Human feces are the solid or semi-solid waste your body expels after digesting food. They’re roughly 75% water and 25% solid matter, and they contain far more than just leftover food. A healthy adult on a typical Western diet produces about 100 to 106 grams of stool per day, with men producing slightly more than women on average.
What Feces Are Made Of
The solid portion of stool is a surprisingly complex mix. About 30% consists of dead bacteria, the remnants of the trillions of microorganisms that live in your gut and help you digest food. Another 30% is indigestible plant material like fiber. Fats, including cholesterol, make up 10 to 20%, and minerals like calcium and iron compounds account for another 10 to 20%. Protein rounds out the remaining 2 to 3%.
When researchers measure the total dry mass of stool, bacteria actually account for about 55% of it. That means more than half of what makes your stool solid isn’t food at all. It’s microbial biomass, both living and dead, reflecting the enormous bacterial population that inhabits your large intestine. These bacteria aren’t just passengers. While in your colon, they break down remaining nutrients and produce essential compounds like vitamin K.
How Your Body Forms Stool
Stool formation is the final stage of digestion, and most of it happens in the large intestine. By the time food reaches your colon, your small intestine has already absorbed the majority of nutrients. What arrives in the large intestine is a liquid slurry of indigestible material, water, and bacteria.
Your colon’s primary job is to reclaim water. It draws fluid from that liquid waste back into your bloodstream, gradually transforming it from a watery mixture into the formed stool you eventually pass. How long waste spends in the colon directly affects how firm or loose it becomes. Longer transit times mean more water absorption and harder stool. Shorter transit times mean less water is pulled out, producing softer or even liquid stool.
Why Stool Is Brown (and Smells the Way It Does)
The characteristic brown color comes from a pigment called stercobilin. Your body constantly recycles old red blood cells, and one byproduct of breaking down the oxygen-carrying molecule in those cells is bilirubin, a yellow-green compound that gets secreted in bile. As bilirubin travels through your intestines, gut bacteria chemically transform it into stercobilin, which is brown. The same family of pigments, processed slightly differently, is what makes urine yellow.
The distinctive smell comes primarily from a compound called skatole, produced when gut bacteria break down tryptophan, an amino acid found in many protein-rich foods. Skatole is the dominant odor compound in mammalian feces. Sulfur-containing gases, produced as bacteria ferment undigested material, contribute additional sharpness to the smell. Diet plays a large role in how strong these odors are, since the bacteria responsible for producing them feed on whatever reaches the colon.
What Normal Stool Looks Like
Doctors use the Bristol Stool Scale, a seven-point chart, to classify stool consistency. The types range from hard, separate lumps (Type 1) to entirely liquid with no solid pieces (Type 7). Types 3 and 4, described as sausage-shaped with surface cracks or smooth and snakelike, represent the healthiest range. These forms indicate that waste is moving through your intestines at a balanced pace, spending enough time in the colon to solidify without sitting there so long that it dries out.
Types 1 and 2, hard pellets or lumpy sausage shapes, suggest constipation. Stool becomes this way when it spends too long in the colon and loses too much water. On the other end, Types 5 through 7, ranging from soft blobs to watery liquid, indicate that material is passing through too quickly for your colon to absorb adequate water. Occasional variation is normal, but consistently falling at either extreme is worth paying attention to.
What Stool Color Can Tell You
Since the brown color depends on bile pigments being processed normally, changes in stool color can signal that something in the digestive chain has shifted. Some color changes are harmless and diet-related. Others point to conditions that need attention.
- Green: Often means food moved through your intestines faster than usual, so bacteria didn’t fully convert bilirubin to its brown form. It can also occur with bacterial infections or irritable bowel syndrome.
- Red: Can indicate bleeding in the lower digestive tract from hemorrhoids, ulcers, fissures, or inflammatory bowel disease. Red foods like beets can also be the cause.
- Black: May signal bleeding higher in the digestive tract, such as the stomach or upper intestine, where blood has time to darken before it’s passed. Iron supplements and bismuth-based medications (like Pepto-Bismol) can also turn stool black without any bleeding.
- Pale, gray, or clay-colored: Suggests a problem with bile production or flow. Since bile pigments are what eventually give stool its color, a lack of bile can point to issues with the liver, gallbladder, or pancreas.
Feces as a Medical Tool
Stool has become increasingly valuable in medicine, particularly through fecal microbiota transplants. In this procedure, processed stool from a healthy donor is introduced into the gut of a patient whose intestinal bacteria have been disrupted, most commonly by severe, recurring infections caused by the bacterium C. difficile. The transplanted bacteria essentially reseed the patient’s gut with a functional microbial community.
Donor screening for these procedures is rigorous. Potential donors are excluded if they’ve had recent major surgery, work with animals (due to the risk of transmitting animal-borne infections), have received live vaccines within six months, or have regular contact with healthcare settings like hospitals or long-term care facilities. The strict criteria reflect how much bioactive material stool contains. It’s not inert waste. It’s a dense sample of someone’s entire gut ecosystem, carrying both its benefits and its risks.