The smell in your farts comes from sulfur compounds, and they make up only about 1% of the gas you release. The other 99% is odorless. Nitrogen alone accounts for more than half of every fart, with hydrogen, carbon dioxide, oxygen, and methane filling out the rest. That tiny fraction of sulfur-based gases is so potent, though, that your nose can pick up hydrogen sulfide at concentrations as low as 0.0005 parts per million.
The Sulfur Compounds Behind the Smell
Hydrogen sulfide is the primary culprit. It’s the “rotten egg” gas, and its production varies widely from person to person, even when people eat the same diet. Your colon is home to specialized sulfate-reducing bacteria that produce hydrogen sulfide in two ways: by breaking down sulfur-containing amino acids (the building blocks of protein) and by chemically reducing sulfate, a compound found naturally in many foods and drinking water.
Hydrogen sulfide isn’t working alone. Another compound called methanethiol contributes a cabbage-like odor, and the two together account for most of what you’d describe as a foul-smelling fart. There’s also skatole, a nitrogen-based compound produced when gut bacteria break down the amino acid tryptophan. Skatole has an extremely low odor threshold, meaning you can smell it at vanishingly small concentrations. It’s the same compound responsible for much of the smell of animal manure.
Why Certain Foods Make It Worse
Foods high in sulfur give your gut bacteria more raw material to work with. The classic offenders include cruciferous vegetables like broccoli, cauliflower, cabbage, and Brussels sprouts. Eggs, red meat, garlic, onions, and dried fruits preserved with sulfites also increase sulfur gas production. Beer and wine contain sulfates as well.
High-protein meals are a double hit. Your gut bacteria ferment undigested protein in the lower part of your colon, producing both sulfur gases and skatole. This tends to happen more when your diet is low in fiber, because the bacteria in your colon run out of their preferred fuel (carbohydrates) and shift toward breaking down protein instead. A meal that’s heavy on steak but light on vegetables is a reliable recipe for more pungent gas.
Beans and lentils, while famous for causing gas, primarily produce hydrogen and carbon dioxide, which are odorless. They make you gassy in volume, but not necessarily in smell. It’s the sulfur-rich foods and protein-heavy meals that change the odor profile.
Your Gut Bacteria Decide the Smell
Everyone’s gut microbiome is different, and that’s a major reason the same meal produces barely noticeable gas in one person and room-clearing farts in another. The key players are sulfate-reducing bacteria, particularly species in the Desulfovibrio genus. These bacteria thrive on sulfate and pump out hydrogen sulfide as a waste product. Other microbes, including species of Fusobacterium and certain strains of E. coli, generate hydrogen sulfide by dismantling sulfur-containing amino acids.
The balance of your gut bacteria shifts based on your long-term diet, antibiotic use, and other factors. Someone whose microbiome has a higher proportion of sulfate-reducing bacteria will consistently produce smellier gas, regardless of any single meal.
Food Intolerances and Malabsorption
If your farts are persistently foul-smelling, a food intolerance could be amplifying the problem. Lactose intolerance is the most common example. When your body doesn’t produce enough of the enzyme that breaks down lactose (the sugar in milk), that undigested lactose passes into your colon, where bacteria ferment it into fatty acids, carbon dioxide, hydrogen, and methane. The extra fermentation activity and changes in your gut environment can intensify odor.
Broader malabsorption conditions push even more undigested material into the colon for bacteria to feast on. Celiac disease and inflammatory bowel disease cause general malabsorption of multiple nutrients. Pancreatic insufficiency, where the pancreas doesn’t produce enough digestive enzymes, has a similar effect. Fat malabsorption in particular sends undigested fats into the colon, producing especially foul-smelling gas and greasy stools. Carbohydrate malabsorption works the same way: unabsorbed carbs get fermented by colonic bacteria, producing excess gas and short-chain fatty acids.
Persistent, unusually smelly gas paired with bloating, diarrhea, or changes in stool consistency can be a sign that something beyond diet is going on.
How to Reduce Fart Odor
The most direct approach is reducing sulfur-rich foods in your diet, at least temporarily, to see if the smell improves. Cutting back on red meat, eggs, and cruciferous vegetables while increasing fiber from low-sulfur sources like oats, rice, and most fruits can shift your gut bacteria’s activity away from sulfur gas production. More fiber also keeps bacteria busy fermenting carbohydrates instead of protein, which reduces skatole production.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol, has been shown to significantly reduce hydrogen sulfide in the colon. The bismuth component binds directly to hydrogen sulfide gas, converting it into an insoluble compound that doesn’t produce odor. Research has confirmed that it’s the bismuth, not the salicylate, doing the work. It also binds to methanethiol to a lesser degree.
Probiotics may help by shifting the balance of your gut bacteria, though the evidence is less clear-cut than for dietary changes. Eating slowly, chewing thoroughly, and avoiding carbonated drinks reduce the total volume of gas in your digestive system, which can make each episode less noticeable even if the sulfur content stays the same.