Protein enters the stomach: the first thing gastric acid (HCl, pH ~ 1.5-2) does is denature the protein — the tertiary structure unfolds, exposing peptide bonds to enzymes.Pepsin is activated at low pH — converted by acid hydrolysis from pepsinogen (the inhibitory peptide is removed). Pepsin cleaves peptide bonds adjacent to aromatic amino acids (Phe, Trp, Tyr), breaking large proteins into long peptide fragments of 10-50 aa.
The stomach completes 10-15% of protein digestion — not the biggest share, but the kickoff. Inadequate stomach acid (, atrophic gastritis of aging, gastrectomy) → every downstream step takes a discount.
2 · Duodenum · pancreatic arsenal
In the duodenum, pancreatic bicarbonate neutralizes stomach acid, trypsin sets off a self-activation cascade, and a set of pancreatic enzymes cleaves nearly any protein at different positions.Chyme enters the duodenum → pancreatic juice bicarbonate neutralizes stomach acid → pH rises to ~ 6.5-7.5 → trypsin is activated, triggering a self-activation cascade:
· Trypsin — cleaves peptide bonds after Lys / Arg (basic amino acids) · Chymotrypsin — cleaves after aromatic amino acids (Phe / Trp / Tyr) · Elastase — cleaves after small amino acids (Ala / Gly) · Carboxypeptidase A / B — strips off single amino acids one by one from the C-terminus
Together this enzyme set can cleave nearly any protein — they target peptide bonds at different positions and complement each other.
The pancreas secretes them as proenzymes (so it does not digest itself); only on entering the small intestine are they activated by enterokinase, opening the self-activation cascade.
3 · Brush border · final cuts + uptake
Brush-border enzymes cut short peptides into amino acids and 2-3 aa remnants, which PEPT1 and many transporters carry into the enterocyte and on to the portal vein.The apical brush border of enterocytes carries the final enzyme set:
· Aminopeptidase — cleaves single amino acids from the N-terminus · Dipeptidases / tripeptidases — break short peptides into free amino acids or 2-3 aa remnants
Absorption: · PEPT1 — di-/tri-peptide transporter, can carry 2-3 amino-acid remnants into the cell at once (more efficient than single-AA transport) · Many amino-acid transporters — separate carriers for neutral, acidic, basic AAs (~ 20 in total) · B0AT1, ASCT2, EAAT3 etc. handle distinct amino-acid families
This parallel multi-channel setup ensures that even if one amino acid is overloaded, others can still get through.
Enterocytes also use some amino acids themselves (mainly glutamine, the energy source for intestinal epithelium); the rest enters the portal vein to the liver.
4 · Liver gate · BCAA bypasses
Newly absorbed amino acids go to the liver first, but the branched-chain amino acids are barely metabolized there and pass straight into the systemic circulation, mainly for use in muscle.The portal vein carries newly absorbed amino acids to the liver first — the liver is the hub for most amino-acid metabolism:
Most amino acids enter the systemic circulation only after hepatic metabolism — the liver is the 'first filter station.'
But there is a famous exception: branched-chain amino acids (: Leu, Ile, Val) — the liver barely metabolizes them, letting them pass directly into the systemic circulation, where they are mainly oxidized in muscle or used for synthesis.
This is why: · Whey protein (highest leucine content) is best post-workout — leucine sails straight to muscle and triggers · Slow-digesting protein (casein) releases gradually, suitable for pre-sleep · Plant protein needs ~ 1.3× the dose of animal protein per workout, because of slightly lower leucine ratio + slower intestinal absorption
So 'protein = one thing' is an oversimplification — digestion speed + amino-acid profile + hepatic filtering together decide which protein works best in which context.