Carbohydrate digestion and absorption
Also known as: starch digestion, SGLT1, GLUT5, GLUT2, disaccharidases
Starch and disaccharides are broken down by salivary and pancreatic amylase and then by brush-border enzymes into glucose, galactose and fructose. These are absorbed by SGLT1 and GLUT5 and leave the enterocyte by GLUT2 into the portal blood.
Overview
The main dietary carbohydrates are starch (amylose with α-1,4 links; amylopectin with α-1,4 links and α-1,6 branch points), sucrose, lactose and some free glucose and fructose. Humans lack enzymes for β-linked polysaccharides such as cellulose. This dietary fibre reaches the colon, where bacteria ferment it to short-chain fatty acids.
Luminal phase. Salivary α-amylase starts starch hydrolysis in the mouth and continues inside the swallowed bolus until gastric acid inactivates it. Pancreatic α-amylase in the duodenum does most of the work. It cleaves interior α-1,4 bonds to produce maltose, maltotriose and α-limit dextrins, which retain the α-1,6 branch points. Amylase cannot release free glucose.
Brush-border phase. Enzymes anchored in the microvillous membrane finish the job. Maltase-glucoamylase removes glucose from the ends of α-1,4 chains. Sucrase-isomaltase splits sucrose to glucose and fructose, and its isomaltase part cleaves α-1,6 branch points. Lactase splits lactose to glucose and galactose. Trehalase splits trehalose (from mushrooms).
Absorption. Glucose and galactose enter enterocytes by SGLT1, which carries 2 Na⁺ with each sugar. This is secondary active transport, powered by the Na⁺ gradient that the basolateral Na⁺/K⁺-ATPase maintains. Fructose enters by facilitated diffusion through GLUT5. All three monosaccharides leave across the basolateral membrane through GLUT2 and reach the liver in the portal blood. When luminal glucose is high, GLUT2 is also inserted into the apical membrane. Absorption is mainly in the duodenum and jejunum and is nearly complete.
Parts & subdivisions
- Mouth and stomach (inside the bolus). Hydrolyses α-1,4 bonds.
- Duodenum. Products: maltose, maltotriose, α-limit dextrins.
- Maltase-glucoamylase, sucrase-isomaltase, lactase, trehalase.
- Na⁺-coupled uptake of glucose and galactose; facilitated uptake of fructose; basolateral exit of all three.
Function
- Converts dietary starch and sugars into absorbable monosaccharides.
- Couples Na⁺ and water absorption to glucose uptake.
Clinical significance
Lactose intolerance
Lactase non-persistence affects most adults worldwide. Undigested lactose holds water osmotically and is fermented in the colon, causing bloating, flatulence, cramps and diarrhoea. Breath hydrogen rises and stool pH is low. Secondary lactase deficiency follows enteritis or coeliac disease.
Oral rehydration solution
Because SGLT1 couples Na⁺ with glucose, and water follows, a glucose–salt solution is absorbed even when secretion is massive, as in cholera. WHO low-osmolarity ORS contains about 75 mmol/L each of Na⁺ and glucose. It has saved millions of lives.
α-Glucosidase inhibitors
Acarbose inhibits brush-border α-glucosidases, which slows glucose absorption and blunts post-prandial hyperglycaemia in type 2 diabetes. Flatulence is the typical side effect.
Test yourself
Which transporter is responsible for apical uptake of glucose by enterocytes?
- GLUT2
- GLUT5
- PepT1
- SGLT1
Answer: SGLT1
SGLT1 co-transports 2 Na⁺ with each glucose or galactose molecule (secondary active transport). GLUT5 takes up fructose. GLUT2 carries all three monosaccharides out across the basolateral membrane. PepT1 carries di- and tripeptides. Oral rehydration therapy depends on SGLT1.
Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.