Digestion of dietary protein in the gastrointestinal tract of young pigs. pH values in the gastric (stomach) juice of neonatal pigs and postweaning growing pigs are 3 to 5 and 2 to 3, respectively. pH values in the lumen of the small intestine of young pigs are 6 to 7. All diet-derived AA undergo various degrees of catabolism by luminal bacteria and some of them are oxidized by enterocytes. AA = amino acids; GSH = glutathione; NEAA = nutritionally nonessential AA; NM = nitrogenous metabolites; NT = nucleotides; PepT1 = H+ gradient-driven peptide transporter 1.