Isotopic approaches to the estimation of protein requirements in burn patients.

Determination of the protein requirement of the severely burned patient has been hampered by the lack of an appropriate means of evaluating the effects of various levels of intake. We have used two new isotopic techniques in an attempt to assess the effects of two levels of protein intake in six severely burned adult patients. A crossover experimental design enabled us to study each patient at the end of two 3-day dietary regimens. All diets were isocaloric and provided approximately 25% more calories than the measured energy expenditure (means = 40 Kcal/kg X day). In one regimen, each patient received 2.2 g protein/kg X day; during the other treatment period they received 1.4 g protein/kg X day. The patients were studied in the fed state and after 10-12 h of fasting. Leucine kinetics were determined by means of the primed-constant infusion of (1-13C)-leucine. We were able to distinguish the oxidation of plasma leucine from the oxidation of leucine derived from intracellular protein at the site of the deamination of leucine (predominantly muscle) by simultaneously determining both leucine and alpha-ketoisocaproic acid enrichment. Also, rates of whole-body protein synthesis and catabolism were calculated by means of another stable-isotope technique involving the infusion of (15N2)-urea. Finally, a third means of estimating net protein catabolism based on urinary N-excretion data was used at the same time that the isotopic studies were performed. The 13C-leucine data and the N-excretion data indicated that a balance between protein synthesis and catabolism could be achieved with a protein intake of 1.4 protein/kg X day. When protein intake was increased to 2.2 g protein/kg X day, neither isotopic method indicated a further beneficial effect on net protein synthesis, although the absolute rates of protein synthesis and catabolism were stimulated. The N-excretion data, however, indicated a significant improvement in net protein synthesis with higher protein intake. Regardless of the level of protein intake, the underlying alterations in protein metabolism that occurred as a response to burn injury persisted.