Loose Stools with Undigested/Semi-digested Food

Pathophysiological Framework

Causes - Differential Diagnosis

1. Pancreatic Exocrine Insufficiency (PEI)

• Seen in chronic pancreatitis, pancreatic cancer, cystic fibrosis
• Steatorrhea appears only when lipase output falls below 10% of normal (Schwartz's Surgery)
• Stools are large, bulky, fat-laden (>30 g fat/day), foul-smelling, oily, difficult to flush
• Often associated with abdominal pain and diabetes

2. Celiac Disease (Gluten-Sensitive Enteropathy)

• Autoimmune villous atrophy of the proximal small intestine
• Diarrhea with steatorrhea + weight loss + bloating; extraintestinal: dermatitis herpetiformis, iron/folate deficiency anemia, osteomalacia
• Diagnosis: anti-tissue transglutaminase IgA (anti-TTG IgA) + duodenal biopsy

3. Small Intestinal Bacterial Overgrowth (SIBO)

• Excess bacteria in small bowel deconjugate bile salts - impaired micelle formation
• Causes steatorrhea + B12 deficiency + bloating; often in setting of structural abnormalities, dysmotility, or post-surgical anatomy
• Diagnosed by glucose/lactulose H2 breath test or small bowel aspirate culture

4. Inflammatory Bowel Disease (Crohn's Disease)

• Transmural inflammation, particularly ileitis, reduces absorptive surface
• Undigested food, diarrhea, abdominal pain, weight loss, possible blood
• May present with perianal disease, fever, extraintestinal manifestations (arthritis, uveitis, erythema nodosum)

5. Giardiasis

• Most common protozoan cause of malabsorption worldwide
• Loose, greasy, foul-smelling stools; bloating, flatulence, abdominal cramps; NO blood in stool
• Exposure history: travel, contaminated water, children
• Diagnosis: stool PCR or antigen ELISA; empirical metronidazole trial is reasonable

6. Rapid Intestinal Transit / Dysmotility

• Hyperthyroidism, carcinoid syndrome, diabetic autonomic neuropathy, VIPoma
• Reduced contact time = incomplete digestion and absorption
• Even with rapid transit alone, mild steatorrhea up to 14 g fat/day can occur (Harrison's)

7. Bile Acid Malabsorption (BAD)

• Ileal disease/resection (<100 cm) leads to bile acid loss into colon, causing secretory + osmotic diarrhea
• Watery diarrhea with urgency; may have some fat malabsorption

8. Whipple's Disease

• Tropheryma whipplei infection; rare but classic
• Steatorrhea + arthralgia + lymphadenopathy + weight loss + CNS changes
• Duodenal biopsy: PAS-positive macrophages in lamina propria

9. Tropical Sprue

• Acquired enteropathy in endemic regions (tropics)
• Pale, bulky, frothy, foul-smelling stools; glossitis, weight loss, B12/folate deficiency (Sleisenger & Fordtran)

10. Short Bowel Syndrome

• Post-surgical reduction of absorptive surface
• Diarrhea worsens with eating (unlike secretory diarrhea which persists with fasting)

11. Lactase / Disaccharidase Deficiency

• Unabsorbed sugars cause osmotic diarrhea + fermentation gas
• Watery diarrhea, bloating after dairy/sugars - typically NOT steatorrhea
• H2 breath test after lactose challenge

Key Features on History

• Relationship of diarrhea to food ingestion (worsening with eating suggests malabsorption/short bowel; secretory diarrhea persists with fasting)
• Presence of nocturnal episodes (organic > functional)
• Undigested food or fat in stool
• Consistency (Bristol Stool Scale)
• Weight loss (points to organic malabsorption)
• Floating/oily/foul-smelling stools (classic steatorrhea)
• Travel history, medication history, family history of celiac/IBD

Diagnostic Approach

Algorithm for management of chronic diarrhea - Harrison's 22E

Initial Workup

• CBC - anemia (iron, B12, folate deficiency)
• CRP/ESR - inflammation
• Anti-TTG IgA + total IgA - celiac screen
• Stool fat test (72h fecal fat on 70-100 g/day fat diet) - best screening test for malabsorption
  • Stool fat >20 g/day → evaluate pancreatic function (stool elastase, imaging)
  • Stool fat 14-20 g/day → evaluate small bowel (biopsy, imaging, SIBO breath test)
  • Stool fat <14 g/day → rapid transit or functional cause more likely
• Stool PCR/ELISA for Giardia + ova and parasites
• Serum albumin, electrolytes, LFTs, TSH
• Stool calprotectin - screens for intestinal inflammation

Second-line Investigations

• CT/MR enterography - mucosal disease, structural abnormalities, lymphoma
• Upper endoscopy with duodenal biopsy - celiac, Whipple's, Giardia trophozoites
• Colonoscopy to terminal ileum - IBD, microscopic colitis, villous adenoma
• H2 breath test - SIBO, lactase deficiency
• Serum 7αC4 or fecal bile acids - bile acid diarrhea
• Stool elastase-1 - pancreatic exocrine insufficiency (sensitivity ~85-90% for severe PEI)
• Hormonal screen (VIP, gastrin, serotonin, calcitonin) if secretory neuroendocrine tumor suspected

Red Flags Requiring Urgent Investigation

• Weight loss >5 kg
• Nocturnal diarrhea
• Rectal bleeding
• Age >50 at onset
• Family history of colorectal cancer or IBD
• Anemia, hypoalbuminemia, edema
• Abdominal mass

Summary Table