Introduction
The anabolic potential of a protein source is dependent on factors related to protein digestibility, amino acid kinetics, and amino acid composition. The leucine trigger hypothesis predicts that the magnitude (amplitude and rate) of postprandial increase in blood leucine concentrations, termed leucinemia, serves to regulate the magnitude of postprandial muscle protein synthesis (MPS) response to an ingested protein source. The aim of this qualitative systematic review was to systematically examine the leucine trigger hypothesis by comparing a range of supplemental protein and/or whole-food protein sources in young and older adults at rest and post-exercise.
 
Methods
This qualitative systematic review extracted data from studies that combined measurements of postprandial blood leucine concentrations and rates of MPS following ingested protein at rest and following exercise in young and older adults. Data relating to blood leucine concentration profiles and postprandial MPS rates were extracted from all studies and reported as providing sufficient or insufficient evidence for the leucine trigger hypothesis.
 
Results
Overall, 16 of the 29 eligible studies provided sufficient evidence to support the leucine trigger hypothesis for explaining divergent postprandial rates of MPS in response to different ingested protein sources. Of these 16 studies, 13 were conducted in older adults (8 of which conducted measurements post-exercise) and 14 studies included the administration of isolated proteins.
 
Conclusions
This systematic review underscores the merits of the leucine trigger hypothesis for the explanation of the regulation of MPS. However, our data indicate that the leucine trigger hypothesis confers most application in regulating the postprandial response of MPS to ingested proteins in older adults. Consistent with our hypothesis, we provide data to support the idea that the leucine trigger hypothesis is more relevant within the context of ingesting isolated protein sources rather than protein-rich whole foods. Future mechanistic studies are warranted to understand the complex series of modulatory factors beyond blood leucine concentration profiles within a food matrix that regulate postprandial rates of MPS.