Does reduced lactase activity cause lactose malabsorption?

Lactase biology strongly supports the claim: it is a specialized epithelial enzyme required for normal digestion and absorption of dietary lactose.

Mechanism and localization

• Lactase, or lactase-phlorizin hydrolase, is an integral glycoprotein of the apical microvillar brush-border membrane of absorptive small-intestinal villus enterocytes. Its catalytic activity faces the intestinal lumen, where it hydrolyzes lactose into glucose and galactose, the absorbable monosaccharides.
• Expression is linked to enterocyte maturation: lactase is minimal in crypt cells and more prominent in differentiated villus cells. It is synthesized as a precursor and processed through the Golgi into its mature brush-border form. Activity/distribution varies along the small bowel, with relatively greater jejunal expression or activity reported than in more proximal or distal segments.

Clinical significance

• Reduced brush-border lactase activity directly causes lactose malabsorption: lactose is inadequately hydrolyzed, remains unabsorbed in the small intestine, and reaches the colon.
• There, bacterial fermentation generates hydrogen and other metabolites, while unabsorbed lactose contributes an osmotic load. These processes can produce bloating, gas, abdominal pain, or diarrhea, but symptoms are not inevitable.
• Thus, lactose malabsorption is an objective absorptive outcome, whereas lactose intolerance requires symptoms and varies with lactose dose, colonic microbiota, and visceral sensitivity.

Testing and practical interpretation

• A lactose hydrogen-breath test can identify malabsorption through downstream colonic fermentation; a rise in breath hydrogen of at least 20 ppm above baseline after a lactose challenge supports lactose malabsorption.
• Reduced lactase activity alone does not establish whether low activity is primary adult-type hypolactasia or secondary to small-intestinal mucosal disease.

Bottom line

• The claim is strongly supported: lactase is a small-intestinal brush-border enzyme, and reduced activity causes lactose malabsorption by preventing lactose conversion to absorbable glucose and galactose. Symptoms, however, are a separate and variable consequence.