Faecal Matter: Its Composition, Health Clues, and Uses

Faecal matter is roughly three-quarters water and one-quarter solid material, yet that solid fraction is packed with biological information and practical value that most people never consider. What you flush away contains bacteria, undigested food residues, metabolic byproducts, and shed intestinal cells in proportions that shift with your diet, your health, and the microbial community living in your gut. Clinicians already use stool to screen for cancer, diagnose gut infections, and assess organ function. Researchers and engineers, meanwhile, are finding uses for faecal matter that range from transplanting it into sick patients to harvesting it for renewable energy.

What Stool Is Actually Made Of

Water dominates. Across dozens of studies, the median moisture content of human faeces sits around 75%, though individual samples range anywhere from about 53% to 92% depending on diet, hydration, and gut transit time.¹² People who eat more fibre tend to have wetter stool because non-degradable fibre absorbs water in the colon; vegetarian diets push moisture content toward roughly 79%, while higher-protein, lower-fibre diets bring it closer to 73%.¹

The remaining quarter of solid material is overwhelmingly organic, somewhere between 84% and 93% by weight. Within that organic fraction, bacteria are the single largest component. One carefully controlled study of healthy adults on a typical British diet found that bacteria made up about 55% of total faecal solids, fibre accounted for roughly 17%, and soluble material about 24%.³ That means a large share of what you see in the toilet is not leftover food at all but the bodies of trillions of gut microbes, both living and dead. The rest of the dry matter includes undigested carbohydrates and plant material, proteins, a small proportion of fats, and an inorganic fraction of calcium phosphate, iron phosphate, shed epithelial cells, and mucus.¹

The Microbial Community Inside

Because bacteria make up more than half the dry weight of stool, faecal analysis has become the easiest window into the gut microbiome. The large intestine is an anaerobic environment where obligate anaerobes thrive, and two major bacterial groups dominate: Bacteroidetes and Firmicutes, which together account for more than 90% of the gut’s bacterial community.⁴ The exact ratio between these groups varies from person to person and changes with age, diet, medication use, and disease. Common genera in the colon include Bacteroides, Bifidobacterium, Clostridium, and Ruminococcus, at densities reaching up to a billion cells per gram of tissue.⁴ This microbial community produces the short-chain fatty acids, vitamins, and metabolic byproducts that influence everything from immune function to mood, which is why stool sampling has become central to microbiome research.

What Shape and Consistency Tell You

The Bristol Stool Form Scale, a seven-point visual chart running from hard lumps (type 1) to entirely liquid (type 7), was designed as a quick proxy for how fast food is moving through the colon. That proxy works well, especially in people with constipation. In constipated adults, stool form correlated strongly with whole-gut transit time, and a Bristol score below 3 predicted delayed transit with about 85% sensitivity and 82% specificity.⁵ Looser stools and more frequent bowel movements track with faster colonic motility, while hard, infrequent stools point toward slow transit.⁶

An important caveat: in healthy adults with no gut complaints, stool form did not correlate with measured transit time at all.⁵ So if your stool occasionally looks a bit different from the textbook “ideal” type 3 or 4, that alone does not signal a problem. Bristol scores are most informative when something already feels off. An earlier study also confirmed that stool form was a better correlate of gut transit than either defecation frequency or total stool output, which means consistency matters more than how often you go.⁷

Colour is another clue, though one you should interpret cautiously. The normal brown colour comes from stercobilin, a pigment produced when gut bacteria break down bilirubin from bile. Very pale or clay-coloured stool can indicate a bile duct blockage. Black, tarry stool may point to bleeding in the upper digestive tract. Bright red typically means bleeding lower down, such as from haemorrhoids or colonic polyps. Green stool is usually harmless, often caused by eating leafy vegetables or by food moving through the colon faster than usual, leaving bile pigments only partially processed.

Stool Lab Tests and What They Catch

Beyond visual inspection, laboratory analysis of faeces has become a frontline diagnostic tool for several conditions. Three tests stand out for their widespread clinical use.

Faecal calprotectin is a protein released by white blood cells during intestinal inflammation. Measuring it in stool helps clinicians distinguish inflammatory bowel disease from irritable bowel syndrome without needing to jump straight to colonoscopy. A meta-analysis pooling 17 studies found that the test picked up IBD with about 86% sensitivity and 92% specificity.⁸ A normal calprotectin result is particularly reassuring: when the background prevalence of IBD is low, the negative predictive value is nearly 100%, meaning a normal result makes IBD very unlikely. Different test kits perform slightly differently, so the specific brand used can matter.⁹

Faecal immunochemical tests, commonly called FIT, detect tiny amounts of blood in stool to screen for colorectal cancer. A Cochrane review comparing the older guaiac-based stool blood tests with newer immunochemical ones found that FIT was substantially better at catching cancer: about 76% sensitivity for colorectal cancer at a standard cut-off, compared with roughly 39% for the older test.¹⁰ FIT also outperformed guaiac tests for detecting advanced precancerous growths, which is the whole point of screening: catching problems before they become cancer.

Faecal elastase-1 measures how well the pancreas is doing its job of producing digestive enzymes. Low levels suggest pancreatic exocrine insufficiency, a condition where the pancreas does not make enough enzymes to properly break down food. A systematic review found that at the standard threshold, the test had a pooled sensitivity of 94% and was especially accurate in people with cystic fibrosis, where sensitivity reached 98%.¹¹

Sniffing Out Disease

The smell of stool is not just unpleasant; it carries chemical information. Volatile organic compounds emitted from faeces are the end products of microbial metabolism, and their patterns shift in disease states. Researchers have paired electronic noses and machine-learning algorithms with stool gas analysis to distinguish healthy people from those with conditions like colorectal cancer.¹² One study found that volatile profiles from stool could separate colorectal cancer patients from healthy controls with 85% sensitivity and 87% specificity, and even picked up precancerous advanced adenomas at a lower but still meaningful rate.¹³ The technology is still experimental, but the idea that a non-invasive “smell test” could supplement or even replace some screening methods is genuinely promising.

As for the everyday stink, the compounds most responsible are not what most people assume. A gas chromatography and mass spectrometry study identified methyl sulfide compounds, specifically methanethiol, dimethyl disulfide, and dimethyl trisulfide, as the dominant odour-causing chemicals. Skatole and indole, long blamed for faecal odour in textbooks, actually produce more of a mothball-like smell and are secondary contributors at best.¹⁴ Small amounts of hydrogen sulfide also contribute. Diet matters: sulphur-rich foods like eggs, meat, and cruciferous vegetables feed the bacterial pathways that produce these sulphur compounds, which is why some meals make the bathroom noticeably worse the next day.

Fecal Microbiota Transplantation

Perhaps the most striking medical use of faecal matter is transplanting it directly from a healthy donor into a sick patient’s gut. Fecal microbiota transplantation, or FMT, has become an established treatment for recurrent Clostridioides difficile infection, a potentially life-threatening condition in which harmful bacteria take over the colon, usually after antibiotics have wiped out the normal microbial community. Case reports and trials consistently show that FMT can resolve symptoms that antibiotics alone cannot.¹⁵

The field has recently moved beyond raw donor stool. Standardized live biotherapeutic products like Rebyota and Vowst now offer manufactured alternatives that eliminate the variability of individual donor samples.¹⁶ These products are essentially curated collections of beneficial bacteria derived from screened donor stool, packaged and dosed in a controlled way.¹⁷

Researchers are also exploring FMT for metabolic conditions. A pilot randomized trial found that FMT delivered by rectal enema significantly improved insulin resistance in people with metabolic syndrome over six weeks, along with improvements in fasting blood glucose and a marker of systemic inflammation.¹⁸ Earlier work had hinted at the same connection, with transplants from lean donors temporarily boosting insulin sensitivity in recipients with metabolic syndrome, though those effects were modest and did not persist long-term.¹⁹ The evidence here is still early-stage and limited to small trials, so FMT for obesity or diabetes is nowhere near clinical practice yet.

Tracking Epidemics Through Sewage

Wastewater carries the pooled faecal output of entire communities, and that makes it a powerful surveillance tool. Because many pathogens shed in stool days before symptoms appear, testing sewage can flag outbreaks earlier than waiting for people to show up at hospitals. This idea got a massive boost during the COVID-19 pandemic, when cities worldwide began monitoring SARS-CoV-2 levels in wastewater to track infection trends at a population level.²⁰ The same infrastructure can detect other waterborne pathogens like Cryptosporidium and Giardia, as well as monitoring for drug-resistant bacteria or even tracking community-level drug use. Wastewater surveillance provides information on pathogen spread, evolution, and outbreak risk that clinical testing alone cannot capture, because it includes people who never seek care.²¹

Manure as Fertiliser

Animal faeces have been used to enrich soil for thousands of years, and the practice remains widespread. Poultry manure, for example, is rich in nitrogen, phosphorus, potassium, calcium, and magnesium. A field study found that applying it to soil boosted concentrations of nitrogen by 82%, phosphorus by 46%, and calcium by 137% compared to untreated plots, while also increasing crop yield and improving the mineral content of harvested grain.²² Long-term studies of cattle manure confirm lasting effects: a single heavy application can double the available phosphorus in soil nearly a decade later, and manure-treated fields show improved water infiltration rates during the growing season.²³

These benefits come with caveats, though. Treated sewage sludge, often called biosolids, still contains low concentrations of persistent organic pollutants, per- and polyfluoroalkyl substances (PFAS), heavy metals, pharmaceutical residues, and microplastics.²⁴ Wastewater treatment plants are themselves a major route by which microplastics enter agricultural soils through biosolid application, and methods to mitigate this contamination are still being developed.²⁵ Applying biosolids responsibly means keeping concentrations below thresholds that would harm soil organisms, crops, or the humans eating those crops.

Biogas and Energy Recovery

Faecal matter is organic, wet, and full of microbes, which makes it a natural feedstock for anaerobic digestion, the process that produces biogas. Human faeces alone can generate biogas, but yields improve substantially when the material is mixed with co-substrates. One study found that co-digesting human faeces with cow dung produced about 70% more biogas than human faeces on their own, likely because the mixture achieves a better carbon-to-nitrogen ratio and supports faster bacterial breakdown.²⁶ Other combinations, like mixing human faeces with rice straw at a 50-50 ratio, also optimise methane output.²⁷ In parts of the world where both sanitation and energy access are challenges, converting latrine waste into cooking fuel addresses two problems at once.

Forensic DNA From Faeces

Stool left at a crime scene can identify a suspect. Because faeces contain shed intestinal cells from the person who produced them, it is possible to extract human nuclear DNA and generate a genetic profile. Using specialised extraction kits that neutralise the PCR-inhibiting compounds unique to faeces, forensic scientists have successfully typed DNA from stool samples exposed to a range of environmental conditions.²⁸ The technique works for animal faeces too. Optimised protocols for canine stool, for instance, now allow reliable DNA genotyping from as little as 30 to 50 milligrams of dried material collected from the outer surface of a stool sample.²⁹ Some cities have used this approach to enforce dog-fouling laws by matching pavement deposits to registered pets.

Ancient Coprolites and What They Preserve

Fossilised or desiccated faeces, called coprolites, are among the most information-dense artefacts available to archaeologists. A coprolite contains DNA from the person who produced it as well as DNA from whatever plants and animals they ate.³⁰ That makes them a direct record of ancient diets that does not depend on the biases of food-preparation debris or written accounts. Beyond diet, coprolites reveal parasite loads, gut microbiome composition, and, increasingly, host health and migration history through genomic analysis. Researchers consider them an underutilised proxy for understanding past environments, diets, and human health, partly because many excavations still treat them as unimportant finds and discard them.³¹ Analytical methods now span macroscopic, microscopic, and biomolecular techniques, and the field continues to expand as ancient DNA technology improves.

In animal ecology, faecal analysis fills a parallel role. Examining the volatile compounds in wild rabbit droppings, for instance, has identified fatty acids and phenols that may serve as territorial scent signals, giving ecologists a non-invasive way to study social behaviour without capturing or disturbing animals.³² For many wildlife species, scat collection is the only practical method for estimating population size, mapping home ranges, and monitoring diet across seasons. What starts as waste ends up being one of the most versatile biological samples available to science.