Supplemental enteral arginine is metabolized to ornithine in injured patients

Arginine has been added to immune enhancing diets that may improve patient outcomes, but little is known about the metabolic fate of supplemental arginine. We hypothesize that supplemental enteral arginine in injured patients is metabolized to ornithine by increased activity of the enzyme arginase. Twenty-five adult patients with injury severity scores <20 received up to 14 days of enteral nutrition supplemented with arginine (30 g/day) or placebo in a prospective, randomized, blinded study. Plasma arginine, citrulline, and ornithine concentrations and peripheral blood mononuclear cell (PBMC) arginase activity were measured at baseline and on days 1, 3, 5, 7, 10, and 14. Clinical data collected included demographics, injury patterns, lengths of stay, and infectious complications. Data were analyzed using ANOVA and t test. PBMC arginase activity was elevated in all patients. In the supplemented group, plasma arginine concentrations increased at days 7, 10, and 14 when compared to baseline (P < 0.05) and were higher at day 14 when compared to those of controls (P < 0.05). Citrulline concentrations in both groups were unchanged over time. Ornithine concentration increased within 24 h of arginine supplementation and remained elevated when compared to baseline (P < 0.01). Ornithine concentration in the supplemented group was higher at days 1, 3, 5, and 7 when compared to that of controls (P < 0.05). There were no differences in clinical outcomes. Supplemental enteral arginine is absorbed in injured patients and increases arginine levels. Supplemental arginine appears to be metabolized to ornithine. Increased arginase enzyme activity in peripheral blood mononuclear cells may be a contributor.