Why Does Poop Smell? The Science of Fecal Odor

The characteristic odor of feces comes from volatile organic compounds (VOCs) produced by anaerobic gut bacteria fermenting undigested food residues (proteins, carbohydrates, fats).

Major Odor-Contributing Compounds

Factors Influencing Odor

• Diet: High protein (meat, eggs) → more sulfur compounds; high fiber → more SCFAs; cruciferous vegetables → sulfur VOCs.
• Gut Microbiome Composition: Varies by individual; dysbiosis alters VOC profile.
• Transit Time: Constipation → more fermentation → stronger odor. Diarrhea → different profile.
• Medications: Antibiotics (disrupt flora), metformin, iron supplements.
• Health Conditions: Malabsorption (foul, greasy stool), GI bleeding (metallic), infections (C. diff – characteristic horse-manure odor).

Other Fecal Components

Bile pigments (stercobilin – brown color), water (~75%), bacteria (dead/live), undigested fiber, fat, inorganic salts, mucus, sloughed epithelial cells.

Sudden, persistent changes in stool odor—especially with other symptoms (blood, weight loss, diarrhea)—warrant medical evaluation.

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Compound	Odor Description	Source
Hydrogen sulfide (H₂S)	Rotten egg	Sulfur-containing amino acids (cysteine, methionine)
Methanethiol / Methyl mercaptan	Rotten cabbage, garlic	Methionine degradation
Dimethyl sulfide	Cooked corn, sweet	Methionine, choline
Indole	Fecal, mothballs	Tryptophan fermentation
Skatole (3-methylindole)	Strong fecal, floral at low conc.	Tryptophan fermentation
Ammonia (NH₃)	Pungent, urine-like	Protein/urea deamination
Short-chain fatty acids (butyrate, propionate, acetate)	Rancid, sweaty	Carbohydrate fermentation
Phenols, cresols	Medicinal, tarry	Tyrosine/phenylalanine fermentation