What are the etiologies of loose bowel movements (diarrhea)?

Causes of Loose Bowel Movements

Loose bowel movements result from five major pathophysiologic mechanisms: colonic neoplasia/inflammation, small bowel inflammation, small bowel malabsorption, pancreatic maldigestion, and motility disorders. 1

Primary Etiologic Categories

Colonic and Inflammatory Causes

• Inflammatory bowel disease (ulcerative colitis and Crohn's disease) produces inflammatory diarrhea characterized by bloody or mucoid liquid stools. 2
• Microscopic colitis is an important cause that must be ruled out, particularly in patients with watery diarrhea and normal colonoscopy appearance. 2
• Colorectal neoplasia presents with new-onset diarrhea and must be excluded early in the investigative scheme due to its prevalence and potential seriousness. 1
• Infectious gastroenteritis from viral, bacterial, or parasitic pathogens accounts for most acute cases but can occasionally persist beyond four weeks. 3, 4

Small Bowel Malabsorption

• Celiac disease is the most common small bowel enteropathy in Western populations, with prevalence between 1:200 and 1:559, frequently presenting with steatorrhea manifesting as pale, bulky, malodorous stools. 1, 2
• Bile acid malabsorption causes diarrhea especially in patients with diabetes, after ileal resection, or following cholecystectomy, with up to 10% of post-cholecystectomy patients affected. 1, 2
• Bacterial overgrowth occurs in patients with surgical blind loops (Billroth II, Roux-en-Y), small bowel strictures from Crohn's disease or radiation, or conditions causing intestinal stasis. 1

Pancreatic Maldigestion

• Chronic pancreatitis is a primary cause of steatorrhea presenting as pale, bulky, malodorous yellow stools due to inadequate fat digestion. 5
• Pancreatic carcinoma and cystic fibrosis similarly impair pancreatic enzyme secretion, leading to fat malabsorption with significant impact on mortality and nutritional status. 5

Endocrine and Systemic Disorders

• Hyperthyroidism causes diarrhea through endocrine effects on intestinal motility, accelerating intestinal transit time. 1, 2
• Diabetes mellitus produces diarrhea via multiple mechanisms: autonomic neuropathy, bacterial overgrowth, bile acid malabsorption, and medication effects (especially metformin). 1, 2
• Parathyroid disease and adrenal disorders may predispose to diarrhea through various endocrine effects and autonomic dysfunction. 1

Iatrogenic and Medication-Related Causes

• Up to 4% of chronic diarrhea cases are medication-induced, particularly from magnesium-containing products, antihypertensives, NSAIDs, theophyllines, antibiotics, antiarrhythmics, and antineoplastic agents. 1, 2
• Food additives such as sorbitol and fructose can cause osmotic diarrhea. 1, 2
• Alcohol abuse causes diarrhea through rapid gut transit, decreased intestinal disaccharidase activity, and reduced pancreatic function. 1

Post-Surgical Diarrhea

• Upper GI surgery may result in vagal nerve damage or gastric capacity reduction, causing rapid gastric emptying with osmotic diarrhea and dumping syndrome. 1
• Small and large bowel resection reduces absorptive capacity; at the extreme, this results in short bowel syndrome characterized by chronic diarrhea, dehydration, electrolyte abnormalities, and malnutrition. 1
• Even limited right hemicolectomy may result in diarrhea through reduced absorptive surface area. 1

Functional and Motility Disorders

• Irritable bowel syndrome (IBS) affects 9-12% of the population and is characterized by reported changes in stool frequency or form, though stool weight typically does not increase. 1
• Functional diarrhea represents a common cause of chronic loose stools without the abdominal pain characteristic of IBS. 6

Special Clinical Scenarios

Faecal Incontinence with Overflow

• Faecal impaction with overflow diarrhea should be considered especially in the elderly, those with cognitive or behavioral issues, learning difficulties, or neurological/spinal disease. 1
• Clinical judgment rather than marker studies is recommended to confirm this diagnosis. 1

Secretory vs. Osmotic Mechanisms

• Secretory diarrhea results from bacterial toxins, reduced absorptive surface area, luminal secretagogues (bile acids, laxatives), circulating secretagogues (hormones, drugs, poisons), or compromised regulation of intestinal function. 7
• Osmotic diarrhea occurs when nonabsorbable, osmotically active substances are ingested, including carbohydrate malabsorption syndromes and laxative abuse. 7, 6

Critical Diagnostic Distinctions

The British Society of Gastroenterology emphasizes that symptoms persisting longer than four weeks suggest a non-infectious etiology and merit further investigation. 1 The broad range of conditions makes it difficult to separate causes on clinical grounds alone, necessitating a systematic approach that prioritizes exclusion of serious organic disease such as colonic neoplasia early in the investigative scheme. 1