Human Muscle Protein Synthetic Responses during Weight-Bearing and Non-Weight-Bearing Exercise: A Comparative Study of Exercise Modes and Recovery Nutrition

Abstract

Effects of conventional endurance (CE) exercise and essential amino acid (EAA) supplementation on protein turnover are well described. Protein turnover responses to weighted endurance exercise (i.e., load carriage, LC) and EAA may differ from CE, because the mechanical forces and contractile properties of LC and CE likely differ. This study examined muscle protein synthesis (MPS) and whole-body protein turnover in response to LC and CE, with and without EAA supplementation, using stable isotope amino acid tracer infusions. Forty adults (mean ± SD, 22 ± 4 y, 80 ± 10 kg, VO 2peak 4.0 ± 0.5 L ∙ min(-1)) were randomly assigned to perform 90 min, absolute intensity-matched (2.2 ± 0.1 VO2 L ∙ m(-1)) LC (performed on a treadmill wearing a vest equal to 30% of individual body mass, mean ± SD load carried 24 ± 3 kg) or CE (cycle ergometry performed at the same absolute VO2 as LC) exercise, during which EAA (10 g EAA, 3.6 g leucine) or control (CON, non-nutritive) drinks were consumed. Mixed-muscle and myofibrillar MPS were higher during exercise for LC than CE (mode main effect, P < 0.05), independent of dietary treatment. EAA enhanced mixed-muscle and sarcoplasmic MPS during exercise, regardless of mode (drink main effect, P < 0.05). Mixed-muscle and sarcoplasmic MPS were higher in recovery for LC than CE (mode main effect, P < 0.05). No other differences or interactions (mode x drink) were observed. However, EAA attenuated whole-body protein breakdown, increased amino acid oxidation, and enhanced net protein balance in recovery compared to CON, regardless of exercise mode (P < 0.05). These data show that, although whole-body protein turnover responses to absolute VO2-matched LC and CE are the same, LC elicited a greater muscle protein synthetic response than CE.

Trial registration: ClinicalTrials.gov NCT01714479.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. Experimental design.

Load carriage (LC) and conventional endurance (CE) exercise muscle protein synthesis and whole-body protein turnover protocols.

Fig 2. Stable isotope enrichments.

Plasma 2H5-phenylalanine (A), plasma 13KIC (B), muscle intracellular 2H5-phenylalanine (C), and breath 13CO2 (D) isotopic enrichments during recovery from a 90 min, metabolically matched load carriage (LC) or conventional endurance (CE) exercise bout, with and without (control, CON) essential amino acid (EAA) supplementation. Data are mean ± SD, n = 10 per group. Drink-by-time interactions were observed, EAA different than CON, P < 0.05.

Fig 3. Protein synthesis.

Mixed-muscle (A), myofibrillar (B), and sarcoplasmic (C) muscle protein synthesis (MPS) during exercise and mixed-muscle (D), myofibrillar (E), and sarcoplasmic (F) MPS in recovery from a 90 min, metabolically matched load carriage (LC) or conventional endurance (CE) exercise bout, with and without (control, CON) essential amino acid (EAA) supplementation. Data are mean ± SD, n = 10 per group. †Mode main effect; LC different than CE, P < 0.05. *Drink main effect; EAA different than CON, P < 0.05.

Fig 4. Whole-body protein turnover.

Flux, synthesis, breakdown, oxidation, and net protein balance during recovery from a 90 min, metabolically matched load carriage (LC) or conventional endurance (CE) exercise bout, with and without (control, CON) essential amino acid (EAA) supplementation. Data are mean ± SD, n = 10 per group. *Drink main effect; EAA different than CON, P < 0.05.

Fig 5. Amino acids and insulin.

Effects of load carriage, conventional endurance exercise, and essential amino acid supplementation on plasma essential amino acids (A), branched-chain amino acids (B), leucine (C), and insulin (D) during recovery. Data are mean ± SD, n = 10 per group. CE, conventional endurance exercise; LC, load carriage; CON, control; and EAA, essential amino acids. Drink x time (D x T) interaction for EAA, BCAA (branched-chain amino acids), leucine, and insulin. *Different from baseline for EAA groups, #from baseline for CON groups, and for EAA groups compared to †CON, D x T, P < 0.05.