Diarrhoea: understand it, place it and react the right way

Healthy state

Diarrhoea means soft to liquid stools, usually with more frequent bowel movements and a strong urge. The yardstick today is stool form, not a fixed amount: on the Bristol scale Bristol stool form scale A classification of stool into seven types, from hard lumps (1) to liquid (7). Types 3 and 4 are considered ideal. More in the chapter “Constipation: more than too little fibre”, types 3 and 4 (sausage-shaped, smooth to slightly cracked) are considered ideal and types 6 and 7 (mushy to watery) count as diarrhoea.

Normally surprisingly large amounts of fluid flow through the gut every day. On top of what you drink come saliva, gastric juice, bile and pancreatic juice, about nine litres in total. The small intestine absorbs most of it, the large intestine takes back the rest and draws water from the chyme until a formed mass results. Only about 100 to 200 millilitres remain in the stool. Even a small shift in this balance turns firm stool into liquid stool.

Absorption and secretion balance each other. The lining actively takes up fluid and salts, but can also release some when needed. A salt-sugar transporter in the small intestinal wall pulls sodium and glucose into the cell together, and water follows. The oral rehydration solution for diarrhoea is based on this principle.

Time also plays a role. The way through the small intestine takes about three to five hours; in the large intestine the contents usually stay between half a day and two days. Waves of movement push the contents on and leave enough time for water to be taken back. After eating, the stomach triggers movement in the large intestine via a reflex (gastrocolic reflex), which is why many people need the toilet after a meal.

The gut flora Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” helps: from fibre, bacteria make short-chain fatty acids Short-chain fatty acids Acids such as acetate, propionate and butyrate that gut bacteria make from fibre. They nourish the gut wall and act throughout the body. More in the chapter “Butyrate and other short-chain fatty acids”, which serve the large intestine as an energy source and promote water absorption. A stable flora also keeps pathogens at bay.

If the diarrhoea passes after a few days, it is usually harmless. If it lasts longer than four weeks, it counts as chronic and should be assessed.

Disrupted state: causes and effects

Diarrhoea arises in four ways that can overlap. Osmotic: unabsorbed sugars such as lactose, fructose or sorbitol draw water into the gut. Secretory: the lining itself releases water and salts, for example with cholera toxin or too many bile acids Bile acids Substances made by the liver that make fat digestible. About 95 percent are reabsorbed in the last part of the small intestine; gut bacteria transform the rest. More in the chapter “Bile acids: when bile causes diarrhoea” in the large intestine. Inflammatory: a damaged lining secretes fluid, often with blood or mucus. And via movement: if the gut is too fast, there is too little time to take water back. A hint about the route: if diarrhoea stops when you fast, that suggests an osmotic mechanism; if it carries on anyway, more a secretory one.

Acute diarrhoea is usually due to an infection or spoiled food. Viruses such as norovirus and rotavirus are the most common cause. Bacteria (such as Campylobacter, Salmonella or certain E. coli strains) and single-celled organisms such as Giardia come in depending on region and travel; some produce toxins, others invade the lining and cause fever and bloody stool. Travellers’ diarrhoea is a case of its own.

After antibiotics or a hospital stay, Clostridioides difficile Clostridioides difficile A bacterium that can spread after antibiotics and cause severe diarrhoea with its toxins. More in the chapter “After antibiotics: C. difficile and faecal transplants” can exploit the gap in the gut flora and cause often watery, foul-smelling diarrhoea. This should be tested for and differs from the harmless diarrhoea phase that antibiotics can cause even without the germ.

Chronic diarrhoea has different causes. Common are diarrhoea-predominant IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis”, bile acid diarrhoea, insufficiently digested sugars, coeliac disease Coeliac disease An autoimmune disease in which gluten damages the lining of the small intestine. It is diagnosed with a blood test and usually a biopsy while you are still eating gluten. More in the chapter “Lactose, fructose, gluten: making sense of intolerances”, small intestinal bacterial overgrowth Small intestinal bacterial overgrowth (SIBO) Too many bacteria in the small intestine, where normally few live. Bloating and diarrhoea are typical; it is usually diagnosed with a breath test. More in the chapter “When the small intestine is too densely colonised” and microscopic colitis, which is visible only in tissue under the microscope and particularly affects older women. In IBS, nerves, stress and mast cells Mast cells Immune cells beneath the skin and mucous membranes, filled with histamine and other messengers. They raise the alarm in allergies, infections and stress. More in the chapter “Mast cells: alarm cells in the gut, histamine and the disputed MCAS” play a part: the gut reacts more sensitively and faster without anything being visible. An overactive thyroid, diabetes and pancreatic weakness (fatty stools) also come into question.

Medications often play a part too: antibiotics, acid blockers, metformin, magnesium supplements, some painkillers and chemotherapy. In everyday life it is often sugar alcohols Sugar alcohols (polyols) Sweeteners such as sorbitol, xylitol or maltitol, which also occur in fruit. They are barely absorbed and can cause bloating and diarrhoea. More in the chapter “Lactose, fructose, gluten: making sense of intolerances” in “sugar-free” products, a lot of coffee or alcohol that drive the gut.

A special case is overflow diarrhoea: with stubborn constipation, hard stool builds up and liquid stool trickles past it. This mainly affects older people and can look like diarrhoea, but it is the constipation that is treated.

In infants, small children and older people, dehydration threatens fastest: they have fewer reserves and notice thirst later.

Two common assumptions persist: that diarrhoea automatically means infection, and that a bowel cleanse is healthy. Neither is true as stated. Not every diarrhoea has a pathogen, and there is no proven benefit in regular bowel emptying without disease – while water loss and shifted salts threaten.