Knowledge of the stimulatory effects of enteral and parenteral (IV) feeding on the synthesis and turnover of trypsin would help in the management of acute pancreatitis, as the disease is caused by the premature activation of trypsin. To investigate this, we have labeled IV infusions with 1-¹³C-leucine and enterals with ²H-leucine and measured isotope enrichment of plasma, secreted trypsin and duodenal mucosal proteins over 6h by duodenal perfusion/aspiration and endoscopic biopsy. 30 healthy volunteers were studied during fasting (n=7), IV feeding (n=6), or post-pyloric enteral feeding (duodenal polymeric n=6, elemental duodenal n=6, jejunal elemental n=5)). All diets provided 1.5 g/kg/d protein and 40 kcal/kg/d energy. Results demonstrated that in comparison to fasting, enteral feeding increased the rate of appearance (71(4) vs. 91(5) minutes, p=0.01), and secretion (546(80) vs. 219(37) units/h, p=0.01) of newly labeled trypsin, and expanded zymogen stores (1660(237) vs. 749(133) units, p=0.03). These differences persisted whether the feeds were polymeric or elemental, duodenal or jejunal. In contrast, IV feeding had no effect on basal rates. Differential labeling of the plasma amino acid pool by enteral and IV isotope infusions suggested that 35% of absorbed amino acids were retained within the splanchnic bed during enteral feeding, and that mucosal protein turnover increased from a fasting rate of 34(6)%/d to 108(8)%/d (p<0.05) in comparison to no change after IV feeding. In conclusion, all common forms of enteral feeding stimulate the synthesis and secretion of pancreatic trypsin, and only parenteral nutrition avoids it.