U01.01.109 Lactase deficiency

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Learning Objectives

  • Identify the three types of lactase deficiency and their underlying causes.
  • Explain the biochemical basis for osmotic diarrhea and gas production in lactose intolerance.
  • Recognize the diagnostic findings of stool pH and hydrogen breath tests.
  • Distinguish between primary and secondary lactase deficiency based on intestinal biopsy findings.

1. Normal Physiology vs. Deficiency

Lactase is an enzyme located on the intestinal brush border (the microvilli of the small intestine). Its role is to hydrolyze lactose, a disaccharide found in milk, into the monosaccharides glucose and galactose for absorption.

  • Primary Deficiency: An age-dependent decline in enzyme activity after childhood. It is the most common form and is highly prevalent in Asian, African, and Native American populations.
  • Secondary Deficiency: Occurs when the brush border is damaged. Common causes include gastroenteritis (e.g., rotavirus), Celiac disease, or Crohn’s disease.
  • Congenital Deficiency: A rare, autosomal recessive genetic defect where the enzyme is absent from birth.

2. Clinical Findings and Pathophysiology

When lactose remains undigested, it acts as an osmotically active solute in the lumen of the gut and becomes a substrate for colonic bacteria.

  • Osmotic Diarrhea: Undigested lactose draws water into the colon.
  • Bloating, Flatulence, and Cramps: Colonic bacteria ferment the lactose, producing hydrogen gas (H_{2}) and organic acids.

Diagnostic Test Result Reasoning
Hydrogen Breath Test Increased H_{2} content Bacterial fermentation of undigested lactose produces gas.
Stool pH Decreased pH (Acidic) Fermentation produces organic fatty acids.
Intestinal Biopsy Normal Mucosa In primary/hereditary deficiency, the structure is normal, but the enzyme is low.

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3. Management and Treatment

Management focuses on reducing the lactose load in the gastrointestinal tract while maintaining calcium intake.

  • Dietary Modification: Avoidance of dairy products or switching to lactose-free milk.
  • Enzyme Replacement: Oral lactase pills (taken before dairy consumption) can help the patient digest the lactose.

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High-Yield Point:

In secondary lactase deficiency, the intestinal biopsy will show abnormal mucosa (e.g., villous atrophy in Celiac disease), whereas in primary deficiency, the biopsy appears completely normal.

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