Feces
Feces, also known as stool or excrement, are the semisolid or solid residues expelled from the digestive tract of humans and other vertebrates after the absorption of nutrients, consisting primarily of water, undigested food particles, bacterial biomass, and desquamated epithelial cells.[1][2] In humans, feces form mainly in the large intestine through the reabsorption of water and electrolytes from residual chyme, resulting in a composition of roughly 75% water and 25% dry solids, with bacterial matter accounting for 25-54% of those solids alongside indigestible fibers like cellulose, proteins, fats, and minerals.[1][2][3] Variations in consistency, color, and odor—typically brown due to stercobilin from bilirubin breakdown—reflect diet, hydration, transit time, and microbial activity, with a median pH around 6.6.[2][4] Biologically, feces encapsulate the gut microbiome, a diverse community of trillions of microorganisms that modulate digestion, immunity, and metabolism, enabling diagnostic applications such as fecal microbiota analysis for conditions like inflammatory bowel disease or via transplantation to restore dysbiotic flora.[5][6] From an environmental and public health perspective, unmanaged feces contribute to pathogen transmission—including bacteria, viruses, and parasites—contaminating water sources and soil, while proper treatment can mitigate risks and repurpose nutrients as fertilizer, underscoring the tension between waste as a health hazard and a resource in nutrient cycling.[7][2][8]Biological Fundamentals
Definition and Composition
Feces are the semi-solid or solid waste material expelled from the intestines of animals after the digestion and absorption of nutrients from ingested food, consisting of undigested residues, microbial biomass, and epithelial cells sloughed from the gut lining.[1] This material forms through the incomplete breakdown of organic matter in the digestive tract, with composition reflecting dietary inputs and microbial activity rather than metabolic byproducts alone.[9] In humans and many mammals, feces comprise approximately 75% water by wet weight, with the dry fraction dominated by bacteria (25-54% of dry weight), indigestible polysaccharides such as cellulose fiber (up to 30%), residual proteins and fats (5-20%), and inorganic salts including nitrogenous compounds and phosphorus.[10] [11] Human feces harbor roughly $10^{11} to $3 \times 10^{11} bacterial cells per gram of wet weight, predominantly anaerobic species from phyla like Firmicutes and Bacteroidetes, whose diversity and abundance shift with host diet and age.[12] [13] Across animal species, fecal composition varies systematically with trophic level and gut anatomy; carnivores produce lower-fiber, higher-protein residues due to efficient protein hydrolysis, while herbivores exhibit elevated indigestible fiber content (often >50% dry weight from lignocellulose) alongside specialized microbial consortia for fermentation.[14] [15] Recent empirical data from 2025 reveal microplastics as emergent contaminants in human feces, present at concentrations altering microbial diversity by promoting biofilm-forming opportunists and disrupting metabolic pathways, with chronic ingestion linked to reduced Bacteroidetes abundance.[16] [17] These particles, ingested via food chains, comprise <1% of dry mass yet exert disproportionate effects on community structure in vitro and ex vivo models.[18]| Major Component (Human Feces, Wet Weight Basis) | Approximate Proportion | Key Sources |
|---|---|---|
| Water | 75% | Undiluted measurements from fecal analysis[11] |
| Bacteria (viable and dead) | 25-54% of dry weight (~8-13% wet) | Microscopy and culturing enumerations[10] [12] |
| Indigestible fiber and residues | 20-30% of dry weight | Dietary tracer studies[9] |
| Proteins, fats, salts (N, P) | 5-20% of dry weight | Biochemical assays[11] |