The smell comes from sulfur. Specifically, your gut bacteria produce tiny amounts of sulfur-containing gases as they break down food, and these gases are potent enough to stink even at very low concentrations. The main culprit is hydrogen sulfide, the same compound responsible for the rotten-egg smell. Your farts also contain two other sulfur gases, methanethiol and dimethyl sulfide, but hydrogen sulfide is present at roughly five times the concentration of the others. The vast majority of gas you pass is actually odorless, made up of nitrogen, oxygen, carbon dioxide, and sometimes methane. It’s that small sulfur fraction that makes all the difference.
What Creates the Smell Inside Your Gut
Your large intestine is home to trillions of bacteria, and some of them specialize in breaking down sulfur-containing compounds from your food. Bacteria like Desulfovibrio convert sulfate (a compound found naturally in many foods and drinking water) into hydrogen sulfide. Other species, including Fusobacterium and certain strains of E. coli, generate hydrogen sulfide by breaking down sulfur-containing amino acids like cysteine and taurine. The more sulfur-rich material these bacteria have to work with, the more odorous gas they produce.
Everyone’s mix of gut bacteria is a little different. Some people naturally harbor more sulfate-reducing bacteria than others, which partly explains why two people can eat the same meal and have very different results. Your personal microbiome is a major factor in how bad your gas smells on any given day.
Foods That Make It Worse
Diet is the single biggest lever you have over gas odor. Foods high in sulfur give your gut bacteria more raw material to convert into hydrogen sulfide. The most common offenders fall into two categories: cruciferous vegetables and high-protein animal foods.
Cruciferous vegetables include broccoli, cauliflower, Brussels sprouts, cabbage, kale, bok choy, and turnips. These are nutritious foods, but they’re naturally rich in sulfur compounds. If you eat a large serving, your gut bacteria will have a field day.
On the protein side, red meat is the highest in sulfur, followed by eggs (both yolk and white), pork, fish, and poultry. Dairy products, soy, whey protein powder, and bone broth also contribute. This is why people who start a high-protein diet often notice their gas gets noticeably worse. The gut bacteria ferment sulfur-containing amino acids and produce that classic rotten-egg smell. Interestingly, the total volume of gas you pass may not change much on a high-protein diet. It’s the smell that shifts, because the proportion of sulfur gases increases.
Some less obvious sources include supplements like glucosamine sulfate, MSM, and alpha lipoic acid. Food additives containing sulfites or sulfates (check labels for sodium bisulfite or potassium metabisulfite) can also feed sulfate-reducing bacteria in the colon.
How Much Gas Is Normal
There’s no official medical baseline for how often people pass gas. A commonly cited number in older textbooks is about 14 times per day, but a recent study using sensor-equipped underwear found the real average is closer to 32 times per day, with some participants logging as few as 4 and others as many as 59. So if you feel like you’re gassing more than you should be, the honest answer is that the range of normal is enormous. The smell, not the frequency, is usually what signals something worth paying attention to.
Medical Reasons for Unusually Foul Gas
Sometimes persistently terrible-smelling gas points to a digestive problem rather than just a bad meal. The common thread is malabsorption: when your body fails to properly break down and absorb nutrients in the small intestine, those undigested nutrients reach the colon, where bacteria ferment them aggressively.
Lactose intolerance is the most common example. If you lack the enzyme to digest lactose, the sugar passes intact into your colon, where bacteria ferment it into gas and short-chain fatty acids. The result is bloating, cramping, and foul-smelling flatulence after consuming dairy. Celiac disease and inflammatory bowel disease cause broader malabsorption affecting multiple nutrients, which can produce particularly smelly gas along with other symptoms like diarrhea and weight loss.
Fat malabsorption is another culprit. When fats aren’t absorbed in the small intestine, they reach the colon and produce greasy, especially smelly stools and gas. This can happen with pancreatic insufficiency, cystic fibrosis, or gallbladder and liver conditions.
Small intestinal bacterial overgrowth (SIBO) occurs when bacteria that normally live in the large intestine colonize the small intestine, where they start fermenting food before it’s fully absorbed. This produces excess gas and often changes its odor profile. Parasitic infections like giardiasis can also alter gas, producing sulfurous, egg-smelling burps alongside digestive upset. If your gas has changed dramatically and doesn’t improve with dietary changes, especially if it comes with abdominal pain, diarrhea, unexplained weight loss, fever, or blood in your stool, that’s worth investigating with a healthcare provider.
How Doctors Test for It
If your provider suspects a malabsorption issue or bacterial overgrowth, the most common first step is a hydrogen breath test. It’s noninvasive: you drink a solution containing a specific sugar (usually lactose or lactulose), then breathe into a collection device at intervals over a few hours. The test measures hydrogen and methane in your breath. In a healthy digestive system, breath hydrogen levels stay below 16 parts per million. A rise of more than 20 ppm above baseline suggests the sugar is being fermented by bacteria instead of absorbed normally. For SIBO specifically, that rise needs to happen within 90 minutes, indicating fermentation is occurring in the small intestine rather than the colon where it belongs.
About 15% to 30% of people have gut bacteria that produce more methane than hydrogen, so most clinics now measure both gases simultaneously to avoid missing a diagnosis.
How to Reduce the Smell
The most effective approach is dietary. Cutting back on high-sulfur foods for a week or two can help you identify which ones are the biggest contributors for you personally. You don’t necessarily need to eliminate broccoli or eggs forever, but knowing your triggers lets you manage portions and timing.
If you’re eating a lot of protein through supplements like whey powder, switching to a plant-based protein (other than soy) may help, since animal proteins tend to be higher in sulfur-containing amino acids.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol, can reduce gas odor by binding to hydrogen sulfide in the colon. It works as what’s sometimes called an “internal deodorant.” It’s not a long-term solution, but it can help in situations where you want short-term relief.
Eating more slowly and chewing thoroughly reduces the amount of air you swallow, which won’t change the smell much but can reduce overall gas volume. Cooking cruciferous vegetables rather than eating them raw also makes them easier to digest, potentially reducing the amount of undigested material that reaches your colon bacteria. Probiotics are widely marketed for gas, but their effects on odor specifically are inconsistent and depend heavily on the strain.