Development of Oral Amino Acid Tracers to Study Protein Turnover in Humans

Overview

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\<feeding\<exercise \& feeding). The secondary objective of the proposed study is to develop and validate non-invasive models to measure whole-body amino acid oxidation and net balance in response to feeding and resistance exercise. It is hypothesized that whole-body net balance, as determined by a novel oral tracer model (i.e. L-\[1-13C\]leucine) , will align with traditional intravenous tracer methodology (i.e. L-\[5,5,5-2H3\]leucine) and reveal the expected physiological changes in whole-body protein turnover in response to feeding and resistance exercise (i.e. fasted\<feeding\<exercise \& feeding).