Protein requirement of healthy school-age children determined by the indicator amino acid oxidation method

Background: The current Dietary Reference Intake (DRI) recommendations for protein requirements in children are based on a factorial estimate and have not been directly determined. Objective: The objective of the current study was to determine the protein requirement in healthy, school-age children by measuring the oxidation of L-[1-C-13]-phenylalanine to (CO2)-C-13 [label tracer oxidation (F (CO2)-C-13)] in response to graded intakes of protein. Design: Seven healthy children (6-11 y old) each randomly received a minimum of 7 protein intakes (range: 0.1-2.56 g . kg(-1) . d(-1)) for a total of 56 studies. The diets provided energy at 1.7 times the resting energy expenditure and were made isocaloric by using carbohydrate. Protein was given as an amino acid mixture on the basis of the egg-protein pattern, except for phenylalanine and tyrosine intakes, which were maintained constant across intakes. The mean protein requirement was determined by applying a 2-phase linear regression crossover analysis on F (CO2)-C-13 data, which identified a breakpoint (requirement) at minimal F (CO2)-C-13 in response to graded amounts of protein intake. Results: Mean and population-safe (upper 95% CI) protein requirements were determined to be 1.3 and 1.55 g . kg(-1) . d(-1), respectively. These results are significantly higher than the mean and population-safe protein requirements currently recommended by the DRI 2005 for macronutrients (0.76 and 0.95 g . kg(-1) . d(-1), respectively). Conclusion: To our knowledge, this study was the first to directly estimate protein requirements in children by using stable isotopes and indicated that current recommendations are severely underestimated. Am J Clin Nutr 2011; 94: 1545-52.