Farts come from two sources: air you swallow and gas produced by bacteria in your large intestine. Most people pass gas about 15 times a day, though anywhere from 3 to 40 times falls within the normal range. Your body produces between 400 and 2,000 milliliters of gas daily, and nearly all of it is odorless.
Swallowed Air
Every time you eat, drink, or swallow saliva, a small amount of air travels down into your digestive tract. Most of it comes back up as a burp, but whatever makes it past the stomach continues through the intestines and exits the other end. Certain habits increase the amount of air you swallow: eating too fast, talking while eating, chewing gum, sucking on hard candy, drinking through a straw, and drinking carbonated beverages. Smoking also introduces extra air into the gut.
Stress and anxiety can amplify the problem. Both affect your breathing rate and can create a nervous swallowing pattern that pulls in more air than usual. Ill-fitting dentures have a similar effect because they trigger extra saliva production, which means more frequent swallowing.
Bacterial Fermentation in the Colon
The bigger source of gas is your gut bacteria. Your small intestine absorbs most nutrients from food, but certain carbohydrates resist digestion and pass into the colon intact. There, trillions of bacteria break them down through fermentation, producing hydrogen, carbon dioxide, and sometimes methane as byproducts. This is the same basic process that makes bread rise or beer fizzy, just happening inside your body.
Five odorless gases make up about 99% of every fart: nitrogen, oxygen, carbon dioxide, hydrogen, and methane. The exact mix varies wildly from person to person. Nitrogen can range from 11% to 92% of the total, hydrogen from 0% to 86%, and methane from 0% to 54%. Some people produce virtually no methane at all, depending on which bacterial species dominate their gut.
What Makes It Smell
The remaining 1% is what you actually notice. The characteristic rotten-egg smell comes from hydrogen sulfide, a colorless, pungent gas produced when gut bacteria break down sulfur-containing amino acids from your food. Bacteria in the genera Fusobacterium, Desulfovibrio, and others generate hydrogen sulfide either by dismantling sulfur-rich proteins or by chemically reducing sulfate, a compound found naturally in many foods and drinking water.
Foods high in sulfur compounds tend to produce smellier gas. Eggs, cruciferous vegetables like broccoli and cabbage, garlic, onions, and red meat are common culprits. The volume of gas you produce and the odor intensity are controlled by completely separate mechanisms, which is why a large, loud fart can be odorless while a small, quiet one clears the room.
Foods That Produce the Most Gas
The foods that generate the most flatulence are those rich in fermentable carbohydrates, often grouped under the acronym FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols). These short-chain carbohydrates are poorly absorbed in the small intestine, so they arrive in the colon where bacteria feast on them and produce gas.
- Legumes and lentils are high in a carbohydrate called galacto-oligosaccharides that humans lack the enzyme to break down.
- Dairy products contain lactose, which causes gas in anyone who doesn’t produce enough of the enzyme lactase.
- Wheat, rye, and barley contain fructans, another fermentable carbohydrate.
- Certain fruits like apples, pears, and watermelon are high in excess fructose and sorbitol.
- Vegetables like onions, garlic, asparagus, and artichokes are particularly rich in fructans and mannitol.
- Sugar-free candy and gum contain sugar alcohols like sorbitol and xylitol that travel to the colon largely undigested.
Sugar Alcohols and Artificial Sweeteners
Sugar-free products deserve their own mention because they catch so many people off guard. Sugar alcohols like sorbitol, xylitol, and erythritol are only partially absorbed in the small intestine. Whatever isn’t absorbed reaches the colon, where bacteria ferment it into gas. At high levels, sorbitol can cause bloating, cramps, and diarrhea on top of the flatulence.
Research from UC Davis found that specific gut bacteria (from the Clostridia group) normally help break down sorbitol before it causes trouble. People who lack enough of these bacteria are more sensitive to even small amounts of sugar alcohols. This explains why some people can chew sugar-free gum without issue while others feel bloated after a single piece.
Lactose Intolerance and Enzyme Gaps
Your body needs specific enzymes to break down certain sugars before they reach the colon. When those enzymes are missing or in short supply, the undigested sugar passes straight to your gut bacteria, which ferment it and produce gas. Lactose intolerance is the most common example. People with this condition don’t produce enough lactase, the enzyme that splits lactose into simpler sugars for absorption. When they consume dairy, symptoms like bloating, gas, and cramping typically start within 30 minutes to 2 hours.
Roughly 68% of the global population has reduced lactase production after childhood, making this a widespread cause of gas. A similar mechanism applies to beans: humans don’t produce alpha-galactosidase, the enzyme needed to break down the complex sugars in legumes, so those sugars always end up being fermented by colonic bacteria.
Medical Conditions That Increase Gas
When gas becomes excessive or painful, a medical condition may be amplifying the normal process. Small intestinal bacterial overgrowth (SIBO) occurs when bacteria that normally live in the colon colonize the small intestine in large numbers. Food gets fermented earlier in the digestive process than it should, producing gas in a part of the gut that isn’t designed to handle it. SIBO can be triggered by anything that slows the movement of food through the intestines: diabetes, certain autoimmune conditions, prior abdominal surgery, or long-term use of acid-suppressing medications.
Irritable bowel syndrome (IBS) often involves heightened sensitivity to normal amounts of gas combined with increased fermentation of FODMAPs. People with IBS don’t necessarily produce more gas than average, but their intestines react more strongly to the stretching that gas causes. Celiac disease damages the lining of the small intestine, reducing its ability to absorb nutrients and allowing more undigested food to reach the colon for fermentation. Crohn’s disease can have a similar effect and sometimes coexists with SIBO.
Why Some People Fart More Than Others
The variation in gas production between individuals is enormous, and it comes down to three factors: diet, gut bacteria, and digestive efficiency. Two people can eat the same meal and produce vastly different amounts of gas because their microbial populations differ. Your personal collection of gut bacteria is as unique as a fingerprint, shaped by your genetics, diet history, antibiotic use, and environment.
Fiber intake plays a major role. A sudden increase in fiber, even from healthy foods like whole grains and vegetables, can temporarily spike gas production because your gut bacteria need time to adjust to the new substrate. People who eat high-fiber diets consistently tend to produce gas more efficiently, with less bloating, because their bacterial populations have adapted. Speed of eating matters too. Bolting down a meal introduces more swallowed air and sends larger, less thoroughly chewed food particles to the colon, giving bacteria more material to ferment.