Evaluating the Leucine Trigger Hypothesis to Explain the Post-prandial Regulation of Muscle Protein Synthesis in Young and Older Adults: A Systematic Review.
Level 1 - systematic review of randomized trials
Systematic review of 29 human intervention studies
PubMed 34307436 · doi:10.3389/fnut.2021.685165
What was done
This qualitative systematic review evaluated the "leucine trigger" hypothesis—the concept that the magnitude and rate of post-prandial blood leucine rise governs the muscle protein synthesis (MPS) response—following protein ingestion at rest and after exercise. Authors extracted data from studies measuring both post-prandial blood leucine profiles and MPS rates in young and older adults, categorizing findings by age group, exercise status, and protein format (isolated protein vs. protein-rich whole foods).
What was found
Across 29 eligible studies, 16 (55%) provided sufficient evidence to support the leucine trigger hypothesis. Of the 16 supportive studies, 13 were conducted in older adults (8 measuring post-exercise MPS) and 14 evaluated isolated protein sources. The abstract reports no pooled quantitative effect sizes or specific concentration values.
Why it matters
The findings indicate that rapid blood leucine elevation is not a universal driver of MPS, but is primarily relevant for older adults and when consuming isolated protein supplements rather than whole-food protein sources.
Limits
The review relies on qualitative categorization rather than quantitative meta-analysis. The abstract does not report participant sample sizes, specific criteria for "sufficient evidence," risk of bias assessments, or detailed whole-food matrix comparisons.
Cited by
- context Approximately 2.5 to 3 grams of the amino acid leucine is required per meal to trigger muscle protein synthesis.