Diarrhea happens when your intestines either absorb too little water or push too much water out, resulting in loose or liquid stools. The World Health Organization defines it as three or more loose stools per day. To understand why it happens, it helps to know how much fluid your gut handles normally and what disrupts that process.
How Your Gut Normally Handles Water
Your digestive system processes a staggering amount of fluid every day. Between what you drink, what you eat, and the digestive juices your body produces (saliva, stomach acid, bile, pancreatic fluid), roughly 8 to 10 liters of liquid pass through your intestines daily. That’s more than two gallons.
The small intestine does most of the heavy lifting, absorbing the vast majority of that fluid. It pulls water in by first transporting sodium and other electrolytes across the intestinal lining. Water follows the sodium passively. One particularly efficient route involves sugar absorption: every time a glucose molecule gets transported into an intestinal cell, it brings along sodium ions and about 260 water molecules. This single mechanism accounts for an estimated 5 liters of water absorption per day.
After the small intestine finishes, about 1.5 liters reach the colon, which absorbs most of what’s left. In the end, only about 100 milliliters of water, less than half a cup, leaves your body in stool. Diarrhea is what happens when any part of this finely tuned system breaks down.
Three Ways the System Breaks Down
Not all diarrhea works the same way. The underlying mechanism falls into three broad categories, and they can overlap.
Secretory Diarrhea
In secretory diarrhea, your intestinal lining actively pumps water and electrolytes into the gut instead of absorbing them. The classic example is cholera. The cholera toxin triggers intestinal cells to flood chloride ions into the intestinal space. That negative electrical charge pulls sodium after it, and water follows both. Your gut essentially reverses direction, becoming a faucet instead of a sponge.
This same mechanism drives traveler’s diarrhea (caused by certain strains of E. coli) and diarrhea from Staph toxins in food poisoning. Some rare tumors can also trigger it by overproducing hormones that stimulate the same secretory pathways. A hallmark of secretory diarrhea: it continues even if you stop eating entirely, because the problem isn’t about what’s in your gut. It’s about what your gut lining is doing.
Osmotic Diarrhea
Osmotic diarrhea works on a simpler principle. When something in your gut can’t be absorbed, it holds water in the intestinal space by osmotic pressure, the same force that makes a salty sponge draw in moisture. The extra water stays in the bowel and comes out as loose stool.
Lactose intolerance is a common example. If you can’t break down lactose (milk sugar), it sits in the gut unabsorbed, pulling water with it. Sugar alcohols like sorbitol, xylitol, and mannitol do the same thing. Sorbitol can cause bloating and cramps at doses of 5 to 20 grams per day, and doses above 20 grams reliably cause diarrhea. For perspective, a single pack of sugar-free gum can contain 20 or more grams of sorbitol. Certain laxatives, like lactulose and magnesium citrate, work by deliberately triggering this mechanism.
Unlike secretory diarrhea, osmotic diarrhea stops when you stop consuming the offending substance.
Inflammatory (Exudative) Diarrhea
When the intestinal lining is damaged or inflamed, it can leak blood, mucus, pus, and protein-rich fluid into the bowel. These inflammatory products add volume to the stool on their own, but the damaged lining also loses its ability to absorb water and electrolytes normally. Infections with bacteria like Salmonella, Shigella, and Campylobacter work this way, as do conditions like inflammatory bowel disease (Crohn’s disease and ulcerative colitis) and diverticulitis.
Why Oral Rehydration Works Even During Diarrhea
Here’s something counterintuitive: even when your gut is actively secreting fluid (as in cholera), the sodium-glucose absorption pathway often still works. A specific transporter on your intestinal cells moves one glucose molecule and two sodium ions together across the cell membrane. Water follows. This is the scientific basis for oral rehydration solutions, which contain a precise mix of salt and sugar. Without glucose present, intestinal sodium isn’t actively absorbed. That’s why drinking plain water during severe diarrhea is far less effective than drinking a salt-and-sugar solution.
This discovery, that a simple glucose-salt drink could treat even cholera-level diarrhea, has been called one of the most important medical advances of the 20th century.
Acute vs. Chronic Diarrhea
Most diarrhea is acute, lasting a few days and caused by a virus, bacteria, or something you ate. The mechanisms above explain most of these episodes. Chronic diarrhea, lasting four weeks or more, involves different processes.
Inflammatory bowel disease (IBD) causes ongoing mucosal inflammation that damages the intestinal lining. Even when someone with IBD feels fine between flares, their gut typically shows elevated inflammatory markers and immune cell activity. The diarrhea here is largely exudative and tied to measurable tissue damage.
Irritable bowel syndrome with diarrhea (IBS-D) works differently. It’s classified as a functional disorder, meaning the gut looks structurally normal on examination. The problem appears to involve visceral hypersensitivity, where the nerves in the gut overreact to normal stimuli like stretching or gas. Some people with IBS-D, particularly those whose symptoms started after an infection, show low-grade inflammation, but it’s far milder than what’s seen in IBD. A stool marker called calprotectin can help distinguish the two: it runs high in IBD and normal to low in IBS.
How Loose Is “Diarrhea”?
Clinicians use the Bristol Stool Scale, a visual chart that grades stool from type 1 (hard, separate lumps) to type 7 (entirely liquid with no solid pieces). Types 6 (mushy with ragged edges) and 7 (watery) correspond to diarrhea. Type 5 (soft blobs with clear edges) falls in a gray zone. The scale correlates with how fast material moves through your colon: the faster the transit, the less water gets absorbed, and the looser the stool.
Signs of Dangerous Dehydration
Because diarrhea diverts water and electrolytes that your body normally reclaims, dehydration is the primary risk. In adults, warning signs include extreme thirst, dark urine, dizziness, fatigue, and a skin pinch test where you gently pinch the skin on the back of your hand and it doesn’t snap back flat immediately. Sunken eyes or cheeks also indicate significant fluid loss.
In infants and toddlers, the signs are different: no wet diapers for three hours or more, crying without tears, a sunken soft spot on the skull, or unusual drowsiness. Young children dehydrate faster because of their smaller fluid reserves, making even a day or two of diarrhea potentially serious.