Journal Title (Medline/Pubmed accepted abbreviation): Am J Clin Nutr
Page number: 809-818
doi : 10.3945/ajcn.111.017061
Background: Effects of resistance exercise and consumption of essential amino acids (EAA) on skeletal muscle protein metabolism during recovery are well known (increase in muscle protein synthesis [MPS] and enhancement of anabolic response via mTORC1 pathway, respectively). However, clinical evidence is limited and/or conflicting for the effects of endurance exercise and consumption of EAAs on muscle protein metabolism.
Hypothesis: Leucine supplementation during steady-state exercise would spare endogenous leucine stores and affect MPS, mTORC1 signaling, and whole-body protein utilization during exercise recovery.
Subjects: Ten recreationally fit (peak oxygen consumption [VO2peak ], 40 to 50 mL/kg/min) military personnel who performed endurance and resistance exercise 3 to 4 days/week (mean age, 24 ± 2 yr; height, 174 ± 3 cm, weight, 76 ± 6 kg, body mass index, 25 ± 2 kg/m2, body fat, 22% ± 2%) participated in this study. Analyses are based on 8 volunteers who had complete data.
Experimental design: Randomized, double-blind, cross-over
Treatments Protocol: Participants were randomized to receive 500 mL of either an EAA (1.87 g leucine) or a leucine-added EAA (L-EAA; 3.5 g leucine) isonitrogenous drink with small amounts of isotope-labelled leucine and phenylalanine added during a 60-minute moderate, steady-state cycling exercise (60% VO 2peak, determined previously). Equal portions (125 mL) were drunk in 20-minute intervals during the exercise. All participants completed the exercise under both conditions 5 days apart. A bolus injection of 2.35 μmol/kg [13C]bicarbonate primed the bicarbonate pool before isotope infusions started. Whole-body protein synthesis, breakdown, oxidation, and turnover, as well as MPS, were calculated from infusions of L–[1– 13C]leucine (from 30 min before exercise to 195 min after exercise) and L–[2H5]phenylalanine (from 30 min before exercise to 300 min after exercise), from muscle biopsies (at baseline and 30 and 210 min after exercise from the vastus lateralis), and from breath samples (at baseline and 120 and 300 min after exercise). Blood (collected at baseline and every 15 min after exercise) and breath samples were used to measure insulin, glucose, and amino acid levels. Muscle biopsies were also used to measure levels of phosphorylated proteins in the mTORC1 signaling pathway.
These results show that leucine added to an optimal EAA supplement can enhance MPS during recovery from endurance exercise. Enhanced MPS after L-EAA supplement may be, in part, because of altered whole-body protein turnover and plasma amino acid availability to muscles. However, muscle intracellular amino acid kinetics and availability were not measured in this study to determine if upregulation occurred from the plasma availability. Intracellular signaling by the mTORC1 pathway appeared to be unaffected by the addition of leucine supplementation and limits the interpretation of the MPS changes. A larger number of participants may be needed to detect any statistical differences in intracellular signaling proteins. Moreover, other leucine-sensitive signaling proteins may need to be analyzed.