Can the intestine absorb starch or disaccharides directly?

No. Only monosaccharides are transported across the enterocyte, so starch and disaccharides must first be digested to glucose, galactose, and fructose by amylase and brush border enzymes.

Why does adding glucose help rehydration during diarrhoea?

Glucose is absorbed together with sodium through SGLT1, and water follows the absorbed sodium osmotically, so a glucose-and-salt solution drives fluid uptake even when secretory diarrhoea is present.

Carbohydrate Absorption

Carbohydrate absorption is the process by which dietary sugars, reduced to monosaccharides by luminal and brush border enzymes, are transported across the small intestinal epithelium into the blood. Because the enterocyte can absorb only monosaccharides, digestion to glucose, galactose, and fructose is the obligatory prelude to their carrier-mediated uptake.

Definition

Carbohydrate absorption is the transepithelial transport of dietary monosaccharides — principally glucose, galactose, and fructose — across small intestinal enterocytes, following their release from starch and disaccharides by amylase and brush border hydrolases.

Scope

This entry covers the digestion of starch and disaccharides to monosaccharides and the membrane transporters that move those sugars across the enterocyte. It explains the sodium-coupled uptake of glucose and galactose and the facilitated uptake of fructose. It is a reference description of normal physiology and not clinical guidance.

Core questions

How are dietary starch and disaccharides reduced to absorbable monosaccharides?
Why must carbohydrates be monosaccharides before they can be absorbed?
How does the enterocyte transport glucose and galactose against their concentration gradients?
How is fructose absorbed, and how do sugars exit the cell into the blood?

Key concepts

Monosaccharides (glucose, galactose, fructose)
Salivary and pancreatic amylase
Brush border alpha-glucosidases (sucrase-isomaltase, maltase-glucoamylase)
Sodium-glucose cotransporter SGLT1 (secondary active transport)
Facilitative transporter GLUT5 (fructose)
Basolateral exit via GLUT2
Coupling of glucose uptake to the sodium gradient

Mechanisms

Luminal amylase cleaves starch to oligosaccharides and limit dextrins, which the brush border alpha-glucosidase complexes — sucrase-isomaltase and maltase-glucoamylase — together with maltase and other disaccharidases reduce to glucose, galactose, and fructose. Glucose and galactose enter the enterocyte across the apical membrane through SGLT1, which couples the inward movement of two sodium ions per sugar molecule to the downhill sodium gradient maintained by the basolateral Na+/K+-ATPase; this secondary active transport lets the cell concentrate these sugars. Fructose enters by facilitated diffusion through GLUT5 down its concentration gradient. All three monosaccharides leave the cell across the basolateral membrane through GLUT2 into the interstitium and blood. The tight coupling of glucose and sodium absorption is the physiological basis of sodium-glucose co-uptake exploited by oral rehydration solutions.

Clinical relevance

The sodium dependence of intestinal glucose transport explains why glucose enhances sodium and water absorption, the principle behind oral rehydration therapy. Defects in the relevant carriers or in specific disaccharidases leave their sugar substrates unabsorbed and osmotically active in the lumen. This entry describes normal mechanism for reference and is not diagnostic or treatment advice.

Evidence & guidelines

The molecular physiology of intestinal sugar transport is established in transporter cloning and electrophysiological studies and consolidated in major reviews and textbooks; as a normal-physiology topic it is not governed by clinical practice guidelines.

History

The idea that intestinal sugar uptake is powered by the sodium gradient rather than by ATP directly was formulated by Robert Crane in the early 1960s as the sodium-cotransport hypothesis, later vindicated by the cloning of SGLT1 and detailed kinetic study of the human sodium-glucose transporters. Parallel work on the brush border alpha-glucosidases clarified how starch is finished to glucose at the absorptive surface.

Key figures

Ernest M. Wright
Robert K. Crane
Buford L. Nichols

Seminal works

wright-2011
nichols-2003

Frequently asked questions

Can the intestine absorb starch or disaccharides directly?: No. Only monosaccharides are transported across the enterocyte, so starch and disaccharides must first be digested to glucose, galactose, and fructose by amylase and brush border enzymes.
Why does adding glucose help rehydration during diarrhoea?: Glucose is absorbed together with sodium through SGLT1, and water follows the absorbed sodium osmotically, so a glucose-and-salt solution drives fluid uptake even when secretory diarrhoea is present.