Journal Title (Medline/Pubmed accepted abbreviation): J Nutr
Page numbers: 1970-1976
doi (if applicable): N/A
Summary of Background and Research Design
Background:Although all essential amino acids (EAAs) stimulate muscle protein synthesis in animal and human models, leucine has unique properties in animal models that have raised interest in leucine as a maintenance therapy for human muscle mass. Specifically, leucine stimulates the cell hypertrophy mammalian target of rapamycin complex 1 [mTORC1] pathway and decreases muscle protein breakdown and its associated cellular signaling and mRNA expression. However, studies in humans to evaluate the effects of leucine on muscle mass have yielded mixed results, with uncertainties regarding the direct contribution of leucine among the EAA to net-protein balance.
Hypothesis/purpose of study:The purpose of this study was to determine the effects of different leucine concentrations in an isonitrogenous EAA mixture on muscle protein turnover (synthesis and breakdown) and associated cellular signaling. Specifically, to investigate the ability of leucine concentrations higher than that contained in a typical high-quality protein supplement to increase muscle protein synthesis and mTORC1 signaling and decrease muscle protein breakdown and associated signalling in young adults.
Subjects: Fourteen participants (6 men, 8 women) who were not currently engaged in any regular exercise training (age 27 to 34 yr, mean weight 72 kg, mean body fat 26%)
Experimental design:Two-cohort, non-blinded
Treatments and protocol:Volunteers refrained from vigorous physical activity for 48 hours before the study and fasted overnight. All studies were performed at a similar time of day. Following a baseline blood sample, a primed continuous tracer infusion of L-[13C6] phenylalanine and L-1-13C leucine was run for 2.5 hours before a baseline biopsy of the vastus lateralis muscle was taken. An infusion of indocyanine green (ICG) was started in the femoral artery for 10 minutes, with 4 sets of blood samples obtained every 15 minutes to determine blood flow (via ICG concentration), amino acid enrichment, glucose, and insulin concentration. A second biopsy was taken following the final blood sample. Participants then ingested either a control (CTRL; 18% leucine or 1.8 g) or leucine-enriched (LEU; 35% leucine or 3.5 g) solution of essential amino acids (10 g total essential amino acids). Biopsies as well as ICG infusions and blood samples were taken at 60, 120, and 180 minutes following ingestion. A small amount of L-[13C6] phenylalanine and L-1-13C leucine was added to the ingested solutions to minimize changes in blood enrichment of the tracers.
Summary of research findings:
- Blood flow was not different across time or between treatment groups.
- Glucose concentration was decreased from baseline at both 120 and 180 minutes postingestion in the LEU group and at 180 minutes postingestion in the CTRL group (P < .05 for all).
- There were no differences between groups at 180 minutes.
- Plasma insulin concentrations were elevated from baseline at 30 and 60 minutes in the LEU group and only at 30 minutes in the CTRL group (P < .05 for all).
- The treatment groups did not differ in most of the amino acid concentrations postingestion.
- Arterial leucine concentration and leucine delivery to the leg increased from baseline in both groups postingestion but was higher in the LEU group for up to 135 minutes compared with CTRL (P < .05) as assessed through the leg muscle amino acid kinetic model.
- Leucine transport into the muscle increased from baseline at 90 minutes postingestion (P < .05), but there were no differences between treatment groups.
- Intracellular leucine availability increased from baseline in both groups at 60 minutes postingestion and was significantly elevated in the LEU group compared with the CTRL group at 45 minutes postingestion (P < .05).
- The fractional synthetic rate of proteins (between biopsies) calculated for the 180-minute period was elevated from baseline in the LEU group (P = .05), but not in the CTRL group.
- However, hourly means of muscle protein synthesis were elevated from baseline in both groups at 60 minutes postingestion (P < .05), with no differences between treatment groups.
- Muscle protein breakdown decreased from baseline at 60 minutes postingestion only in the LEU group (P < .05).
- There were no group differences in net protein balance.
- Phosphorylation of most cellular signalling proteins was not different between the treatment groups, although there were often differences within each group relative to baseline.
- As an exception, the mTOR:ULK1 complex, which indicates less potential for protein breakdown, was elevated at 180 min for the LEU group versus both baseline and the CTRL group.
- NOTE: Based on the cell signaling results reported in the text, there were discrepancies in Table 3. It appears that the value for 4E-BP1 (Thr37/46) should have had a† symbol in addition to the * footnote for the LEU group at 60 minutes. Also, the 60-minute value for the CTRL group regarding eEF2 (Thr56) should have had an * footnote.
Interpretation of findings/Key practice applications:
This study indicates that a leucine concentration typical of high-quality proteins can elicit a muscle protein response in young adults similar to that of higher leucine concentrations, although the higher leucine concentration transiently decreased muscle protein breakdown (possibly through decreased autophagy) and tended to enhance mTORC1 signaling. However, the small observed changes in cellular signaling and muscle protein breakdown did not translate into improvements in muscle protein synthesis or overall protein net balance. It appears that the ability to transport leucine into the leg muscle, rather than availability of leucine to the leg, was a limiting factor that explains the failure of the larger dose of leucine to promote significantly greater overall muscle protein balance than the smaller dose. Further studies, however, are warranted to evaluate whether the small, but nonsignificant, improvements in net muscle protein synthesis/balance with higher leucine concentrations translate into meaningful improvements in skeletal muscle mass over a longer time period. Nevertheless, this study is in agreement with other studies that indicate ~1.8 g leucine in an amino acid mixture is sufficient for maximal protein anabolic response in healthy, nonexercising, young adult subjects.