Feces: Composition, Normal Characteristics & Clinical Significance
Feces (stool) is the solid waste expelled from the rectum, composed of:
- Water (~75%)
- Bacteria (~30% of dry weight) — mostly anaerobic (Bacteroides, Firmicutes); living + dead
- Undigested fiber (cellulose, resistant starch)
- Fat (fatty acids, soaps)
- Inorganic salts (calcium, phosphate, magnesium)
- Protein (bacterial, sloughed epithelial cells, mucus)
- Bile pigments (stercobilin → brown color)
Normal Characteristics (Bristol Stool Scale)
- Type 3–4: Sausage-shaped, smooth/soft, easy to pass — ideal.
- Color: Medium to dark brown (stercobilin).
- Frequency: 3/day to 3/week — wide normal range.
- Odor: Characteristic (indole, skatole, H₂S from bacterial fermentation).
Abnormal Signs Warranting Evaluation
| Finding | Possible Significance |
|---|---|
| Black/tarry (melena) | Upper GI bleed |
| Bright red (hematochezia) | Lower GI bleed, hemorrhoids |
| Pale/clay | Biliary obstruction, liver disease |
| Greasy, foul, floating (steatorrhea) | Fat malabsorption (pancreatic, celiac, bile acid) |
| Pencil-thin | Rectal stenosis, cancer |
| Mucus/blood | IBD, infection, cancer |