maltose

Maltose (glucose-α(1→4)-glucose) is a key intermediate product of starch digestion. Dietary starch (amylose and amylopectin) is first hydrolyzed by salivary amylase (begins digestion in the mouth) and pancreatic amylase (continues in the duodenum) into smaller oligosaccharides including maltose, maltotriose, and α-limit dextrins (branched fragments containing α-1,6 bonds). These products are further broken down by brush-border enzymes on enterocytes: maltase (glucoamylase) cleaves maltose and maltotriose into free glucose, while sucrase-isomaltase handles both sucrose and the α-1,6 bonds in limit dextrins. The resulting monosaccharides (glucose, fructose, galactose) are absorbed via SGLT1 (glucose/galactose, Na+-dependent) and GLUT5 (fructose) transporters on the apical membrane, then exit via GLUT2 into the bloodstream.

Clinical relevance: Congenital or acquired deficiencies in brush-border disaccharidases (e.g., sucrase-isomaltase deficiency) can impair maltose/starch digestion, leading to osmotic diarrhea, bloating, and flatulence due to unabsorbed carbohydrates being fermented by colonic bacteria. This is tested clinically via hydrogen breath tests. Maltose is also used in some clinical reagents; notably, certain point-of-care glucose monitoring strips using glucose dehydrogenase pyrroloquinoline quinone (GDH-PQQ) methodology can falsely read elevated glucose in patients receiving icodextrin (a maltose/maltotriose-containing peritoneal dialysis solution), an important patient safety consideration in USMLE Step 2 CK contexts (icodextrin-related pseudohyperglycemia).

Biochemically, maltose is a reducing sugar (free anomeric carbon) and can participate in the Maillard reaction, relevant to food chemistry and glycation studies (e.g., HbA1c formation is glucose-based, not maltose, but similar reducing sugar chemistry applies).