Leucine is an essential amino acid, which serves a special purpose of signaling cell growth in muscle in addition to making up protein. Essential amino acids like leucine need to be eaten every day from our foods like meat, dairy, eggs, beans, and nuts, as they cannot be made by our bodies. The current recommendation for leucine in the diet of school-age children are based upon adult needs. This study will measure the leucine requirement in children aged 6-10 years old using an non-invasive technique with special diets, safe stable isotopes, and simple breath collections. Creating guidelines based on measured requirements for leucine in this age group is essential for supporting optimal growth in healthy children.

Purpose:

- The purpose of this study is to determine the requirement of the essential amino acid leucine, in school-aged children (6-10 years old), with the minimally invasive indicator amino acid oxidation (IAAO) protocol.

Hypothesis:

- The leucine requirement for healthy school-age children is higher than current 2005 recommendation of 40-49 mg/kg/d when measured using the IAAO method.

Justification:

- Protein is essential for growth, tissue repair, and maintenance of physiological functions throughout the lifecycle. Dietary protein is particularly important during childhood to support a healthy growth trajectory and lean body mass development. Despite the importance of protein to support early life growth and development, childhood protein requirements are largely extrapolated from adult values, and not on pediatric-specific data. Consequently, the current leucine recommendations for children of 40-49 mg/kg/d may not accurately reflect the physiological needs of growth.

Objectives:

• Primary Objective: To determine the leucine requirement in healthy school-aged children using IAAO.
• Secondary Objective: To quantify urinary flux of L-[1-13C] phenylalanine as an additional measure of amino acid metabolism during the experimental protocol.

Research Design:

• Randomized crossover metabolic design based on the IAAO technique to estimate the minimum dietary requirement for leucine. The repeated measures study design involves 7 children (participants) completing 1 pre-study visit and 5 IAAO study visits each with a randomized leucine intake.
• After a 10h fast, each child (participant) is randomized to receive 1 of 7 leucine intake levels (10, 20, 30, 40, 50, 60 or 80 mg/kg/d). On the study day, the diet is administered hourly as eight isonitrogenous, isocaloric meals. Each meal consists of two protein-free cookies and a formulated drink prepared with crystalline amino acids, a flavoured protein-free liquid (powdered drink mix), and corn oil. Multivitamin supplements are provided throughout the study to ensure adequate micronutrient intake. The study diet provides an energy intake equivalent to 1.7 x resting energy expenditure and 1.5 g/kg protein.
• The principle underlying IAAO is that when the intake of an essential amino acid is insufficient, protein synthesis cannot proceed, and the remaining amino acids, unable to be stored, are oxidized. This physiological process can be leveraged by administering a stable isotope-labeled "indicator" amino acid, L-[1-13C]-phenylalanine. When requirements are met, the indicator is incorporated into body protein, but when an amino acid is limiting, the indicator is oxidized, and its labeled carbon appears as 13CO2 in the breath.

Inclusion Criteria:

• Healthy children
• Aged 6 to 10 years at enrollment
• Body weight between the 3rd and 85th percentile according to WHO Child Growth Standards
• Assent from the child and written informed consent from parent(s) and/or legal guardian(s)

Exclusion Criteria:

• Diagnosis of any acute or chronic medical condition
• Current use of prescription medication that may affect protein or energy metabolism
• Recent weight loss
• Recent illness (fever, vomiting, severe runny nose within the past 5 days)
• Inability to comply with study procedures

There is no plan to share data with other researchers