Diarrheal stool is mostly water, typically 85% to 95% by weight, compared to about 75% water in a normal, formed stool. The rest is a mix of electrolytes, bacteria, mucus, shed intestinal cells, undigested food particles, and sometimes fat or immune cells. What makes diarrhea different from a healthy bowel movement isn’t so much that it contains foreign substances, but that the normal balance of water absorption in your gut has been disrupted.
Water: The Dominant Ingredient
Your digestive system processes a surprisingly large amount of fluid every day. Between what you drink, what you eat, and what your stomach and intestines secrete on their own, roughly 9 to 10 liters of fluid pass through your gut daily. Normally, your small intestine and colon reabsorb 99% of that liquid. Even a tiny shift, just a 1% drop in absorption, can tip the balance and produce diarrhea.
A healthy adult produces about 100 to 200 grams of stool per day. Diarrhea is formally defined as stool output exceeding 200 grams per day. The extra weight is almost entirely water. That’s why the stool becomes loose or liquid: the solid components haven’t necessarily changed much, but they’re now suspended in far more fluid than usual.
Electrolytes Lost in Diarrheal Fluid
Dissolved in all that extra water are electrolytes, the salts your body needs for nerve signaling, muscle function, and fluid balance. Diarrheal fluid contains significant concentrations of sodium (around 54 milliequivalents per liter) and potassium (around 33 milliequivalents per liter), along with chloride and bicarbonate. These losses are the reason diarrhea causes dehydration so quickly. You’re not just losing water; you’re losing the minerals that help your cells retain and use that water.
Potassium loss can become serious during prolonged or severe episodes, potentially affecting heart rhythm and muscle function. Bicarbonate loss can shift your blood’s pH toward acidic, a condition called metabolic acidosis. This is why oral rehydration solutions contain specific amounts of salts and sugar, not just plain water. They’re designed to replace what diarrheal fluid actually carries out of your body.
Bacteria and Other Microbes
All stool, healthy or not, contains enormous quantities of bacteria. Your colon is home to trillions of microorganisms, and they make up a substantial portion of solid stool matter. In diarrhea, these bacteria are flushed out more rapidly and in larger volumes. When an infection is causing the diarrhea, the stool may also contain the specific pathogen responsible, whether that’s a bacterium like Salmonella, a virus like norovirus, or a parasite like Giardia.
Some bacterial infections produce toxins that actively block your intestines’ ability to absorb sodium and potassium. This disrupts the chemical gradient your gut relies on to pull water out of your intestinal contents and back into your body. The result is secretory diarrhea, where the intestines essentially pour fluid into the stool rather than absorbing it.
Mucus, Immune Cells, and Blood
When diarrhea is caused by inflammation, whether from an infection, Crohn’s disease, ulcerative colitis, or another inflammatory condition, the stool often contains visible mucus, white blood cells, and sometimes blood. White blood cells in stool are a direct sign that your immune system is actively fighting something in the digestive tract. Mucus production ramps up as the intestinal lining tries to protect itself from damage.
Your intestinal lining also sheds cells constantly as part of normal turnover, but inflammation accelerates this process. In severe cases, the lining can become damaged enough to bleed, producing stool streaked with red or dark blood. Inflammatory diarrhea tends to come with additional symptoms like abdominal pain, fever, and fatigue, distinguishing it from the more purely watery types.
Undigested Food and Fiber
Diarrhea often contains visible fragments of undigested food. This is usually high-fiber plant matter, like corn kernels, leafy greens, or seed husks, which your body can’t fully break down even under normal conditions. When stool moves through the intestines faster than usual, even foods that would normally be well-digested can pass through partially intact because they simply don’t spend enough time in contact with digestive enzymes and absorptive surfaces.
This reduced transit time is itself one of the three main mechanisms behind diarrhea. When food and fluid rush through too quickly, there isn’t enough contact between the intestinal lining and the contents for proper absorption to occur.
Fat in Diarrheal Stool
Some types of diarrhea contain unusually high amounts of fat, a condition called steatorrhea. Normal stool contains a small amount of dietary fat, but when fat absorption is impaired (due to conditions like celiac disease, chronic pancreatitis, or surgical removal of part of the intestine), more than 7 grams of fat per day can end up in the stool. This produces greasy, pale, foul-smelling stool that may float or leave oily residue in the toilet.
Unabsorbed fat doesn’t just pass through passively. It actively stimulates the colon to secrete more water and electrolytes, making the diarrhea worse. Unabsorbed bile acids, which normally help digest fat, have a similar effect when they reach the colon intact.
Osmotic vs. Secretory: Two Different Recipes
The composition of diarrheal stool actually varies depending on what’s causing it, and doctors can distinguish between the two main types by measuring the solutes dissolved in a stool sample. The key measurement is called the fecal osmotic gap.
In osmotic diarrhea, something in the gut is holding onto water. This could be an unabsorbed sugar like lactose (in lactose intolerance), artificial sweeteners like sorbitol or xylitol, or certain medications like magnesium-based antacids. The stool in these cases contains high concentrations of these unabsorbed substances pulling water in, and the osmotic gap is large, typically above 125 milliosmoles per kilogram.
In secretory diarrhea, the intestines themselves are pumping out excess fluid, usually triggered by bacterial toxins, hormonal imbalances, or bile acid irritation. The stool in secretory diarrhea is rich in sodium and other electrolytes, and the osmotic gap is small, below 50 milliosmoles per kilogram. A practical difference: osmotic diarrhea usually stops when you stop eating the offending substance, while secretory diarrhea often continues even during fasting.