Lactase is a brush‑border enzyme that cleaves lactose, a disaccharide, into absorbable monosaccharides (galactose and glucose).
In the case of lactase deficiency, an almost complete absence of lactose digestion is observed, resulting in decreased absorption in the small intestine (particularly the jejunum).
The transfer of osmotically active amounts of lactose into the large intestine leads to the osmotic binding of water → diarrhea with a high osmotic gap
↓ Stool pH (< 6): due to lactose fermentation by colonic bacterial flora
Increased peristalsis due to increased intestinal filling → abdominal pain
Metabolism of lactose via the physiological bacterial flora of the colon
Formation of short‑chain fatty acids that exacerbate diarrhea
Increased gas formation and flatulence
Clinical features
Diagnostics
Hydrogen breath test
The amount of hydrogen in the expired air increases after administering lactose in the fasting state.
Procedure
Fasting for 8–12 hours
Ingestion of lactose
Measurement of breath hydrogen levels at baseline and at 30‑minute intervals over 3 hours
Breath hydrogen levels > 20 ppm are considered diagnostic of lactose intolerance.