Farts smell bad primarily because of gases produced by bacteria in the large intestine that break down food. These bacteria produce sulfur-containing compounds, most notably hydrogen sulfide, which has a rotten egg smell. The amount and type of bacteria, as well as the foods consumed—especially those rich in sulfur like broccoli, cabbage, and meat—affect the intensity of the odor. Other factors such as food intolerances (e.g., lactose or fructose intolerance), constipation, and certain medical conditions can also contribute to smelly farts.
Why Farts Smell Bad
- The large intestine contains bacteria that break down undigested food and produce gases, including sulfur compounds.
- Hydrogen sulfide gas, which smells like rotten eggs, is the main culprit behind foul-smelling farts.
- Sulfur-rich foods such as cruciferous vegetables (broccoli, Brussels sprouts, cabbage), meat, and eggs increase hydrogen sulfide production.
- Food intolerances like lactose intolerance cause more undigested sugars to reach the colon, fueling bacterial fermentation and smelly gases.
- Medications, constipation, and digestive disorders like celiac disease or irritable bowel syndrome can exacerbate the smell.
Key Foods Increasing Smelly Gas
- Vegetables: Broccoli, cauliflower, cabbage, Brussels sprouts, kale
- Proteins: Meat, eggs, nuts
- Dairy: Milk, cheese (especially if lactose intolerant)
- Foods high in certain sugars: Beans, peas, fruits with fructose
The smell varies because not all gases produced during digestion are odorous; gases like nitrogen, carbon dioxide, methane, and hydrogen are mostly odorless. It's the sulfur compounds that cause the bad smell when released.
In summary, the bad smell of farts comes mainly from the presence of hydrogen sulfide gas produced by gut bacteria digesting sulfur-containing foods and other undigested substances in the colon.
