UBC Theses and Dissertations
Determination of threonine requirements and the metabolic availability of threonine from food sources in healthy school-aged children Radonic, Peter William
Threonine, an indispensable amino acid, is required for protein synthesis throughout the body. Due to its quantitative importance in gut mucosal proteins, consuming adequate amounts of threonine is essential for proper digestive and immune function during growth. Currently, the dietary reference intakes (DRIs) recommend an estimated average requirement (EAR) and recommended dietary allowance (RDA) of 19 and 24 mg/kg/d for threonine intake in 6-10y children based on factorial (mathematical) calculations. In addition, it is unknown to what extent dietary threonine is available for protein synthesis from food protein sources in school-aged children. The primary objective of this thesis was to determine the dietary requirement for threonine in 6-10y healthy children using the minimally invasive Indicator Amino Acid Oxidation (IAAO) method. The secondary objective was to compare threonine metabolic availability from casein (animal protein) and soy (vegetable protein) in the same set of children. Six healthy Canadian children (three boys: three girls) aged 7.5 ± 1.4 y randomly received 6-9 test threonine intakes each, ranging from 1-50 mg/kg/d, with an amino acid mixture patterned after egg protein. Study day diets were complete with protein provided at 1.5 g/kg/d, energy provided at 1.7x the resting energy expenditure. To determine threonine requirements, the oxidation of L-[1-¹³C]-phenylalanine to ¹³CO₂ (F¹³CO₂) in response to the test intakes was used. To determine threonine metabolic availability, the children were tested at 0 mg/kg/d, and at 11 mg/kg/d from three different sources (threonine as crystalline amino acid, casein and soy), and the oxidation of L-[1-¹³C]-Phenylalanine to ¹³CO₂ measured. Threonine requirement was determined to be 21.9 mg/kg/d (95% CI: 10.5 – 33.4 mg/kg/d). The metabolic availability of threonine was determined to be 96.6% and 83.4% from casein and soy, respectively. Threonine requirements in school-aged children were determined for the first time directly, and the results are ~15% higher than current DRI recommendations of EAR 19 mg/kg/d. In addition, the metabolic availability of threonine was found to be higher in casein compared to soy, which needs to be considered when making dietary recommendations.
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