Frequent Diarrhea

Diarrhoea is defined as passing three or more loose or liquid stools a day, or more frequently than is normal for you. The critical division is by duration, because it determines the entire approach. Acute diarrhoea lasting under two weeks is almost always infectious — viral in most cases, particularly norovirus and rotavirus, and bacterial in a minority. It is self-limiting, and the priority is fluid and electrolyte replacement rather than stopping the diarrhoea. Oral rehydration solution outperforms water alone because the sodium-glucose cotransport mechanism drives fluid absorption even in an inflamed gut. Antibiotics are unnecessary and often harmful in most cases, and loperamide should be avoided where there is fever or bloody stool, since slowing transit prolongs contact with the pathogen. Chronic diarrhoea, defined as lasting more than four weeks, is a different problem entirely and requires a diagnosis. The commonly missed causes are worth naming because they are all treatable. Bile acid malabsorption is substantially under-recognised and responds dramatically to a bile acid sequestrant. Microscopic colitis produces watery diarrhoea with a completely normal-looking colonoscopy — it is diagnosed only if biopsies are taken. Coeliac disease is easily excluded by serology and frequently is not. Exocrine pancreatic insufficiency, lactose intolerance and inflammatory bowel disease complete the common list. Medication is an underappreciated cause. Metformin, magnesium-containing supplements and antacids, proton pump inhibitors, SSRIs, colchicine and sugar-free products containing sorbitol all commonly cause diarrhoea, and a careful medication review resolves a meaningful proportion of cases without further investigation.

• •Three or more loose or watery stools daily
• •Urgency and, in some cases, incontinence
• •Abdominal cramping
• •Nocturnal diarrhoea that wakes you — a red flag for organic disease
• •Bloating and excess wind
• •Signs of dehydration: thirst, reduced urine, dizziness on standing
• •Weight loss if chronic
• •Pale, greasy, floating stool suggesting fat malabsorption

• •Viral gastroenteritis — norovirus, rotavirus
• •Bacterial infection — Campylobacter, Salmonella, E. coli, C. difficile
• •Antibiotic-associated disruption of gut flora
• •Bile acid malabsorption, frequently after cholecystectomy or ileal disease
• •Coeliac disease
• •Microscopic colitis
• •Inflammatory bowel disease — Crohn disease and ulcerative colitis
• •Lactose or fructose intolerance
• •Exocrine pancreatic insufficiency
• •Medications: metformin, magnesium, PPIs, SSRIs, colchicine
• •Hyperthyroidism
• •IBS with diarrhoea predominance
• •Sorbitol and other sugar alcohols in sugar-free products

Diarrhoea arises through four mechanisms, and identifying which is operating narrows the differential efficiently. Osmotic diarrhoea occurs when unabsorbed solute holds water in the lumen — lactose in lactase deficiency, sorbitol from sugar-free products, magnesium salts. Its defining feature is that it stops when you stop eating the offending substance, and it ceases with fasting. Secretory diarrhoea occurs when the intestinal epithelium actively secretes fluid, driven by bacterial toxins, bile acids reaching the colon, or hormone-secreting tumours. It continues during fasting and tends to be high volume, which is the key distinguishing question. Inflammatory diarrhoea results from mucosal damage — inflammatory bowel disease, invasive infection, microscopic colitis — and typically involves blood, mucus, urgency and raised faecal calprotectin. Motility-related diarrhoea reflects accelerated transit, as in hyperthyroidism, diabetic autonomic neuropathy or IBS-D. Bile acid malabsorption deserves emphasis as the archetypal missed diagnosis. Bile acids normally reabsorb in the terminal ileum; when that fails — after gallbladder removal, ileal resection, Crohn disease, or idiopathically — they spill into the colon and act as potent secretagogues. Patients are frequently labelled IBS-D for years before a SeHCAT scan or an empirical trial of colestyramine reveals the answer.