogen, methane, and carbon dioxide gets absorbed through the intestinal wall into your bloodstream. The blood carries these gases to your lungs, where you exhale them. This is actually how doctors test for certain digestive conditions: breath tests can measure hydrogen and methane levels to assess what’s happening in your gut. Carbon dioxide produced in the upper small intestine is especially likely to take this route, getting reabsorbed before it ever reaches the colon.
The gas that isn’t absorbed continues moving through the intestines and is released as flatulence, or in some cases, trapped temporarily, causing bloating and discomfort.
When Gas Points to a Bigger Problem
A noticeable increase in gas, especially paired with bloating, cramping, or diarrhea, can signal that something beyond normal digestion is going on. One common culprit is small intestinal bacterial overgrowth, or SIBO, where bacteria that normally live in the colon colonize the small intestine instead. Because the small intestine isn’t designed to host large bacterial populations, even ordinary meals get fermented too early in the digestive process, producing excess gas higher up in the gut.
SIBO is more common in people with conditions like Crohn’s disease, celiac disease, chronic pancreatitis, or liver disease. It also occurs more frequently after surgeries that remove the valve between the small and large intestine. Lactose intolerance and fructose malabsorption are simpler explanations for excess gas: your small intestine can’t break down specific sugars, so they pass intact to the colon where bacteria ferment them aggressively.
A sudden, sustained change in how much gas you produce, or a shift in its character (persistent foul smell, constant bloating, pain), is worth investigating. In most cases, though, gas is simply the byproduct of a healthy gut doing exactly what it’s supposed to do: feeding trillions of bacteria that help you digest food.