Tryptophan Requirement in School-Age Children Determined by the Indicator Amino Acid Oxidation Method is Similar to Current Recommendations

The requirement for dietary tryptophan in school-age children has never been empirically derived. The objective of our study was to determine the tryptophan requirement of school-age children using the indicator amino acid oxidation technique. Volunteer healthy school-age children, between 8 and 12 y, were enrolled and the oxidation of l-[¹³C]-phenylalanine to ¹³CO₂ measured in response to graded intakes of dietary tryptophan. Seven children (3 boys, 4 girls) participated in the study and received randomly assigned tryptophan intakes ranging from 0.5 to 9.75 mg.kg⁻¹.d⁻¹ for a total of 36 studies. The diets provided energy at 1.5 times each subject's resting energy expenditure and were isocaloric. Protein was provided as an amino acid mixture on the basis of the egg protein pattern, and phenylalanine and tyrosine were maintained constant across the protein intake concentrations at 25 and 40 mg.kg⁻¹.d⁻¹. All subjects were adapted for 2 d before the study day to a protein intake of 1.5 g.kg⁻¹.d⁻¹. The mean tryptophan requirement was determined by applying a mixed-effect change-point regression analysis to F¹³CO₂ (label tracer oxidation in ¹³CO₂ breath) which identified a breakpoint in the F¹³CO₂ in response to graded amounts of tryptophan. The mean [estimated average requirement (EAR)] and upper 95% CI, (approximating the RDA) of tryptophan requirements were estimated to be 4.7 and 6.1 mg.kg⁻¹.d⁻¹, respectively. Our results are similar to the current recommended EAR and RDA of 5 and 6 mg.kg⁻¹.d⁻¹ for healthy growing children based on the factorial calculation. Clinical Trials Registration No. NCT02018588.