What causes a stool pH of 5.5?

Causes of Stool pH of 5.5

A stool pH of 5.5 is most commonly caused by carbohydrate malabsorption, where unabsorbed sugars are fermented by colonic bacteria producing organic acids that lower the stool pH. 1

Primary Causes of Acidic Stool (pH 5.5)

Carbohydrate Malabsorption

• Stool pH values below 5.6 (and especially below 5.3) are strongly indicative of carbohydrate malabsorption, as demonstrated in experimental studies 1
• Unabsorbed carbohydrates (such as lactose, fructose, sorbitol) are fermented by colonic bacteria, producing short-chain fatty acids and lactic acid that lower stool pH 1, 2
• Common carbohydrate malabsorption conditions include:
  • Lactose intolerance
  • Fructose malabsorption
  • Sorbitol intolerance (from sugar-free foods)
  • Small intestinal bacterial overgrowth (SIBO)

Bacterial Fermentation Patterns

• At acidic pH levels (5.2-5.5), there is increased production of lactic acid, succinic acid, and formic acid by colonic bacteria 2
• Bifidobacterium populations, which are major lactate producers, increase significantly in the presence of undigested polysaccharides 2
• The balance between lactate production and utilization is disrupted at low pH, leading to lactate accumulation 2

Secondary Causes and Clinical Implications

Medication-Induced Changes

• Certain medications can alter gut transit time or bacterial flora, affecting carbohydrate absorption:
  • Antibiotics (disrupting normal flora)
  • Proton pump inhibitors (altering upper GI pH)
  • Laxatives (particularly those containing lactulose or sorbitol) 3

Pathological Conditions

• Helicobacter pylori infection can affect gastric pH, potentially influencing downstream digestion 3
• Inflammatory bowel conditions may alter intestinal transit and absorption capacity
• Malabsorption syndromes (celiac disease, pancreatic insufficiency) can lead to increased undigested carbohydrates reaching the colon 3

Clinical Significance of Stool pH

• Acidic stool pH (≤5.5) is generally associated with better clinical outcomes in critically ill patients compared to alkaline stool pH 4
• Alkaline stool pH (>7.0) is associated with increased risk of C. difficile infection 5
• Normal stool pH ranges from 6.0 to 7.5 in healthy adults 1

Diagnostic Considerations

Stool Testing Interpretation

• Fecal osmotic gap can help differentiate osmotic from secretory diarrhea:
  • Osmotic gap >50 mOsm/kg with pH <5.6 suggests carbohydrate malabsorption 1
  • Low fecal pH with high osmotic gap is characteristic of osmotic diarrhea caused by unabsorbed carbohydrates 1

Potential Pitfalls in Interpretation

• Factitious diarrhea (laxative abuse) can alter stool pH and should be considered in patients with chronic unexplained diarrhea 3
• Stool samples should be analyzed promptly as pH can change with prolonged exposure to air 3
• Dietary factors (high fiber intake, probiotic consumption) can influence stool pH independent of pathological conditions 6

Management Implications

• Identifying carbohydrate malabsorption through stool pH can guide dietary modifications:
  • Reduction of specific carbohydrates (lactose, fructose, FODMAPs)
  • Enzyme supplementation when appropriate
• Treatment of underlying conditions (SIBO, H. pylori) may normalize stool pH 3
• Monitoring stool pH can be useful in assessing treatment response in certain conditions 4