Flatulence: Why You Have So Much Gas and What Helps

Where gas comes from

Gas in the bowel comes from two sources:

1. Swallowed air: while eating fast, drinking through a straw, chewing gum, smoking, drinking fizzy drinks, or with loose dentures. Most is belched out; some travels through.
2. Fermentation: bacteria in the large bowel break down carbohydrates that were not absorbed in the small intestine, producing hydrogen, carbon dioxide and, in some people, methane. Small amounts of sulfur-containing gases give flatus its smell.

Passing gas more after meals and in the evening is normal.

Common causes of excessive flatulence

Foods

Lactose intolerance

Many adults make too little lactase, the enzyme that digests milk sugar. Dairy then causes gas, bloating, cramps and loose stools within a few hours. It is especially common in people of East Asian, African, Hispanic and Native American descent. See lactose intolerance.

Increasing fiber

Adding fiber quickly causes gas for a few weeks while gut bacteria adapt. Increase gradually and drink enough water.

Constipation

Stool sitting longer in the colon ferments more, causing smelly gas and bloating.

Medical causes

• Irritable bowel syndrome: gas, bloating and belly pain linked to bowel movements, with diarrhea, constipation or both. People with IBS often feel normal amounts of gas more intensely.
• Small intestinal bacterial overgrowth (SIBO): excess bacteria in the small bowel ferment food early, causing bloating, gas and diarrhea.
• Celiac disease: immune reaction to gluten, with gas, diarrhea, weight loss and anemia.
• Fructose malabsorption and other carbohydrate intolerances.
• Pancreatic insufficiency: poor fat digestion causes greasy, foul, floating stools and gas.
• Giardia and other gut infections: sulfurous belching, gas and diarrhea after travel or drinking untreated water.
• Medicines: metformin, acarbose, lactulose, fiber supplements, some antibiotics and orlistat.

Why gas smells

Most gas is odorless. The smell comes from tiny amounts of hydrogen sulfide and other sulfur compounds, produced from protein and sulfur-rich foods such as eggs, meat, cabbage, garlic and onions, and from some supplements. Very foul gas with greasy stools suggests fat malabsorption. Sulfur-smelling belches with diarrhea suggest giardia.

How to reduce flatulence

Eating habits

• Eat slowly and chew well with your mouth closed.
• Avoid gum, straws and fizzy drinks.
• Eat smaller, more frequent meals.

Diet changes

• Keep a food and symptom diary for 2 weeks to find your triggers.
• Reduce the worst gas-producing foods, rather than cutting out all fiber.
• Soak and rinse dried beans, or use canned beans, and introduce them gradually.
• Try reducing dairy for 2 weeks if you suspect lactose intolerance.
• Avoid sugar-free products with sorbitol, mannitol or xylitol.
• For IBS, a low FODMAP diet guided by a dietitian can reduce gas and bloating substantially.

Over-the-counter products

• Lactase enzyme tablets or lactose-free milk for lactose intolerance.
• Alpha-galactosidase (taken with the first bite) for beans and vegetables.
• Simethicone may ease bloating by breaking up gas bubbles, though evidence is limited.
• Activated charcoal pads or underwear can reduce odor.

Follow label directions for all products.

Other measures

Regular exercise, especially walking after meals, helps move gas through the gut. Treating constipation reduces trapped gas and odor.

When to see a doctor

See a doctor if gas comes with weight loss, blood in your stool, ongoing diarrhea or constipation, severe pain, or if it has recently changed. They may test for celiac disease, check stool for infection, arrange breath tests for lactose intolerance or SIBO, or refer you for colonoscopy if you have warning signs. The flatulence condition page explains testing and medical treatment in more detail.

Frequently asked questions

How much gas is normal?

Most healthy people pass gas about 10 to 20 times a day, and up to about 25 times can be normal. The amount varies with diet. Gas is a sign of healthy gut bacteria fermenting fiber, and fiber-rich diets naturally produce more.

Why do I fart so much?

The most common reasons are eating foods that ferment in the gut (beans, cabbage family vegetables, onions, whole grains, sugar-free sweeteners), swallowing air, lactose intolerance, constipation and IBS. A food diary often reveals the main triggers.

Why does my gas smell so bad?

The smell comes from sulfur gases produced when gut bacteria break down sulfur-rich foods such as eggs, meat, cabbage, broccoli, garlic and onions. Constipation makes it worse. Very foul gas with greasy stools or diarrhea can mean malabsorption or infection and should be checked.

What foods cause the most gas?

Beans and lentils, cabbage, broccoli, cauliflower and Brussels sprouts, onions and garlic, wheat and bran, apples and pears, dairy (in lactose intolerance), fizzy drinks, beer, and sugar-free sweets containing sorbitol or xylitol are the most common culprits.

Can excessive gas be a sign of something serious?

Gas on its own is rarely serious. It is more concerning with weight loss, blood in the stool, persistent diarrhea, anemia, or a recent change in bowel habits, which can point to celiac disease, inflammatory bowel disease or, rarely, bowel cancer.