The observation that human stool can sometimes acquire a smell reminiscent of carnivorous feces, such as that of a cat, is an unusual but valid query. This phenomenon highlights a direct link between diet, gut biology, and the resulting waste product. The odor of stool reflects what the body has consumed and how the gut microbiome has processed it. This temporary change in scent is generally traced back to specific metabolic byproducts created when certain macronutrients are broken down by intestinal bacteria.
The Chemistry Behind Fecal Odor
The distinct smell of all mammalian feces is caused by a complex mixture of volatile organic compounds (VOCs) produced by the gut microbiota. As undigested food passes into the large intestine, bacteria ferment the remaining material, particularly proteins and carbohydrates. This fermentation yields gases and compounds that are the source of the characteristic odor.
Among the most potent odorants is skatole (3-methylindole), a primary contributor to the overall fecal scent. Skatole is formed when bacteria break down the amino acid tryptophan, a component of dietary protein. Other compounds include indole, a protein breakdown product, and sulfur-containing gases like hydrogen sulfide (which smells like rotten eggs) and methanethiol.
The intensity and specific nature of the odor depend on the concentration and ratio of these volatile molecules. For instance, short-chain fatty acids, such as butyric and valeric acid, also contribute to the profile. Increased undigested protein and sulfur-rich compounds available for fermentation lead to greater production of pungent gases and molecules like skatole and hydrogen sulfide.
How Diet Influences Stool Scent
The specific “cat poop” smell is often directly connected to a temporary shift toward a diet high in protein and fat. Cats are obligate carnivores; their diet results in a stool odor dominated by protein putrefaction byproducts, including high levels of skatole and indole.
When a human adopts a diet that temporarily elevates protein intake, such as a ketogenic diet, or a large portion of meat, the small intestine may be overwhelmed. The excess, undigested protein reaches the colon, where gut bacteria ferment it into the same pungent compounds—skatole and sulfides—that characterize a carnivore’s waste. This temporarily recreates the chemical signature of carnivorous feces.
The speed at which food moves through the digestive tract also influences the smell. Slower transit time allows bacteria more time to ferment undigested food, leading to a greater accumulation of foul-smelling VOCs. Conversely, rapid transit (such as diarrhea) may lead to less fermentation and a different, sometimes more sour, odor because the material is passed quickly.
Conditions That Cause Exceptionally Foul Smells
While most temporary foul-smelling stool relates to dietary changes, a persistent, intensely rancid or rotten odor can signal an underlying health issue. This foulness is often a sign of malabsorption, where the body fails to properly absorb nutrients, leaving undigested material for the gut bacteria to process.
One common medical cause is steatorrhea, or excess fat in the stool, resulting in a pale, bulky, foul-smelling bowel movement that often floats. This occurs with conditions affecting the pancreas, such as chronic pancreatitis, or diseases damaging the small intestine lining, such as Celiac disease, which hinders fat absorption.
Intestinal infections are another cause of a sudden, severe change in odor, as pathogens disrupt the gut microbiome. Parasitic infections (like giardiasis) or bacterial infections (such as C. difficile) produce distinct odors due to the toxins and gases they release. Inflammatory bowel diseases (IBD), including Crohn’s disease and ulcerative colitis, can also cause foul-smelling stool due to chronic inflammation and poor nutrient absorption.
When to Talk to Your Doctor
A change in stool odor linked to a recent diet change is generally harmless and self-correcting. However, if the foul smell is persistent and not easily explained by diet, it warrants a conversation with a healthcare provider.
Seek medical advice if the odor is accompanied by other concerning symptoms. These red flags include:
- Chronic diarrhea or constipation
- Unexplained weight loss
- Abdominal pain or fever
- The visible presence of blood or mucus in the stool
The appearance of pale, greasy, or floating stool, which suggests fat malabsorption, is also a reason to consult a physician for a thorough evaluation.