Flatulence, or farting, is how your body releases gas from your lower gastrointestinal (GI) tract, and it happens to everyone about 10 to 25 times per day. Most of the time, it’s odorless, but other times, you might stop to ask, “Why do my farts smell so bad?”
The answer to that question, and the causes of smelly farts, can vary, as several factors, especially diet, can cause a foul smell.
What Causes Farts to Smell Bad?
Intestinal gas (flatus) comes partly from air that is swallowed and partly from the breakdown of food in the digestive tract.
“The primary cause of flatulence is the type of food we ingest,” says Adrienna Jirik, MD, a Cleveland Clinic gastroenterologist. “Foods that are not well digested are fermented in the GI tract by eager gut bacteria, which then produce uncomfortable and often smelly gas as a byproduct.”
What you eat determines which species of bacteria thrive in the GI tract, and different types of gut bacteria produce different gases. Those gases determine the odor of flatulence. About 99% of gas consists of odorless nitrogen, carbon dioxide, hydrogen, and methane, but the remaining 1% accounts for 100% of the odor: indole, skatole, and sulfur compounds. Indole and skatole create a fecal-like smell, while hydrogen sulfide is responsible for the characteristic rotten egg odor.Dimethyl sulfide causes a cabbage-like odor, and methanethiol produces a smell like rotting vegetables or garlic.
Common Reasons for Smelly Farts
A number of foods, medical conditions, and other factors can contribute to smelly flatulence. Here’s a look at some of these culprits.
Foods That Cause Smelly Gas
Foods that contain short-chain carbohydrates, which are frequently referred to as high FODMAP foods (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) are difficult to digest and common culprits of flatulence. They include:
- some fruits like apples, pears and cherries
- dairy products
- alcohol
- wheat-containing products and bread
- onions
- beans and legumes
- artificial sweeteners.
Sulfur-rich foods and gas are related, too. Foods such as eggs, meat, and cruciferous vegetables, including broccoli, cabbage and cauliflower, may make odor more noticeable. High-fiber foods can increase the amount of gas.
“What many patients are surprised to learn about is that a diet considered healthy, which includes foods high in fiber, such as beans, lentils, and grains, can also cause gas and flatulence through the same mechanism,” Dr. Jirik says.
Food Intolerances and Sensitivities
Food intolerances and flatulence often go together. With lactose intolerance, for example, the small intestine does not produce enough lactase to fully digest the sugar in milk. The undigested lactose reaches the colon, where bacteria ferment it and produce gas. Gluten intolerance and celiac disease cause gas because the body cannot properly break down the proteins found in wheat, barley, and rye, leading to bacterial fermentation in the gut.
People with irritable bowel syndrome may also be especially sensitive to normal amounts of gas. “Patients with irritable bowel syndrome frequently accompanied by spasms that can be precipitated by eating high-FODMAP foods, may experience augmented symptoms of bloating and flatulence, compared to those who do not have underlying IBS,” Dr. Jirik says.
Constipation and Digestive Slowdown
Constipation and gas odor are linked. Constipation can worsen gas odor because stool and fermenting material remain in the colon longer. A digestive slowdown may also cause bloating, pressure, and more noticeable flatulence. Adequate water, regular movement, and enough dietary fiber can help prevent constipation, although suddenly adding large amounts of fiber may temporarily increase gas. Increase your fiber intake slowly to avoid discomfort.
Medication Side Effects and Gas
Many medications can cause gas as a side effect, including aspirin, antacids, diarrhea medicines, opioid pain medicines, fiber supplements, multivitamins, and iron pills.
Aspirin may irritate the digestive tract, which can contribute to bloating and gas in some people.
Antacids, particularly those containing calcium carbonate, can release carbon dioxide during digestion or alter stomach acidity, leading to increased gas.
Diarrhea medicines, such as loperamide, slow intestinal movement, allowing more time for bacteria to ferment food and produce gas.
Opioid pain medicines slow digestion and frequently cause constipation, which can trap gas and increase bloating.
Fiber supplements are fermented by gut bacteria, producing gas as a natural byproduct, especially when first introduced or taken in large amounts.
Multivitamins, particularly those containing certain minerals or sugar alcohols, may cause digestive upset and gas in some people.
Iron supplements commonly cause constipation, which can slow the passage of gas through the intestines.
Antibiotics may temporarily disrupt the balance of gut bacteria, leading to increased gas production until the microbiome recovers.
Bacterial Overgrowth and Flatulence
Bacterial overgrowth and flatulence are another possible combination.
“Patients with overgrowth of bacteria in their small bowel that normally reside in the large intestine or colon can experience excessive bloating and gas,” Dr. Jirik says. “Similar symptoms can also be found in patients with exocrine pancreatic insufficiency.”
Rare Causes: When to Be Concerned
By itself, smelly gas is not a cause for concern. But a sustained change in flatulence, together with unexplained weight loss, anemia, rectal bleeding or a marked change in bowel habits, can occur with colorectal cancer or another serious condition. Persistent gas may also accompany inflammatory bowel disease, an intestinal obstruction, or a malabsorption disorder. The accompanying symptoms, rather than odor alone, are the important clue.
How to Prevent Smelly Farts
You can make several lifestyle and behavioral changes to help limit or avoid smelly flatulence:
- Look to your diet first. “Keep track of everything you are eating and drinking with a food diary,” Dr. Jirik says. “Stepwise elimination of culprit foods can then follow.” Record meals, symptoms and timing for a week or two. A physician or registered dietitian can help plan a short, structured low-FODMAP trial when appropriate, followed by gradual reintroduction. If dairy seems to be the problem, try a lactase enzyme supplement to help break down lactose whenever you ingest dairy, Dr. Jirik says. Beano is an over-the-counter digestive enzyme supplement that containsalpha-galactosidase, an enzyme that helps break down complex carbohydrates found in foods such as beans, lentils, broccoli, cabbage, Brussels sprouts, and whole grains. By digesting these carbohydrates before they reach the large intestine, Beano can help reduce gas and bloating.
- Address air swallowing. Eat slowly, chew thoroughly, and limit habits that increase swallowed air, such as chewing gum, drinking through straws, and drinking carbonated beverages. “Certain behaviors such as talking a lot while eating, excessive sighing, gum chewing, and using a BiPAP or CPAP machine can cause something called aerophagia, or air swallowing, that can also contribute to increases in flatulence, belching, and bloating just from all of the extra ingested air itself,” Dr. Jirik says. CPAP or BiPAP users should ask their clinician about fit or settings rather than discontinuing treatment.
- Move more. Regular activity can help gas and stool move through the intestines. “Walking after eating can be helpful in decreasing GI tract hypersensitivity and moving excess air out of the GI tract,” Dr. Jirik says.
- Drink water. Water helps move food and waste through the intestines, reducing constipation and allowing gas to pass more easily. Staying hydrated is particularly important when increasing fiber.
When to See a Doctor for Smelly Gas
Make an appointment if unusually foul-smelling or excessive gas persists despite reasonable dietary and habit changes, or if it repeatedly interferes with daily life.
“If flatulence is accompanied by any alarm signs, such as change in bowel habits with significant diarrhea or constipation, rectal bleeding, worsening abdominal pain or distension, weight gain or weight loss, nausea, or vomiting, I would advise seeking a consultation with gastroenterology for further workup,” Dr. Jirik says. Severe or rapidly worsening abdominal pain, repeated vomiting, a markedly swollen abdomen, inability to pass stool or gas, black stools, or substantial bleeding warrant prompt medical attention.
While scientists haven’t yet fulfilled Benjamin Franklin’s satirical request to find a way to make flatulence “not only inoffensive, but agreeable as Perfumes,” they can confirm that the answer to “Why do my farts smell so bad?” is just food, gut bacteria, and digestion doing their normal work.
