Whole Body Net Protein Balance Plateaus in Response to Varying Protein Intakes During Post‐Exercise Recovery: Identification of a Maximal Anabolic Intake

Abstract

Dietary protein is important for the repair and/or rebuilding of skeletal muscle and body proteins and is required to induce a net anabolic environment during recovery from exercise. While the majority of studies assess the effect of protein intake on protein synthesis (S), maximizing net balance [NB; the algebraic difference between S and breakdown (B)] would presumably optimize post‐exercise recovery for athletes. Given the suggestion by some that there is no practical limit to the anabolic response to mixed meal ingestion at rest, the present analysis sought to determine whether net protein balance plateaued in response to varying protein intakes within a mixed macronutrient meal during post‐exercise recovery. After a 2‐d controlled diet, 6 healthy, active males (23 ± 1 y; 82.3 ± 5.7 kg; 71.1 ± 5.1 kg fat‐free mass; mean ± 95% CI) and 7 females (21.4 ± 0.8 y; 68.5 ± 9.3 kg; 53.3 ± 8.6 kg fat‐free mass) consumed a liquid meal (1 g·kg−1 carbohydrate) prior to performing a variable intensity exercise protocol (Loughborough Intermittent Shuttle Test). During the 8‐h post‐exercise recovery period, participants consumed isoenergetic mixed meals providing 0.42 g·kg−1·h−1 of carbohydrate and a variable amount of fat (0.08–0.18 g·kg−1·h−1) and protein (0.02–0.22 g·kg−1·h−1). Protein was provided as crystalline amino acids modeled on the basis of egg protein with the exception of tyrosine (3.33 mg·kg−1·h−1) and phenylalanine (2.5 mg·kg−1·h−1), the latter of which contained 0.46 mg·kg−1·h−1 of l‐[13C]phenylalanine to model steady state phenylalanine kinetics. Breath and urine samples were taken at isotopic steady state to determine phenylalanine oxidation (O) and phenylalanine turnover (Q; corrected for phenylalanine intake), respectively, as an estimate of B. NB was determined from the difference between S (Q – O) and B. Mixed modeling bi‐phase linear regression (R2 = 0.62; P < 0.05) explained a greater proportion of NB variance than linear regression (R2 = 0.49; P < 0.05), indicating NB reached a plateau. NB was not different between males and females (P > 0.05) and increased up to a plateau at 0.13 ± 0.03 g·kg−1·h−1 (0.16 ± 0.02 g·kg−1 fat‐free mass·h−1) or the equivalent of 1.6 ± 0.32 g·kg−1·12h−1 when collapsed across sexes. The breakpoint in NB corresponded to a dietary protein:carbohydrate ratio of 1:3.2. Linear modeling revealed no discernible effect of protein intake on S (P = 0.79) or B (P = 0.17) suggesting the increase in NB was a combination of subtle changes in these kinetics. These data suggest there is a practical limit to the anabolic response to mixed meal ingestion during recovery from variable intensity exercise in active males and females. Athletes should consume ~1.6 g protein·kg−1·d−1 with adequate carbohydrate to maximize whole body NB and thus optimize post‐exercise recovery.Support or Funding InformationSupported by the Ajinomoto Innovation Alliance Program