Leucine-enriched proteins (e.g., dairy) are more effective than plant-based sources for supporting post-exercise rates of muscle protein synthesis in adults. However, the impact of protein quality on protein synthesis and acute markers of growth in active children are unknown. Using a randomized crossover design, the investigators will compare the effects of ingesting 0.35g/kg fat free mass (FFM) of either milk protein concentrate (MPC), rice protein isolate (RPI), or soy protein isolate (SPI) following a bout of variable intensity exercise with whole-body protein kinetics (i.e., oxidation, synthesis, breakdown, and net balance) determined by oral [13C]phenylalanine tracer methodology.

In adults, dairy protein is more effective than non-dairy sources at supporting recovery from exercise; however, it is not well understood if protein quality impacts short-term markers of growth in the same manner in active youth. Therefore, it is essential to understand whether different anabolic responses to varying protein quality persist in children, in order to maximize lean tissue accretion specifically during these periods of exponential growth. Thus, assessing the impact of protein quality on post-exercise protein synthesis and whole-body protein balance in active children would help provide valuable maturation-specific nutritional information. Moreover, these studies investigating the effects of protein quality in adults required the use of invasive techniques (i.e., muscle biopsies and indwelling catheters for blood draws) and are not suitable for vulnerable populations like children. As a result, the study of protein quality in children requires non-invasive alternative approaches and novel methods to advance our understanding of the nutritional best practices for active youth.

The overall objective of this investigation is to examine the impact of protein quality on post-exercise protein synthesis and acute markers of "growth" (i.e., net protein balance) in active youth (children and adolescents). Using the minimally invasive indicator amino acid oxidation (IAAO) methodology, we will determine the ability of milk, rice, and soy proteins to support whole-body protein synthesis and net-protein balance during recovery from an acute bout of sport specific moderate-to-vigorous physical activity.

The primary aim of the present study is to compare the ability of dairy and non-dairy alternatives (i.e., plant-based protein sources) to support post-exercise protein synthesis and net-protein balance in active, growing youth. The investigators hypothesize that the ingestion of milk protein concentrate (MPC) will result in a lower indicator amino acid oxidation (i.e., greatest protein synthesis) and higher net protein balance after exercise in both children and adolescents, compared to an isonitrogenous (i.e., equal protein amount) quantity of soy (SPI) and rice (RPI) protein isolate. This is predicated on the relative deficiency of branched-chain amino acids (BCAA) and other essential amino acids (EAA), specifically lysine and methionine, in RPI and SPI respectively, compared to MPC.

Inclusion Criteria:

• Healthy will be defined as screened by the PAR-Q+ (The Physical Activity Readiness Questionnaire for everyone)
• Regularly active, as defined by being in the top 20% of age specific scores on the iPAQ (the International Physical Activity Questionnaire)
• Between the ages of 8 to 16 years
• An age and sex-specific minimum of 75th percentile Beep Test level

Exclusion Criteria:

• Inability to meet health and physical activity guidelines according to the PAR-Q+ and iPAQ
• Inability to adhere to any of the protocol guidelines
• Biological age outside of -0.5 to +1.5 years from PHV for adolescents, and > -1 years from PHV for children

As the study was a single-blind crossover design, the randomization of the protein type will be blinded to the participants only. The allocation concealment in relation to the liquid meal will be done by providing the same relative protein content (0.35 g/kg FFM) within the same volume of fluid, (8 ml/kg) to match the exercise-induced fluid loses, in opaque bottles. Additional carbohydrate (0.4g/kg) and artificial sweetener will be added to improve the organoleptic properties of the beverages.