Disaccharidase Types and Functions
Brush border enzymes are membrane-bound proteins on enterocyte microvilli that catalyze the final step of carbohydrate digestion. Lactase hydrolyzes lactose to glucose and galactose. Sucrase-isomaltase (SI) cleaves sucrose into glucose and fructose, and α-limit dextrins (products of amylase action) into glucose. Maltase-glucoamylase (MGA) hydrolyzes maltose and maltodextrins. Trehalase cleaves trehalose (a disaccharide in mushrooms and some foods). Each enzyme is specific; lactase does not cleave sucrose, for example.
Congenital and Acquired Deficiencies
Congenital lactase deficiency is rare; more common is lactase persistence (continuing lactase expression into adulthood), which occurs in only ~35% of humans due to reduced lactase expression after weaning. Congenital sucrase-isomaltase deficiency is extremely rare (1:7,600 births) but causes severe osmotic diarrhea from birth. Acquired deficiencies follow enteritis (viral, bacterial, parasitic) that damages the brush border: post-infectious IBS often correlates with transient disaccharidase loss, healing over weeks to months.
Link to Carbohydrate Malabsorption
Undigested disaccharides remain in the intestinal lumen, exerting osmotic pressure: water follows, causing osmotic diarrhea. Colonic bacteria ferment these carbohydrates, producing gases (H2, CO2, CH4) and bloating. Breath hydrogen tests detect malabsorption: unabsorbed carbohydrate is fermented, raising breath H2 above baseline (>20 ppm at 90 minutes post-challenge suggests malabsorption). This non-invasive test guides diagnosis.
Relation to FODMAP Intolerance
FODMAP (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) intolerance overlaps with disaccharidase deficiency but isn't identical. Lactose, sucrose, and sorbitol are FODMAPs; those with disaccharidase deficiency may react to FODMAPs, but FODMAP sensitivity can occur without enzyme deficiency (due to bacterial overgrowth or hypersensitivity). The two are distinct mechanisms that often co-occur in IBS populations.
Diagnosis and Management
Brush border disaccharidase activity is measured from intestinal biopsies (rarely done clinically; mainly research). Hydrogen breath tests are standard for clinical assessment. Management: dietary restriction (lactose avoidance in lactase deficiency), enzyme supplementation (lactase pills, sacrosidase drops), or gradual reintroduction post-infection (as brush border heals). Understanding disaccharidase status prevents unnecessary restrictive diets.