Digestive system ↗Acid unfolds proteins
Gastric acid denatures structures and supports conversion of pepsinogen to pepsin.
DIGESTIVE / PROTEIN · AMINO ACIDS
Protein is dismantled.
Nitrogen must leave safely.
Acid and proteases reduce dietary proteins to peptides and amino acids. Absorbed nitrogen enters a dynamic body pool for synthesis and turnover; amino-nitrogen disposal converges on liver urea formation and kidney excretion.
12-STAGE ROUTE
Digest → distribute
→ remove nitrogen.
The body continually synthesizes and degrades protein. A higher meal input does not map directly to muscle gain; energy state, training, age, health and total dietary pattern matter.
Pepsin begins cleavage
Pepsin generates large peptide fragments in the stomach.
Pancreatic zymogens activate
Proteases are secreted as inactive precursors and activated in the small-intestinal lumen.
Brush-border enzymes finish
Peptidases create amino acids plus di- and tripeptides near the absorptive surface.
Transporters take up cargo
Multiple amino-acid transport systems and PEPT1 move distinct substrates into enterocytes.
Small peptides are hydrolyzed
Most absorbed di- and tripeptides become free amino acids within enterocytes.
Portal blood reaches liver
Amino acids cross the basolateral membrane and travel first to the liver.
Body protein turns over
Liver and peripheral tissues use the circulating pool for proteins and other nitrogen-containing molecules.
Transamination redistributes nitrogen
Aminotransferases exchange amino groups with carbon skeletons; glutamate is a central collector.
Ammonia requires control
Nitrogen is transported in forms including glutamine and alanine because free ammonia is toxic.
Liver synthesizes urea
Mitochondrial and cytosolic urea-cycle steps convert nitrogen into the less toxic, water-soluble molecule urea.
Kidneys excrete urea
Urea enters systemic blood and is filtered and variably reabsorbed before urinary elimination.