[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Gastrointestinal](https://durgical.com/en/physiology/#s-gi) › Carbohydrate digestion and absorption [Physiology](https://durgical.com/en/physiology/) · [Gastrointestinal](https://durgical.com/en/physiology/#s-gi) # 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](https://durgical.com/en/anatomy/duodenum/) · [Jejunum](https://durgical.com/en/anatomy/jejunum/) · [Pancreas](https://durgical.com/en/anatomy/pancreas/) · [Parotid gland](https://durgical.com/en/anatomy/parotid-gland/) ## Key points - Only monosaccharides are absorbed: glucose, galactose and fructose. - SGLT1 (apical) co-transports Na⁺ with glucose or galactose (2 Na⁺ per sugar), driven by the gradient the basolateral Na⁺/K⁺-ATPase creates (secondary active transport). - GLUT5 (apical) carries fructose by facilitated diffusion; GLUT2 (basolateral) moves all three monosaccharides into the blood. - Amylase cuts α-1,4 bonds but not α-1,6 or β bonds; it yields maltose, maltotriose and α-limit dextrins. It cannot digest cellulose. - Lactase is the scarcest disaccharidase and the first one lost (adult lactase non-persistence, mucosal damage in celiac disease or gastroenteritis). - SGLT1 is preserved in secretory diarrhea: the basis of glucose-containing oral rehydration salts. ## 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. ## Memory aid 'Glucose and galactose travel with sodium; fructose goes alone': SGLT1 for the two Gs, GLUT5 for the F, GLUT2 to get out. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/carbohydrate-digestion-and-absorption/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Duodenum](https://durgical.com/en/anatomy/duodenum/) - [Jejunum](https://durgical.com/en/anatomy/jejunum/) - [Pancreas](https://durgical.com/en/anatomy/pancreas/) - [Parotid gland](https://durgical.com/en/anatomy/parotid-gland/) ## Related topics - [Water and electrolyte absorption and how diarrhea happens](https://durgical.com/en/physiology/water-and-electrolyte-absorption-and-how-diarrhea-happens/) - [Patterns of malabsorption](https://durgical.com/en/physiology/patterns-of-malabsorption/) - [Exocrine pancreatic secretion](https://durgical.com/en/physiology/exocrine-pancreatic-secretion/) - [Salivary secretion](https://durgical.com/en/physiology/salivary-secretion/) - [Primary and secondary active transport](https://durgical.com/en/physiology/primary-and-secondary-active-transport/) - [Membrane transport: diffusion, channels and carriers](https://durgical.com/en/physiology/membrane-transport-diffusion-channels-and-carriers/) ## Related conditions - [Lactose intolerance](https://durgical.com/en/conditions/lactose-intolerance/) - [Coeliac disease](https://durgical.com/en/conditions/coeliac-disease/) - [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/) - [Classic galactosaemia](https://durgical.com/en/conditions/classic-galactosaemia/) - [Hereditary fructose intolerance](https://durgical.com/en/conditions/hereditary-fructose-intolerance/) ## In Biochemistry - [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/) - [Galactose](https://durgical.com/en/biochemistry/molecules/galactose/) - [Fructose](https://durgical.com/en/biochemistry/molecules/fructose/) - [Carbohydrate chemistry](https://durgical.com/en/biochemistry/topics/carbohydrate-chemistry/) - [Membrane transport](https://durgical.com/en/biochemistry/topics/membrane-transport/) - [Na⁺/K⁺-ATPase (sodium pump)](https://durgical.com/en/biochemistry/enzymes/na-k-atpase/) - [Amylase as a lab test](https://durgical.com/en/biochemistry/topics/amylase-as-a-lab-test/) [← All of physiology](https://durgical.com/en/physiology/) 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](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look) (OpenStax (Rice University), 2022); [Lactose intolerance: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000276.htm) (MedlinePlus, US National Library of Medicine, 2024); [Celiac Disease](https://www.ncbi.nlm.nih.gov/books/NBK441900/) (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. --- Source: https://durgical.com/en/physiology/carbohydrate-digestion-and-absorption/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/carbohydrate-digestion-and-absorption/