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Carbohydrate digestion begins with salivary amylase in the mouth and continues with pancreatic amylase in the small intestine, cleaving polysaccharides and disaccharides into glucose, fructose, and galactose. Brush-border enzymes (maltase, sucrase, lactase) complete hydrolysis. Active transport via SGLT1 absorbs glucose and galactose; fructose absorption is passive (GLUT5). Absorption rate and completeness determine postprandial glucose response and affect satiety.
Compare digestion rates of simple sugars, disaccharides, and complex carbohydrates by studying blood glucose curves and satiety ratings after consumption. Examine lactase persistence and individual differences in absorption capacity.
From your study of carbohydrate structure and function, you know that dietary carbohydrates range from simple monosaccharides (glucose, fructose, galactose) through disaccharides (sucrose, lactose, maltose) to complex polysaccharides (starch, glycogen, fiber). From your work on nutrient digestion and absorption, you know that large molecules must be broken down to absorbable units before the intestinal epithelium can take them up. Carbohydrate digestion is the process that bridges these two facts: it is a sequential enzymatic disassembly that converts complex carbohydrates down to individual monosaccharides.
Digestion begins in the mouth, where salivary amylase (α-amylase) cleaves internal α-1,4-glycosidic bonds in starch and glycogen, producing shorter chains called maltose (a disaccharide) and dextrins (branched oligosaccharides). This oral phase is brief — food is swallowed quickly — and the enzyme is inactivated by stomach acid once it reaches the stomach. The stomach itself contributes no carbohydrate enzymes; this is why the stomach is not a major site of carbohydrate digestion. The main action resumes in the duodenum, where pancreatic amylase continues cleaving α-1,4 bonds in any remaining starch, producing maltose, maltotriose, and α-limit dextrins (branched fragments that α-amylase cannot fully resolve). The key point: at this stage, even after pancreatic amylase, you still do not have free glucose — you have small oligosaccharides and disaccharides.
The final hydrolysis step occurs at the brush border of the small intestinal epithelium, performed by membrane-bound enzymes named for their substrates. Maltase cleaves maltose into two glucose units; sucrase cleaves sucrose into glucose + fructose; lactase cleaves lactose into glucose + galactose; isomaltase cleaves the α-1,6 branch points of the α-limit dextrins. This is why lactase deficiency causes lactose intolerance — without functional lactase, lactose reaches the colon intact, where gut bacteria ferment it, producing gas, osmotic diarrhea, and bloating. The enzyme rather than the substrate is the rate-limiting step.
Once monosaccharides are free in the intestinal lumen, absorption occurs by two different mechanisms depending on the sugar. Glucose and galactose are absorbed by SGLT1 (Sodium-Glucose Linked Transporter 1), an active transport protein that co-transports one glucose and two sodium ions simultaneously. Because it runs on the electrochemical gradient for sodium (maintained by the Na/K-ATPase on the basolateral side), SGLT1 can transport glucose against a concentration gradient — this is why glucose absorption is so efficient and rapid even when lumen concentrations are low. Fructose, in contrast, uses GLUT5, a facilitated diffusion transporter that moves fructose passively down its concentration gradient. This is slower, saturable, and explains why consuming large amounts of fructose (e.g., from high-fructose corn syrup) can overwhelm GLUT5 capacity, leaving unabsorbed fructose to reach the colon. Once inside the enterocyte, all three monosaccharides exit into the portal bloodstream via GLUT2, a low-affinity, high-capacity bidirectional transporter on the basolateral membrane. The glucose then travels to the liver via the portal vein, triggering the insulin response and downstream metabolic consequences you will study when you examine postprandial glucose regulation.