The Effect of Resistance Training on Muscle Protein Turnover in the Fed State

Abstract

Resistance exercise results in an increase in muscle protein synthesis (MPS); however, in the fasted state net muscle protein balance (synthesis minus breakdown) remains negative. With the ingestion of post-exercise amino acids, or protein, net protein balance becomes positive and protein accretion can occur over time. Previously, we found that resistance training elevated resting, fasted, mixed MPS, but blunted the acute resistance exercise-induced rise in MPS. We aimed to determine how resistance training would affect mixed MPS chronically and acutely, in the fed state. Young men (N = 10, 21 ± 0.47 yrs, BMI = 25.9 ± 1.4 kg·m^-2; means± SD) completed an 8-week unilateral leg resistance training protocol which consisted of knee extension exercise performed at ~80% 1 repetition maximum (1 RM) and resulted in one leg being trained (T), while the contralateral leg served as an untrained (UT) control. Strength gains were 3-fold greater in the T versus the UT leg (P < 0.05). As well, mean fibre cross-sectional area of Type I and Type II fibres increased (P < 0.05) in the T leg only. Following training, in the fed state, subjects underwent primed constant infusions of [ring-d_5 or -_13C_6]phenylalanine or d_3-(alpha)-ketoisocaproic acid in a randomized counterbalance manner to determine rates of MPS using the precursor product equation for incorporation into mixed muscle proteins, sampled via muscle biopsy of the vastus lateral is. MPS was assessed at rest, acutely following a bout of resistance exercise, which was at the same relative intensity for both legs, and approximately 28 h after the resistance exercise bout. Rates of MPS at rest were unchanged following training (P = 0.97). Following the resistance exercise bout, rates of MPS were elevated acutely in both the T and UT legs (4 h post-exercise; P < 0.01), but returned to baseline levels by 28 h post-exercise in the T leg only. We conclude that while resistance training did not alter resting MPS, it did affect the time course of MPS following an acute bout of resistance exercise compared to the UT leg. Hence, when taken with our previous results we conclude that resistance training attenuates the MPS response to an acute bout of resistance exercise when loads are matched at the same relative intensity, but the response is not altered by the nutritional status of the individual.