(P04-021-25) Muscle Protein Synthesis Response to Whole-Foods and Free-Form Essential Amino Acids: An Exploratory Analysis

Objectives: The food matrix influences dietary protein and amino acid digestibility, bioavailability, and their subsequent effects on skeletal muscle protein synthesis (MPS). The purpose of this study was to explore the effects of whole food protein sources and free-form essential amino acids (EAAs) on acute changes in MPS.

Methods: This was a retrospective analysis of seven studies encompassing n=183 healthy participants (48% female; age: 42 ± 20 years, range 18 to 83 years; body mass: 75.9 ± 14.8 kg). Protein interventions included whole foods (beef, pork, eggs, peanut butter, kidney beans, tofu, and plant-based burger) and free-form EAAs. Whole foods were analyzed with and without correction for their true ileal digestibility (TID). Studies were conducted after an overnight fast, in the rested state, and with postprandial MPS measurements recorded over 3 to 6 hours. Stable isotope tracer protocols used primed, constant infusions of ւ-[ring-²H₅]-phenylalanine, ւ-[ring-¹³C₆]-phenylalanine, or ւ-[ring-²H₂]-tyrosine. Muscle fractional synthetic rate (FSR) was calculated using the precursor-product model, and FSR change scores (ΔFSR) used for normalization across studies. Data were analyzed using linear or piecewise non-linear regression, depending on the model that provided the best fit and coefficient of determination (R²). Data are mean (95% CI).

Results: For whole foods effects on MPS, piecewise regression explained 32%, 27%, and 27% of the variance (R² = protein: 0.322, EAAs: 0.268, leucine: 0.269), and identified the following initial thresholds: protein 29.5g (22.8g to 38.0g), EAAs 10.1g (8.2g to 14.7g), leucine 2.0g (1.6g to 2.9g). Correcting for the foods’ TID led to a small increase in the goodness of fit for each model (R² = protein: 0.334, EAAs: 0.286, leucine: 0.279). For free-form EAAs, linear regression explained 0.3% and 8% of the variance (R² = EAAs: 0.003, leucine: 0.080); piecewise non-linear regression was not suitable for the data structure.

Conclusions: Total protein, EAAs, and leucine corrected for their TID all similarly predicted changes in acute MPS from whole food sources. For free-form EAAs, total EAAs and leucine had little to no relationship with acute MPS responses. These exploratory results suggest differential MPS responses to whole foods and free-form EAAs.

Funding Sources: No funding was received for the present project and analysis.