Medicine & USMLE

Lactase Deficiency

Metabolic Disorders
  1. Albinism
  2. Pyruvate Dehydrogenase Deficiency
  3. Pyruvate Kinase Deficiency
  4. G6PD Deficiency
  5. Essential Fructosuria
  6. Hereditary Fructose Intolerance
  7. Galactosemia
  8. Galactokinase Deficiency
  9. Lactase Deficiency
  10. Ornithine Transcarbamylase Deficiency
  11. Phenylketonuria (PKU)
  12. Maple Syrup Urine Disease
  13. Alkaptonuria
  14. Homocystinuria
  15. Cystinuria
  16. Propionic Acidemia
  17. Lesch-Nyhan Syndrome
  18. Systemic Primary Carnitine Deficiency
  19. MCAD Deficiency


Lactase deficiency is caused by the absence of lactase in the intestinal brush border. It results in the body’s inability to digest lactose into its two components, glucose and galactose. The build-up of osmotically active lactose creates an osmotic effect in the intestinal lumen causes osmotic diarrhea. And since lactose is fermented by colonic bacteria, it can also cause bloating and flatulence. There are three types of lactase deficiency, primary, secondary, and congenital. Primary lactase deficiency is most common in Asian, African American, and Native American populations and results in age-dependent decline of the lactase enzyme. Secondary lactase deficiency is caused by gastroenteritis, particularly in the context of a rotavirus or autoimmune disease. In both primary and congenital lactase deficiency, you’ll see normal mucosa on biopsy and in secondary lactase deficiency, you’ll see damaged mucosa on biopsy. Two ways to put lactase deficiency on a differential would be to note acidic stool pH and the lactose hydrogen breath test, both of which are due to lactose fermentation by colonic bacteria.

Key Points

  • Lactase deficiency (lactose intolerance)
    • Pathology
      • Insufficient lactase → dietary lactose intolerance
        • Lactase works at the intestinal brush border
        • Splits lactose (in milk and milk products) into:
          • Glucose
          • Galactose
    • Types
      • Primary
        • Age-dependent decline after childhood
          • Due to absence of lactase-persistence allele
        • Intestinal biopsy shows normal mucosa
        • Common in:
          • Asians
          • Africans
          • Native Americans
      • Secondary
        • Loss of intestinal brush border due to inflammation
          • May be caused by gastroenteritis (e.g. rotavirus), autoimmune disease, etc
        • Intestinal biopsy shows damaged mucosa
      • Congenital
        • Rare, due to defective gene
        • Intestinal biopsy shows normal mucosa
    • Signs and Symptoms
      • Colonic bacteria ferment undigested lactose in intestine causing:
        • Decreased stool pH (acidic stools)
        • Decreased breath pH (lactose hydrogen breath test)
        • Bloating / cramps / flatulence
      • Osmotic diarrhea secondary to undigested lactose
    • Treatment
      • Avoid dairy products
        • Instead use lactose-free products--like lactose-free milk
      • Or take lactase supplements