Abstract
All human tissues are in a constant state of remodelling, regulated by the balance between tissue protein synthesis and breakdown rates. It has been well-established that protein ingestion stimulates skeletal muscle and whole-body protein synthesis. Stable isotope-labelled amino acid methodologies are commonly applied to assess the various aspects of protein metabolism in vivo in human subjects. However, to achieve a more comprehensive assessment of post-prandial protein handling in vivo in human subjects, intravenous stable isotope-labelled amino acid infusions can be combined with the ingestion of intrinsically labelled protein and the collection of blood and muscle tissue samples. The combined application of ingesting intrinsically labelled protein with continuous intravenous stable isotope-labelled amino acid infusion allows the simultaneous assessment of protein digestion and amino acid absorption kinetics (e.g. release of dietary protein-derived amino acids into the circulation), whole-body protein metabolism (whole-body protein synthesis, breakdown and oxidation rates and net protein balance) and skeletal muscle metabolism (muscle protein fractional synthesis rates and dietary protein-derived amino acid incorporation into muscle protein). The purpose of this review is to provide an overview of the various aspects of post-prandial protein handling and metabolism with a focus on insights obtained from studies that have applied intrinsically labelled protein under a variety of conditions in different populations.
Highlights
All human tissues are in a constant state of remodelling, which is regulated by the balance between tissue protein synthesis and breakdown rates[1,2]
Isotope-labelled amino acids is commonly applied to assess basal, post-absorptive or post-prandial muscle protein synthesis rates. This approach does not allow the assessment of dietary protein digestion and amino acid absorption kinetics and the subsequent impact on whole-body protein synthesis, breakdown or amino acid oxidation
A more comprehensive assessment of post-prandial protein handling in vivo in human subjects can be achieved by combining the infusion of stable isotopelabelled amino acids with the ingestion of intrinsically labelled protein and frequent collection of blood and muscle samples
Summary
All human tissues are in a constant state of remodelling, regulated by the balance between tissue protein synthesis and breakdown rates. The combined application of continuous intravenous infusion of stable isotope amino acids and ingestion of intrinsically labelled protein has been used to establish that intrinsically labelled milk protein ingestion stimulates whole-body protein synthesis rates, attenuates whole-body protein breakdown rates and improves whole-body protein net balance in a dose-dependent manner up to doses of at least 45 g(6,25,27). This response can be modulated by the type of protein ingested. When labelled free amino acids are co-ingested, more frequent plasma sampling should be applied to confirm and appropriately characterise the destabilisation of the post-prandial isotope tracer precursor pool enrichment (Fig. 3b)
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