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.