Previous studies have used a combination of oral L-[1-13C]leucine and intravenous labeled L-[5,5,5-2H3]leucine to assess the acute postprandial changes in whole-body protein turnover (12, 19). Intravenous and dietary-labeled amino acid tracers have also been used in tandem to assess rates of myofibrillar protein synthesis in response to bolus protein ingestion and resistance exercise (46). By validating whole-body net balance to myofibrillar protein synthesis, our proposed multi-tracer approach will develop minimally invasive models to study protein turnover in a variety of populations in which traditional infusions and/or repeated blood samples are not possible (i.e. pediatric, free-living populations).

The primary objective of the proposed study is to validate the use of a novel oral tracer model to accurately and reliably measure myofibrillar protein synthesis. It is hypothesized that oral L-[1-13C]leucine and L-[ring-2H5]phenylalanine, ingested as a bolus to mimic an intravenous 'pulse dose' administration (51, 65), will reveal similar rates of myofibrillar protein synthesis when compared to traditional intravenous L-[5,5,5-2H3]leucine infusion. Moreover, it is hypothesized that both methods will reveal the expected graded changes in myofibrillar protein synthesis in response to feeding and resistance exercise (i.e. fasted\

Inclusion Criteria:

• Male
• Currently performing structured physical activity 2-5 days per week

Exclusion Criteria:

• Unable to safely perform exercise as per PARQ+ guidelines
• Currently using tobacco products
• Currently using or have history of anabolic steroid use
• Diagnosed with medical condition including type 2 diabetes, cancer, heart disease
• Unable to abstain from supplement use (HMB, branched chain amino acids, phosphatidic acid) for at least three weeks prior to trial
• currently using medications known to affect protein metabolism e.g. corticosteroids, NSAID, prescription-strength acne medication
• allergic to local anesthetics
• female: Hormonal fluctuations associated with the menstrual cycle have been reported to alter protein metabolism during exercise and may influence indices of the post-exercise myofibrillar protein synthetic response. Accordingly, the study will include males to ensure a stable hormonal environment and to increase the homogeneity of the physiological response.