Protein and Amino Acid Absorption

Protein and amino acid absorption is the process by which dietary protein, broken down by gastric and pancreatic proteases and brush border peptidases, is taken up across the small intestinal epithelium as free amino acids and small peptides. The intestine absorbs not only single amino acids but also di- and tripeptides, using a distinct, broadly specific peptide transporter.

Definition

Protein and amino acid absorption is the transepithelial uptake of free amino acids and di- and tripeptides by small intestinal enterocytes, following digestion of dietary protein by luminal proteases and brush border peptidases.

Scope

This entry covers the digestion of protein to amino acids and small peptides and the membrane transporters that carry these products into the enterocyte. It describes the families of sodium-coupled and exchange amino-acid carriers and the proton-coupled peptide transporter. It is a reference account of normal physiology, not clinical guidance.

Core questions

• How is dietary protein digested to absorbable amino acids and peptides?
• Which transporters carry amino acids across the enterocyte and how are they energised?
• Why does the intestine absorb small peptides as well as free amino acids?
• How do amino acids and the products of peptide hydrolysis leave the cell into the blood?

Key concepts

• Endopeptidases and exopeptidases (pepsin, trypsin, chymotrypsin, carboxypeptidases)
• Brush border peptidases (aminopeptidases, dipeptidases)
• Sodium-coupled amino-acid transporters (e.g., B0AT1)
• Amino-acid exchangers (antiporters)
• Proton-coupled peptide transporter PepT1
• Intracellular hydrolysis of absorbed peptides
• Basolateral amino-acid efflux

Mechanisms

Gastric pepsin and pancreatic endopeptidases and exopeptidases reduce dietary protein to a mixture of free amino acids and short oligopeptides; brush border peptidases trim these further to amino acids and di- and tripeptides. Free amino acids are taken up across the apical membrane by a set of transporters with overlapping substrate specificities, several of which couple amino-acid entry to the inward sodium gradient maintained by the basolateral Na+/K+-ATPase, while exchangers couple the uptake of one amino acid to the efflux of another. Di- and tripeptides are absorbed by the proton-coupled transporter PepT1, which uses the inwardly directed proton gradient at the brush border to drive peptide entry; once inside, most peptides are hydrolysed by cytosolic peptidases to free amino acids. Amino acids then exit across the basolateral membrane through facilitative and exchange carriers into the portal blood. The peptide route is broadly specific and contributes substantially to nitrogen absorption.

Clinical relevance

The existence of multiple, overlapping amino-acid carriers plus a broad-specificity peptide transporter explains why protein absorption is robust even when a single amino-acid carrier is defective, since affected amino acids can still be absorbed as peptides. This entry describes normal physiology for reference and is not diagnostic or treatment advice.

Evidence & guidelines

The transporters and digestive steps described here are established in molecular and physiological studies and standard textbooks; as a normal-physiology topic it is not governed by clinical practice guidelines.

History

Functional studies in the mid-twentieth century distinguished several amino-acid transport systems in the intestine, and the observation that some amino acids are absorbed faster as peptides than as free amino acids pointed to a separate peptide pathway. Molecular cloning later identified the specific amino-acid carriers and the proton-coupled peptide transporter PepT1, turning the classical functional systems into defined membrane proteins.

Key figures

• Stefan Bröer
• Hannelore Daniel

Seminal works

• broer-2008
• daniel-2004

Frequently asked questions

Does the intestine absorb whole proteins?

In adults, essentially no; dietary protein must be digested to free amino acids and small peptides, which are then absorbed by amino-acid carriers and the peptide transporter, with absorbed peptides usually broken down to amino acids inside the cell.

Why does the gut have a peptide transporter in addition to amino-acid carriers?

The proton-coupled transporter PepT1 absorbs a wide range of di- and tripeptides with a single, broadly specific carrier, providing an efficient and resilient route for nitrogen uptake that complements the more substrate-specific amino-acid transporters.