The maintenance of lean body mass, especially skeletal muscle, is vital for optimal health and performance across the lifespan. The protein component of lean body mass is in a constant state of turnover, involving the simultaneous breakdown of old and/or damaged proteins and the synthesis of new proteins. These processes collectively determine if someone gains or loses lean body mass. Eating a protein-rich meal or performing resistance exercise can stimulate protein synthesis to gain lean body mass. Stable isotope "tracers" are amino acid building blocks that are slightly heavier than those naturally found in the body. In research, these are often used to assess changes in protein turnover in response to feeding and/or exercise. However, traditional stable isotope tracer methods involve the intravenous delivery of a tracer with blood sampling and muscle biopsies, which may be cumbersome or unfeasible for some for participants.

The investigators have recently developed and validated a non-invasive 'breath test' in males that measures the efficiency of the body for using amino acids in food to build new body proteins. The principle of this method is that leucine, an essential amino acid that the body must acquire from normal diet, can be used to build new body proteins or as a source of energy (i.e., oxidized). Since leucine is preferentially used in skeletal muscle, skeletal muscle protein metabolism can be non-invasively inferred . Any leucine "tracer" that is oxidized can be detected and measured in the carbon dioxide exhaled. It has been observed that less dietary leucine is oxidized when active males perform a bout of resistance exercise, meaning more was used to build muscle proteins. When performed habitually, resistance exercise can help skeletal muscles grow, compared to a rested-state, resulting in greater leucine retention in the body to build new proteins. Therefore, the purpose of this study is to validate this non-invasive breath test in females to increase the validity of the method in a wider range of populations. Ultimately, the results will further validate this non-invasive tool that can potentially detect whether different populations are sensitive to dietary amino acids and in a position to gain or lose lean body mass.

The objective of the present study is to validate the use of an oral L-[13C]-leucine stable isotope tracer, preferentially metabolized within skeletal muscle, to detect an exercise-induced increase in anabolic sensitivity (i.e., reduction in oxidation) in females. It is hypothesized that since resistance exercise enhances skeletal muscle protein synthesis, anabolic sensitivity measured through leucine retention would be greater with feeding after resistance exercise compared to feeding at rest.

Inclusion Criteria:

• Female sex
• 18-35 years of age
• BMI ≥18.5 kg/m2 and ≤ 30 kg/m2
• Eumenorrheic (self-reported menstruation for prior 3 consecutive months)
• Recreationally active; currently performing structured exercise (e.g. running, weightlifting, team-sport activity) at least once per week for ≥ 6 months before enrolment

Exclusion Criteria:

• Use of oral contraceptives
• Inability to perform physical activity as determined by the PAR-Q+
• Inability to adhere to protocol guidelines (e.g. alcohol, caffeine, habitual diet)
• Regular tobacco use
• Illicit drug use (e.g. growth hormone, testosterone, etc...)
• Diagnosed medical condition under the care of a physician (e.g. type 2 diabetes)
• Inability to abstain from supplements (e.g. protein, creatine, HMB, BCAA, phosphatidic acid, etc...) at least three weeks before the trial
• Individuals on any medications known to affect protein metabolism (e.g. corticosteroids, non-steroidal anti-inflammatories, and/or prescription-strength acne medications)