Flatulence is the medical term for passing gas through the rectum. It’s a normal part of digestion that happens to every person, every day, typically between 14 and 23 times in a 24-hour period. The total volume of gas your body produces daily ranges from about 475 to 1,500 milliliters, with a median around 705 ml.
How Your Body Produces Gas
Gas in your digestive tract comes from two main sources. The first is swallowed air. Every time you eat, drink, or even breathe, small amounts of air enter your stomach. Most of this gets released upward as a burp, but some travels deeper into the intestines.
The second and larger source is bacterial fermentation in your large intestine. Trillions of bacteria live in your colon, and when undigested food residues arrive there, these bacteria break them down and release gas as a byproduct. The volume of gas produced depends on what you eat and which bacteria dominate your gut. Your personal mix of gut bacteria is relatively stable over time but can differ significantly from someone else’s, which is one reason the same meal can make one person gassy and barely affect another.
Once gas forms, your intestines actively push it along. This isn’t a passive process where gas simply floats through. Your gut propels gas independently of food and liquid, using subtle shifts in muscle tone: contracting behind the gas pocket and relaxing ahead of it. Interestingly, gas moves more efficiently when you’re upright than when you’re lying down, because your intestines have to work against the gas’s tendency to float upward.
What’s Actually in the Gas
A meta-analysis of intestinal gas composition found the average breakdown is roughly 65% nitrogen, 14% methane, 10% carbon dioxide, 3% hydrogen, and 2% oxygen. These five gases are all odorless, which is why most flatulence doesn’t actually smell.
The smell, when it happens, comes from trace sulfur-containing compounds. Hydrogen sulfide is the primary culprit, followed by smaller amounts of methanethiol and dimethyl sulfide. Research published in the journal Gut found that the intensity of the odor correlates directly with hydrogen sulfide concentration. These sulfur gases make up a tiny fraction of the total volume but have an outsized impact on how the gas smells.
Why Some Foods Cause More Gas
Foods rich in fermentable carbohydrates are the biggest drivers of flatulence. Beans are the classic example, but the category is much broader. Researchers use the term FODMAPs to describe a group of short-chain carbohydrates that resist digestion in the small intestine and arrive in the colon largely intact, where bacteria feast on them. Common high-FODMAP foods include onions, garlic, wheat, certain fruits, and dairy products (for people who don’t fully digest lactose).
In a controlled study, healthy volunteers on a high-FODMAP diet produced more than four times the intestinal hydrogen compared to a low-FODMAP diet (181 vs. 43 parts per million over 14 hours). Even healthy participants reported increased flatulence on the high-FODMAP diet. For people with irritable bowel syndrome (IBS), the effect was even more pronounced, with hydrogen levels climbing to 242 ppm and additional symptoms like bloating and abdominal pain appearing alongside the gas.
The specific bacteria in your colon also matter. A family of bacteria called Lachnospiraceae is strongly linked to hydrogen production. Methane production, on the other hand, doesn’t depend much on what you eat. It’s almost entirely determined by whether you carry a particular type of microorganism called Methanobacteria in your gut. About one in three people produces significant methane.
Swallowed Air and Everyday Habits
While bacterial fermentation accounts for most flatulence, swallowed air contributes too. Everyone swallows some air just by being alive, but certain habits increase the amount substantially:
- Eating too fast
- Talking while eating
- Chewing gum or sucking on hard candy
- Drinking through straws
- Drinking carbonated beverages
- Smoking
Stress and anxiety can also increase air swallowing, sometimes as an unconscious nervous habit of gulping. Poorly fitting dentures cause your mouth to produce more saliva, which triggers more frequent swallowing and more air intake. People who use CPAP machines for sleep apnea sometimes experience excess gas for the same reason: the machine delivers more air than the body can easily absorb.
When Flatulence Signals Something Else
Flatulence by itself is almost never a sign of a serious problem. But when it’s persistent, excessive, and accompanied by other symptoms like bloating, cramping, diarrhea, or unexplained weight loss, it can point to an underlying condition.
Lactose intolerance and fructose intolerance are among the most common causes of excess gas. In both cases, your body lacks the ability to fully break down a specific sugar, leaving it for bacteria to ferment. Celiac disease, an autoimmune reaction to gluten, can produce similar symptoms. Small intestinal bacterial overgrowth (SIBO), where bacteria colonize parts of the small intestine where they don’t normally thrive, produces extra gas along with diarrhea and sometimes weight loss.
IBS is another frequent culprit. People with IBS don’t necessarily produce more gas than average, but their intestines are more sensitive to normal amounts of gas, leading to greater discomfort. More rarely, persistent gas symptoms can be associated with gastroparesis (slow stomach emptying), constipation, or even a blockage in the digestive tract.