Absorption of Proteins — Core Principles

Core Principles

Protein absorption is the final step in making dietary proteins available to the body. It begins with the extensive breakdown of large protein molecules into smaller, absorbable units: primarily individual amino acids, and also some dipeptides and tripeptides.

This digestion starts in the stomach with pepsin and continues vigorously in the small intestine with pancreatic enzymes like trypsin and chymotrypsin, followed by brush border peptidases. The small intestine, particularly the jejunum and ileum, is the main site of absorption.

Amino acids are largely absorbed via Na+-dependent co-transport, a secondary active process driven by the Na+ gradient maintained by the Na+-K+ ATPase pump. Dipeptides and tripeptides are absorbed by the H+-dependent PEPT1 transporter.

Once inside the enterocytes, dipeptides and tripeptides are further hydrolyzed into amino acids. Finally, these free amino acids exit the enterocytes via facilitated diffusion and some active transport into the capillaries, eventually reaching the liver via the hepatic portal vein.

This energy-intensive process is crucial for providing the building blocks for growth, repair, and various metabolic functions.

Important Differences

vs Absorption of Carbohydrates

Aspect	This Topic	Absorption of Carbohydrates
Absorbable Units	Monosaccharides (glucose, galactose, fructose)	Amino acids, dipeptides, tripeptides
Primary Transport Mechanisms (Apical)	Glucose/Galactose: SGLT1 (Na+-dependent active transport); Fructose: GLUT5 (facilitated diffusion)	Amino acids: Na+-dependent active transport; Di/Tripeptides: PEPT1 (H+-dependent active transport)
Intracellular Processing	Generally no further breakdown within enterocytes (monosaccharides are already simplest form)	Dipeptides and tripeptides are hydrolyzed to amino acids within enterocytes
Basolateral Transport	GLUT2 (facilitated diffusion) for all monosaccharides	Multiple Na+-independent facilitated diffusion systems for amino acids
Energy Dependence	SGLT1 is secondary active (Na+-K+ ATPase dependent), GLUT5 is passive	Both Na+-dependent amino acid transport and PEPT1 are secondary active (Na+-K+ ATPase dependent)

While both carbohydrate and protein absorption occur primarily in the small intestine and involve carrier-mediated transport, their specific absorbable units and transport mechanisms differ. Carbohydrates are absorbed as monosaccharides (glucose, galactose, fructose), utilizing SGLT1 (Na+-dependent) and GLUT5 (facilitated diffusion) transporters. Proteins are absorbed as amino acids, dipeptides, and tripeptides, primarily via Na+-dependent co-transport for amino acids and H+-dependent PEPT1 for small peptides. A key distinction is the intracellular hydrolysis of di/tripeptides into amino acids within enterocytes, which is not required for monosaccharides.