Poop smells because bacteria in your large intestine break down undigested food, especially protein, and release a cocktail of volatile gases in the process. The specific chemicals responsible include hydrogen sulfide (the “rotten egg” smell), skatole, indole, and various sulfur compounds. Everyone’s stool has an odor, but how strong it is depends on what you eat, how long waste sits in your colon, and which bacteria dominate your gut.
The Chemicals Behind the Smell
Your large intestine hosts trillions of bacteria, and their main job is fermenting whatever your small intestine didn’t fully absorb. The byproducts of that fermentation are volatile organic compounds, or VOCs, and they’re what you actually smell. The key players include hydrogen sulfide, which gives stool its sulfurous, rotten-egg quality; skatole and indole, which produce a heavy, fecal muskiness; phenol and p-cresol, which add a sharp, medicinal edge; and ammonia, which contributes a pungent bite. Short-chain fatty acids and various alcohols round out the mix. Your stool’s particular odor profile is essentially a fingerprint of your gut bacteria’s metabolic activity.
How Protein Becomes the Worst Offender
Protein is the single biggest contributor to foul-smelling stool. When protein isn’t fully digested in the small intestine, bacteria in the colon take over, using specialized enzymes to break amino acids apart through a series of chemical reactions. Different amino acids produce different odor compounds. Tryptophan, found in meat, eggs, and dairy, gets converted into skatole and indole by bacteria like Clostridium, Bacteroides, and Bifidobacterium. Tyrosine, another amino acid common in high-protein foods, is fermented into phenol and p-cresol. Sulfur-containing amino acids like cysteine and methionine release sulfur that gets transformed into hydrogen sulfide.
The process also generates ammonia as a byproduct of amino acid breakdown and urea processing. Ammonia doesn’t just smell bad on its own; it also shifts the chemical environment in your colon in ways that can encourage further production of odorous compounds. This is why high-protein diets, especially those heavy in animal products, tend to produce noticeably smellier stool than plant-heavy diets.
Foods That Make It Worse
Red meat and processed meats are among the strongest dietary drivers of stool odor. They’re rich in sulfur-containing amino acids, and processed meats carry additional inorganic sulfur from preservatives. Research has shown that these foods are positively correlated with higher populations of sulfur-metabolizing bacteria in the gut, meaning they don’t just provide raw material for smelly gas production; they actively encourage the growth of bacteria that specialize in making it.
Other common culprits include eggs (high in sulfur-containing amino acids), garlic, onions, and beans. Dairy can intensify odor in people who don’t digest lactose well, since unabsorbed lactose ferments aggressively in the colon. Alcohol also tends to increase stool odor by disrupting normal digestion and feeding certain bacterial populations.
Interestingly, cruciferous vegetables like broccoli, cabbage, and kale contain sulfur compounds called glucosinolates, yet they’re actually associated with lower populations of sulfur-metabolizing bacteria. So while a big serving of broccoli might cause gas, its long-term effect on your gut bacteria may not increase stool odor the way a steak would.
Transit Time Matters
How long waste spends in your colon directly affects how much it smells. When stool moves slowly, bacteria have more time to ferment it, producing higher concentrations of gas. Studies comparing people with normal colonic transit (under 45 hours) to those with slow transit (45 hours or more) found that slow-transit patients had significantly higher volumes of gas in their colons.
Constipation also tends to increase methane production. Methane itself is odorless, but it slows intestinal transit even further, creating a feedback loop: the longer stool sits, the more methane is produced, and the more methane is produced, the slower things move. Meanwhile, all the other odor-producing bacteria keep working. This is why stool that’s been “stuck” for days often smells markedly worse than a regular bowel movement.
On the flip side, diarrhea can also smell unusually strong, but for a different reason. When food moves too quickly through the gut, more undigested material reaches the colon, giving bacteria a larger supply of protein and carbohydrates to ferment all at once.
When Smell Signals a Health Problem
A sudden, persistent change in stool odor can point to a digestive issue worth paying attention to. Fat malabsorption, known as steatorrhea, produces a distinctive smell that’s hard to miss. When your body can’t properly digest fats (due to problems with bile acids, pancreatic enzymes, or the intestinal lining), undigested fat passes into the stool. The result is bulky, pale, oily stools with a particularly foul, greasy odor. Steatorrhea is diagnosed when stool contains more than 7 grams of fat per day, and it can signal conditions like celiac disease, chronic pancreatitis, or bile duct obstruction.
Infections also change stool odor in measurable ways. Different pathogens produce distinct volatile compound profiles. Stool from Campylobacter infections, for example, shows elevated levels of phenols, indoles, and organic acids. Clostridioides difficile (C. diff) infections produce a unique set of volatile compounds that lab analysis can distinguish with high accuracy from healthy stool or other gut conditions. Clinicians have long noted that C. diff stool has a characteristic sweet, sickeningly foul smell, though relying on odor alone isn’t reliable enough for diagnosis.
Parasitic infections like giardia, food intolerances, inflammatory bowel disease, and small intestinal bacterial overgrowth (SIBO) can all produce unusually foul-smelling stool. The common thread is that each condition changes either what reaches the colon or which bacteria are active there, shifting the balance of volatile compounds produced.
Why Baby Poop Smells Different
If you’ve noticed that newborn stool barely smells at all compared to an adult’s, that’s because a newborn’s gut microbiome is still in its earliest stages. The bacteria responsible for producing skatole, hydrogen sulfide, and other odor compounds haven’t fully colonized yet. Breastfed babies in particular tend to have relatively mild-smelling stool because their gut is dominated by Bifidobacterium, which primarily ferments milk sugars rather than protein.
The shift happens when solid foods enter the picture. Introducing table food triggers a rapid rise in bacteria like Bacteroides, Clostridium, and enterococci. As these populations grow, the infant’s gut begins producing the same short-chain fatty acids and protein-fermentation byproducts found in adult stool. By the time a child is eating a full diet of solid food, their gut microbiome and its odor output closely resemble an adult’s, complete with the full range of volatile compounds that make the diaper pail noticeably worse.
Reducing Stool Odor
Since protein fermentation and sulfur metabolism drive most of the smell, the most effective dietary change is reducing intake of red meat, processed meat, and eggs while increasing fiber from plants. Fiber speeds colonic transit, giving bacteria less time to produce odor compounds, and it feeds bacterial populations that produce less offensive byproducts. Staying well hydrated supports regular bowel movements, which also limits the buildup of gas.
Probiotics and fermented foods may help by shifting the balance of gut bacteria toward species that produce fewer sulfur compounds, though the effects vary from person to person. Limiting alcohol and artificial sweeteners (which can cause osmotic diarrhea and increased fermentation) also makes a difference for some people. If your stool odor changes dramatically and stays that way for weeks, especially alongside other symptoms like oily stools, persistent diarrhea, or weight loss, that pattern is worth investigating with a healthcare provider.