The smell comes from sulfur. About 99% of the gas you pass is completely odorless, made up of nitrogen, oxygen, carbon dioxide, hydrogen, and methane. The entire stink is produced by sulfur-containing compounds that make up roughly 50 parts per million of each fart. That tiny fraction is potent enough to clear a room.
The Three Compounds Behind the Smell
Your gut bacteria break down sulfur-containing amino acids (cysteine and methionine) from the protein in your food. This process generates three volatile sulfur compounds, each with its own signature odor. Hydrogen sulfide produces the classic rotten-egg smell. Methanethiol smells like rotten cabbage. Dimethyl sulfide adds a garlic-like note. The ratio of these three compounds varies from person to person and meal to meal, which is why some farts smell dramatically worse than others.
Certain bacteria in your colon are especially prolific sulfur producers. Species in the Desulfovibrio genus, for instance, generate hydrogen sulfide by processing sulfate, a compound found naturally in many foods and drinking water. Other common gut bacteria, including strains of E. coli and Fusobacterium, produce hydrogen sulfide by breaking down sulfur-containing amino acids for energy. The more of these bacteria you have, and the more sulfur you feed them, the worse the smell.
Foods That Make It Worse
High-sulfur foods are the most direct dietary trigger. Cruciferous vegetables like broccoli, cauliflower, cabbage, and Brussels sprouts are rich in sulfur compounds that gut bacteria eagerly ferment. Eggs, red meat, dairy, garlic, and onions are also high in sulfur-containing amino acids. Beer and wine contain sulfites that add to the supply. Even your drinking water can contribute if it has naturally occurring sulfate.
High-protein diets tend to produce smellier gas because protein is the main source of the amino acids that bacteria convert into sulfur compounds. A meal heavy in steak and eggs, for example, delivers a large dose of methionine and cysteine straight to your colon’s bacterial workforce. The volume of gas might not change much, but the odor intensity goes up noticeably.
Fiber-rich and high-FODMAP foods (fermentable sugars found in wheat, legumes, certain fruits, and dairy) increase overall gas production. Research from Monash University found that both healthy people and those with irritable bowel syndrome produced more gas and experienced more abdominal discomfort after high-FODMAP meals. More gas volume means more opportunities for those sulfur compounds to make themselves known.
Food Intolerances and Malabsorption
If your gas has gotten notably worse, a food intolerance could be the reason. Lactose intolerance is one of the most common culprits. 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 aggressively. The result is increased gas volume, bloating, and often a more pungent smell.
The same principle applies to fructose malabsorption, sorbitol (found in sugar-free gums and candies), and other poorly absorbed carbohydrates. Any sugar that reaches your colon intact becomes a feast for gas-producing bacteria. If you notice a pattern between specific foods and especially foul-smelling gas, that connection is worth paying attention to.
When a Gut Imbalance Is Involved
Small intestinal bacterial overgrowth, commonly called SIBO, occurs when bacteria that normally live in your large intestine colonize your small intestine in abnormal numbers. Your small intestine is designed to have relatively few bacteria, kept in check by the rapid flow of food and the presence of bile. When something slows that flow, whether from surgery, certain medications, or digestive conditions, bacteria multiply in the stagnant environment and start fermenting food earlier in the digestive process than they should.
SIBO can produce particularly foul-smelling gas along with bloating, diarrhea, and abdominal pain. The bacteria break down food that would normally be absorbed higher up in the digestive tract, generating excess gas and sometimes interfering with nutrient absorption. Other conditions that disrupt your gut’s bacterial balance, including inflammatory bowel disease and celiac disease, can also shift gas production toward smellier output.
How to Reduce the Smell
The most straightforward approach is adjusting what you eat. Cutting back on high-sulfur foods for a few days is a reasonable experiment. If cruciferous vegetables, eggs, or red meat seem to correlate with your worst episodes, reducing your intake of those foods will lower the raw material available for sulfur gas production. You don’t necessarily need to eliminate them permanently, just be aware of how much you’re consuming at once.
A low-FODMAP diet, developed at Monash University, can help identify specific triggers if you suspect a broader pattern. The approach involves temporarily removing high-FODMAP foods and then reintroducing them one category at a time to pinpoint which ones cause problems. This is especially useful for people with IBS or chronic bloating.
Over-the-counter bismuth subsalicylate (the active ingredient in Pepto-Bismol) has been shown to directly neutralize hydrogen sulfide in the colon. The bismuth component binds to hydrogen sulfide and, to a lesser degree, methanethiol, converting them into an insoluble compound that doesn’t produce odor. Research published in Gastroenterology confirmed that bismuth is the active ingredient responsible for this effect, not the salicylate portion. This isn’t a long-term solution, but it works for occasional situations where you want to minimize odor.
What’s Normal and What Isn’t
Passing gas 14 to 23 times a day is within the normal range. Most of those instances won’t smell like much at all, given that the overwhelming majority of intestinal gas is odorless. An occasional foul-smelling episode after a sulfur-heavy meal is completely expected biology, not a sign that something is wrong.
The pattern to watch for is a sustained change. If your gas has become persistently more foul-smelling than your baseline, or if it’s accompanied by abdominal pain, diarrhea, constipation, or unintentional weight loss, those combinations can point to malabsorption, a bacterial imbalance, or an underlying digestive condition that’s worth investigating. A sudden shift in symptoms, even without pain, is also worth noting, since your gut flora and digestive function can change in response to medication, illness, or dietary shifts you might not have connected to the problem.