How is glucose absorbed in the intestine?

Glucose is taken up across the apical membrane by the sodium-glucose cotransporter SGLT1, which couples glucose entry to sodium moving down its electrochemical gradient, and then exits the cell basolaterally through the facilitative transporter GLUT2.

Why does oral rehydration solution contain both salt and sugar?

Because sodium-glucose cotransport pulls sodium and water into the body alongside glucose, combining glucose with salt enhances intestinal water absorption, which is the physiological basis of oral rehydration therapy.

Nutrient Absorption Mechanisms

Nutrient absorption mechanisms are the membrane-transport processes by which the products of digestion — monosaccharides, amino acids and peptides, fatty acids and monoglycerides — together with water, electrolytes, vitamins, and minerals are moved across the intestinal epithelium. They combine passive diffusion with carrier proteins, channels, and pumps that often harness ion gradients. This topic explains how each major class of nutrient is taken up.

Definition

Nutrient absorption mechanisms are the specific transport processes — diffusive, carrier-mediated, and ion-coupled — that translocate digested nutrients and associated solutes across the apical and basolateral membranes of intestinal enterocytes into the blood and lymph.

Scope

The entry covers the principles of transepithelial transport (passive diffusion, facilitated diffusion, primary and secondary active transport, and endocytosis) and applies them to the absorption of carbohydrates, proteins, and lipids, with reference to representative transporters such as the sodium-glucose cotransporter. It is a physiological reference; nutrient-specific clinical disorders are covered in sibling topics.

Core questions

What transport modes (diffusion, facilitated, active, endocytic) move nutrients across the enterocyte?
How are monosaccharides, amino acids and peptides, and lipids absorbed?
How do ion gradients power secondary active transport such as sodium-glucose cotransport?
How do apical and basolateral transporters cooperate to achieve net absorption?

Key concepts

Passive and facilitated diffusion
Primary and secondary active transport
Sodium-glucose cotransport (SGLT1) and GLUT transporters
Peptide and amino-acid transporters
Micelle formation and fatty-acid uptake
Apical-to-basolateral transcellular routing

Mechanisms

Enterocytes are polarised, with distinct apical (brush-border) and basolateral membranes bearing different transporters, so net absorption requires coordinated uptake at one pole and exit at the other (Kiela & Ghishan, 2016). Carbohydrates are absorbed as monosaccharides: glucose and galactose enter apically by sodium-coupled secondary active transport via SGLT1, driven by the sodium gradient that the basolateral sodium-potassium pump maintains, while fructose uses a facilitated route, and sugars exit basolaterally through GLUT transporters (Wright, Loo, & Hirayama, 2011). Proteins are absorbed as amino acids and small peptides through families of apical carriers, including proton-coupled peptide transport. Dietary lipids are first solubilised into micelles with bile salts, then fatty acids and monoglycerides cross the apical membrane, are re-esterified, and leave as chylomicrons via the lymphatics. Water and electrolytes move both transcellularly and through the regulated paracellular pathway between cells (Turner, 2009).

Clinical relevance

These transport mechanisms explain why specific defects produce specific malabsorption patterns — for example, loss of a brush-border enzyme or transporter impairs one nutrient class while sparing others — and they underpin physiological strategies such as oral rehydration, which exploits sodium-glucose cotransport. The content is explanatory background and does not constitute treatment guidance for any individual.

Evidence & guidelines

The transport mechanisms described here are established through decades of membrane-physiology and molecular cloning studies summarised in authoritative physiological reviews, which this entry cites; there are no disease-management guidelines specific to normal transport physiology.

History

The recognition that intestinal glucose uptake is coupled to sodium — the cotransport hypothesis advanced by Robert Crane in the 1960s — was a landmark in membrane physiology and later provided the rationale for oral rehydration therapy. Molecular cloning then identified the transporters themselves, including SGLT1 and the GLUT family, turning a kinetic concept into defined proteins and broadening the catalogue of carriers for amino acids, peptides, and ions.

Key figures

Ernest M. Wright
Robert K. Crane

Seminal works

wright-2011
kiela-2016

Frequently asked questions

How is glucose absorbed in the intestine?: Glucose is taken up across the apical membrane by the sodium-glucose cotransporter SGLT1, which couples glucose entry to sodium moving down its electrochemical gradient, and then exits the cell basolaterally through the facilitative transporter GLUT2.
Why does oral rehydration solution contain both salt and sugar?: Because sodium-glucose cotransport pulls sodium and water into the body alongside glucose, combining glucose with salt enhances intestinal water absorption, which is the physiological basis of oral rehydration therapy.