What causes green-colored stool?

Causes of Green-Colored Stool

Green stool is most commonly caused by bile pigments that haven't been broken down in the intestines due to rapid intestinal transit, but can also result from food dyes, certain medications, or specific infections.

Primary Causes of Green Stool

1. Rapid Intestinal Transit

• When stool moves too quickly through the intestines, bile doesn't have time to be completely broken down 1
• Normally, bile is produced by the liver, stored in the gallbladder, and released into the small intestine to aid in fat digestion
• Bile starts out green (due to biliverdin) and gradually changes to brown (stercobilin) through bacterial action in the colon
• Conditions causing rapid transit:
  • Diarrhea (infectious or non-infectious)
  • Irritable bowel syndrome with diarrhea (IBS-D)
  • Inflammatory bowel disease flares

2. Bile Acid Malabsorption/Diarrhea

• Higher concentrations of bile acids in green stools (7.7 ± 1.52 mg/g) compared to brown stools (3.76 ± 0.7 mg/g) 2
• Causes include:
  • Ileal disease or resection (Crohn's disease)
  • Idiopathic bile acid diarrhea (affects 25-50% of patients with IBS-D) 3, 4
  • Malabsorption syndromes

3. Diet-Related Causes

• Food with green coloring (natural or artificial)
  • Leafy greens (spinach, kale)
  • Foods with green food dye
  • Iron-rich foods
• Food additives can cause color changes and potentially false positive fecal occult blood tests 5

4. Medications

• Iron supplements
• Antibiotics (alter gut flora that normally convert bile pigments)
• Laxatives containing senna

5. Infections

• Certain bacterial infections (e.g., Salmonella, Campylobacter)
• Clostridium difficile infection can present with various patterns 1
• Parasitic infections (e.g., Giardia)

Diagnostic Approach

When to Be Concerned

• Green stool alone is rarely concerning if transient
• Concerning features that warrant further evaluation:
  • Persistent green diarrhea (>4 weeks) 1
  • Associated symptoms: weight loss, blood in stool, fever, severe abdominal pain
  • Age >50 years with new onset of symptoms 1

Evaluation of Persistent Green Stool

1. Clinical assessment:

   • Detailed history of:
     • Stool characteristics (frequency, consistency, presence of blood/mucus) 1
     • Dietary habits and recent changes
     • Medication use (especially antibiotics, iron supplements)
     • Travel history
     • Family history of GI disorders

2. Basic laboratory testing (if symptoms persist):

   • Complete blood count 1
   • C-reactive protein or ESR if inflammation suspected
   • Stool studies for infectious causes including C. difficile toxin 1
   • Fecal calprotectin if inflammatory bowel disease suspected

3. Further testing (based on clinical presentation):

   • Colonoscopy if alarm symptoms present or age >50 years 1
   • Bile acid malabsorption testing in cases of chronic diarrhea 4

Management

Management depends on the underlying cause:

1. Dietary modifications:

   • Reduce intake of green foods and food dyes if diet-related
   • Consider trial of low-FODMAP diet if IBS suspected

2. For bile acid diarrhea:

   • Bile acid sequestrants (cholestyramine, colestipol, colesevelam) 3, 2
   • Bismuth subsalicylate can be effective in decreasing stool frequency and water content in green diarrhea 2

3. For infectious causes:

   • Targeted antimicrobial therapy based on identified pathogen
   • Supportive care with hydration

4. For medication-induced green stool:

   • Consider alternative medications if possible
   • Reassurance if medication is necessary and green stool is not harmful

Common Pitfalls

• Assuming all green stools are benign: While often harmless, persistent green stools with other symptoms may indicate underlying disease
• Missing bile acid diarrhea: This is an underdiagnosed condition in patients with chronic diarrhea 4
• Overreliance on stool color for diagnosis: Green stools can occur even in serious conditions like biliary atresia 6
• False reassurance from colored stools: Even with pigmented stools, significant hepatobiliary disease can be present 6

Remember that transient green stool is usually benign and often related to diet or temporary changes in intestinal transit time. However, persistent green stool, especially when accompanied by other symptoms, warrants further evaluation to identify potential underlying conditions.