Carbohydrate digestion and absorption !
- Mechanism
- Exam priority: high
Starch and sugars are cut down to glucose, galactose and fructose; only those monosaccharides cross the gut wall.
Updated 2026-11-01
Also called: Disaccharidases, SGLT1 and GLUT5
See it in 3D: Duodenum · Jejunum · Pancreas · Parotid gland
What it is
Dietary carbohydrate arrives mostly as starch, sucrose and lactose. Salivary and pancreatic amylase break starch into short fragments, and brush-border enzymes of the duodenum and jejunum finish cutting them into monosaccharides. Only glucose, galactose and fructose are absorbed, each by its own transporters; fibre and anything undigested reaches the colon, where bacteria ferment it.
How it works, step by step
- In the mouth salivary amylase starts cutting starch; gastric acid inactivates it, but it keeps working inside the bolus.
- In the duodenum pancreatic amylase completes luminal digestion: maltose, maltotriose and α-limit dextrins.
- At the brush border, maltase-glucoamylase, sucrase-isomaltase and lactase release monosaccharides right next to their transporters.
- Glucose and galactose enter with Na⁺ through SGLT1; fructose enters through GLUT5.
- Basolateral GLUT2 moves them into the villus capillary; they travel in the portal vein to the liver.
- The basolateral Na⁺/K⁺-ATPase keeps cell Na⁺ low and so sustains glucose entry; water follows the absorbed solutes.
- Whatever is not absorbed reaches the colon: it draws water (osmotic diarrhea) and bacteria ferment it to short-chain fatty acids, H₂ and CO₂.
Brush-border enzymes and their products
| Enzyme | Substrate | Products |
|---|---|---|
| Lactase | Lactose | Glucose + galactose |
| Sucrase-isomaltase | Sucrose; α-limit dextrins (α-1,6 bonds) | Glucose + fructose; glucose |
| Maltase-glucoamylase | Maltose, maltotriose | Glucose |
| Trehalase | Trehalose (mushrooms) | Glucose |
Clinical notes
- Lactose intolerance: undigested lactose causes osmotic diarrhea, bloating and flatulence; the hydrogen breath test rises and stools are acidic.
- In much of Colombia and Latin America lactase declines after childhood; it is a common trait, not a serious disease.
- Celiac disease and gastroenteritis flatten villi and cause secondary lactase deficiency, which improves as the mucosa heals.
- D-xylose test: absorbed without prior digestion; if low, the problem is mucosal, not pancreatic.
Test yourself
Think of the answer, then tap the question to see it.
Why does adding glucose to an oral rehydration solution increase sodium and water absorption in cholera diarrhea?
Cholera drives chloride secretion but leaves SGLT1 intact. Glucose pulls Na⁺ in through that co-transporter, and water follows the absorbed solute.
Which transporters does fructose need to get from the gut lumen into the blood, and what kind are they?
GLUT5 on the apical membrane and GLUT2 on the basolateral membrane; both are facilitated diffusion, not coupled to sodium.
Why does lactose intolerance cause diarrhea with acidic stools and raised breath hydrogen?
Undigested lactose holds water in the lumen (osmotic diarrhea), and colonic bacteria ferment it into short-chain fatty acids that acidify stool and H₂ that is absorbed and exhaled.
With steatorrhea and a normal D-xylose test, is the problem more likely mucosal or pancreatic?
Pancreatic (maldigestion). D-xylose needs no enzymes, so a healthy mucosa absorbs it normally; mucosal disease lowers its absorption.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Water and electrolyte absorption and how diarrhea happens
- Patterns of malabsorption
- Exocrine pancreatic secretion
- Salivary secretion
- Primary and secondary active transport
- Membrane transport: diffusion, channels and carriers
Related conditions
- Lactose intolerance
- Coeliac disease
- Acute (viral) gastroenteritis
- Classic galactosaemia
- Hereditary fructose intolerance
In Biochemistry
- Glucose
- Galactose
- Fructose
- Carbohydrate chemistry
- Membrane transport
- Na⁺/K⁺-ATPase (sodium pump)
- Amylase as a lab test
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 23.7 Chemical Digestion and Absorption: A Closer Look (OpenStax (Rice University), 2022); Lactose intolerance: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2024); Celiac Disease (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 66). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.