The smell of your farts comes almost entirely from sulfur-containing gases, and they make up a tiny fraction of each one. On average, hydrogen sulfide, methanethiol, and dimethyl sulfide together account for just 50 parts per million of any given fart. But these compounds are so potent that even trace amounts dominate what your nose picks up. When your farts smell worse than usual, something has shifted the balance of sulfur production in your gut.
What Makes Farts Smell
Most intestinal gas is odorless. Carbon dioxide, hydrogen, and methane make up the bulk of what you pass, and none of them have a scent. The stink comes from a small cocktail of sulfur compounds. Hydrogen sulfide produces the classic rotten-egg smell. Methanethiol smells like rotten cabbage. Dimethyl sulfide has a garlic-like quality. On top of those, compounds called indole and skatole add a fecal note to the mix.
Interestingly, researchers analyzing flatulence have also found pleasant-smelling molecules like limonene (a citrus compound) and pinene (found in pine trees). But even at very low concentrations, the sulfur compounds and skatole overpower everything else. Your nose is extraordinarily sensitive to hydrogen sulfide in particular, detecting it at levels far below what would be dangerous.
Sulfur-Rich Foods Are the Biggest Trigger
Hydrogen sulfide can only be produced if your food contains sulfur. That’s the single most important thing to understand about foul-smelling gas. Sulfur shows up in two main places in your diet: the amino acids cysteine and methionine (both found in protein-rich foods) and certain vegetables.
Cruciferous vegetables like broccoli, cauliflower, cabbage, kale, and Brussels sprouts are well-known offenders. These foods contain sulfur compounds that gut bacteria break down, releasing hydrogen sulfide in the process. Garlic, onions, and eggs are also high in sulfur.
Red meat, dairy, and other animal proteins are rich in cysteine and methionine. When you eat more protein than your body absorbs in the small intestine (roughly more than one gram per kilogram of body weight per day), the excess travels to your colon. There, bacteria feast on it and produce hydrogen sulfide as a byproduct. This is why high-protein diets, protein shakes, and meat-heavy meals often lead to particularly foul gas.
Your Gut Bacteria Decide the Smell
The specific bacteria living in your colon determine how much sulfur gas gets produced from the food you eat. Some species, including members of the Desulfovibrio genus, specialize in converting sulfate into hydrogen sulfide. Others, like certain strains of Fusobacterium and Escherichia, break down sulfur-containing amino acids for energy and release hydrogen sulfide as waste. The more of these sulfur-processing bacteria you have, the worse your gas will smell on any given diet.
Your microbiome composition shifts based on what you eat over time. A diet consistently high in animal protein and low in fiber tends to favor sulfur-producing bacteria. Fiber-rich diets promote bacteria that produce more carbon dioxide and hydrogen instead, gases that are odorless. This is why a major dietary change (switching to a high-protein diet, for example) can make your farts noticeably worse for a while as your bacterial population adjusts.
Food Intolerances Make It Worse
If your body can’t properly digest certain foods, those foods reach your colon intact and give bacteria extra material to ferment. Lactose intolerance is a common example. People with low levels of lactase (the enzyme that breaks down milk sugar) can’t fully digest dairy in their small intestine. The undigested lactose passes into the colon, where bacteria break it down and create excess fluid and gas. The result is more frequent, often smellier flatulence along with bloating and diarrhea.
Fructose malabsorption works similarly. So does difficulty digesting certain complex sugars found in beans, lentils, and soy products. Your body doesn’t produce the enzyme (alpha-galactosidase) needed to break down these sugars in the small intestine, so they arrive in the colon undigested and become fuel for gas-producing bacteria. Over-the-counter enzyme supplements like Beano supply this missing enzyme and can reduce gas from these specific foods if taken before a meal.
Other Common Causes
Constipation gives bacteria more time to ferment food in your colon, which increases both the volume and the sulfur content of your gas. If you’ve noticed your farts smell worse when you haven’t had a bowel movement in a day or two, that’s the likely reason.
Swallowed air doesn’t typically cause smelly gas (it’s just nitrogen and oxygen), but carbonated drinks and beer introduce carbon dioxide and can increase overall gas volume, making the smell more noticeable simply because you’re passing gas more often. Alcohol itself can also disrupt your gut bacteria and speed up digestion, pulling more undigested material into the colon.
Certain medications and supplements alter gut chemistry. Iron supplements are notorious for causing foul-smelling gas and dark stools. Some antibiotics temporarily wipe out beneficial bacteria, allowing sulfur-producing species to take over. Artificial sweeteners like sorbitol and sugar alcohols found in “sugar-free” products are poorly absorbed and heavily fermented by gut bacteria.
How Much Gas Is Normal
Recent research using sensor-equipped underwear found that people pass gas an average of 32 times per day, roughly twice the 14 times per day cited in older medical textbooks. Individual variation is enormous: some participants recorded as few as 4 episodes daily, while others hit 59. So if you feel like you’re gassing more than everyone around you, you might be right, but you’re also probably still within a normal range.
The smell matters more than the frequency. Occasional foul-smelling gas after a high-protein meal or a plate of cruciferous vegetables is completely expected. Persistently terrible-smelling gas that doesn’t change regardless of what you eat could point to a food intolerance, bacterial overgrowth, or a digestive condition worth investigating.
How to Reduce the Smell
The most direct approach is reducing sulfur intake. Cut back on red meat, eggs, dairy, and cruciferous vegetables for a few days and see if the smell improves. If it does, you’ve identified your trigger and can adjust portions rather than eliminating these foods entirely.
Increasing fiber gradually (not all at once) encourages the growth of bacteria that produce odorless gases instead of hydrogen sulfide. Whole grains, fruits, and non-cruciferous vegetables are good starting points. The key word is gradually. A sudden spike in fiber will temporarily make gas worse before your microbiome adapts.
If beans and legumes are a major part of your diet, enzyme supplements containing alpha-galactosidase can help your body break down the complex sugars before they reach your colon. These work best when taken right before eating.
Staying hydrated and physically active both help keep digestion moving, which reduces the time bacteria have to ferment food and produce sulfur compounds. Even a daily walk can make a measurable difference in transit time.
Signs Something Else Is Going On
Foul-smelling gas by itself is rarely a sign of a serious problem. But if it shows up alongside other symptoms, it’s worth paying attention. Persistent diarrhea, unexplained weight loss, blood in your stool, ongoing abdominal pain, or a lasting change in bowel habits can signal conditions like celiac disease, inflammatory bowel disease, or infections that need proper evaluation. A combination of terrible-smelling gas with greasy, pale, floating stools often points to fat malabsorption, which has several treatable causes.