Carbohydrate Intolerance
Pathophysiology
- Normal carbohydrate digestion requires intact small intestinal brush border disaccharidases (lactase, sucrase-isomaltase) and specific transporters (SGLT1, GLUT5).
- Enzyme or transporter deficiency leaves unabsorbed carbohydrates in intestinal lumen.
- Unabsorbed sugars pass into large bowel.
- Colonic microflora ferments sugars, producing short-chain organic acids (lactic acid) and gases (hydrogen, methane).
- Organic acids decrease stool pH and create strong osmotic gradient.
- Retained intraluminal water causes osmotic diarrhea; gas causes distension.
Clinical Features
- Explosive, watery diarrhea.
- Frothy, bulky stools.
- Severe perianal excoriation.
- Abdominal distension, cramps, excessive flatulence, bloating.
- Failure to thrive or weight loss if prolonged.
- Resolution of diarrhea upon fasting or dietary elimination.
Etiological Classification
| Category | Specific Disorders | Pathogenesis |
|---|---|---|
| Primary/Congenital | Congenital lactase deficiency | Rare autosomal recessive defect; severe diarrhea at birth. |
| Congenital sucrase-isomaltase deficiency | Autosomal recessive SI gene mutation; symptoms upon sucrose/polymer introduction. | |
| Glucose-galactose malabsorption | Autosomal recessive SLC5A1 mutation; severe neonatal osmotic diarrhea. | |
| Primary hypolactasia (Adult-type) | Gradual age-related loss of lactase activity; common globally. | |
| Fructose malabsorption | Reduced GLUT-5 transporter abundance; common with excessive fruit juice intake. | |
| Secondary/Acquired | Post-infectious enteropathy | Acute gastroenteritis causes mucosal damage. |
| Celiac disease | Villous atrophy reduces absorptive surface and brush border enzymes. | |
| Cow milk protein allergy | Immune-mediated mucosal injury. | |
| Small intestinal bacterial overgrowth | Bacterial fermentation in proximal small bowel. | |
| Malnutrition | Reduced brush border disaccharidases. |
Diagnostic Evaluation
| Investigation | Expected Findings In Carbohydrate Malabsorption |
|---|---|
| Stool pH | Acidic (pH < 5.0 or 5.5) due to organic acids. |
| Stool reducing substances | Positive (>2+). Note: Sucrose requires prior acid hydrolysis to test positive. |
| Stool osmotic gap | > 100 mOsm/kg (calculated as measured osmolality - 2x[Na+ + K+]). |
| Breath hydrogen test | Rise > 20 parts per million (ppm) above baseline after specific carbohydrate load. |
| Small bowel biopsy | Assays specific mucosal disaccharidase concentrations; evaluates villous architecture (differentiates primary from secondary causes). |
| Genetic testing | Identifies mutations (e.g., SLC5A1 for glucose-galactose malabsorption, SI for sucrase-isomaltase deficiency). |
Management Of Specific Disorders
Lactose Intolerance
- Implement low-lactose or lactose-free diet.
- Utilize lactase enzyme supplements.
- Provide alternative calcium sources to meet dietary targets.
- Permit live-culture yogurt and hard cheeses; bacterial lactase aids digestion.
- In secondary deficiency, restrict temporarily; reintroduce gradually after mucosal healing.
Sucrase-Isomaltase Deficiency
- Implement lifelong dietary restriction of sucrose-containing foods and complex glucose polymers.
- Administer sacrosidase (purified yeast enzyme replacement) as adjunct therapy.
Glucose-Galactose Malabsorption
- Mandate rigorous lifelong restriction of glucose and galactose.
- Initiate carbohydrate-free formula supplemented with 6-8% fructose.
- Note: limited starch or sucrose tolerance may develop in later life.
Fructose Malabsorption
- Restrict excessive intake of fruit juices (apple, pear, prune), corn syrup, and sorbitol.
- Avoid artificial sweeteners converting to fructose.