Epidemiology

Etiology

Primary lactase deficiency (lactase nonpersistence)
- Most common cause of lactose intolerance
- Genetic polymorphism in the lactase‑coding gene with subsequent downregulation (absence of lactase-persistence allele)
- Symptoms occur after lactase activity starts to decrease during childhood; the rate of decline is influenced by genetic factors.
Secondary lactase deficiency (acquired)
- Lactase is located distally in the intestinal villi, which are particularly affected by mucosal damage.
- Due to underlying disorders of the small intestine that result in mucosal damage, e.g.:
  - Autoimmune disorders (e.g., gluten-sensitive enteropathy)
  - Infectious gastroenteritis (e.g., due to rotavirus, giardiasis)

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

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.
Stool analysis
- ↑ Stool osmotic gap
- ↓ Stool pH (< 6): due to lactose fermentation by colonic bacterial flora

Avoid or reduce intake of milk products: lactose‑free or lactose‑reduced products
Oral lactase supplements