Irisin Does Not Mediate Resistance Training-Induced Alterations in Resting Metabolic Rate

Purpose: This study aimed to investigate the effects of a 6-month preventive resistance training program on resting metabolic rate (RMR) and its associations with fat-free mass (FFM) and the newly described myokine irisin as two potential mechanistic links between exercise training and RMR. Methods: In a randomized controlled trial, 74 sedentary healthy male and female participants either completed 6 months of high-repetition resistance training 3 d.wk(-1) in accordance with the American College of Sports Medicine recommendations (RT: n = 37; 47 +/- 7 yr; body mass index, 25.0 +/- 3.4 kg.m(-2)) or served as controls (CO: n = 37; 50 +/- 7 yr; body mass index, 24.2 +/- 3.2 kg.m(-2)). Strength (one-repetition maximum), RMR (indirect calorimetry), body fat (caliper method), and serum irisin concentration (enzyme-linked immunosorbent assay) were measured before and after 6 months of training. Results: Training led to an increase in strength (one-repetition maximum leg press, 16% +/- 7%; P < 0.001). RMR increased in RT (1671 +/- 356 vs 1843 +/- 385 kcal.d(-1), P < 0.001) but not in CO (1587 +/- 285 vs 1602 +/- 294 kcal.d(-1), P = 0.97; group-time interaction, P < 0.01). Body weight (RT, -0.5 +/- 2.4 kg; CO, 0.1 +/- 2.3 kg), body fat percentage (RT, -1.1% +/- 2.5%; CO, -0.7% +/- 2.9%), and FFM (RT, 0.4 +/- 2.1 kg; CO, 0.6 +/- 1.9 kg) did not develop differently between groups (group-time interaction: P = 0.29, P = 0.54, and P = 0.59, respectively). Serum irisin concentration increased in CO (70.8 +/- 83.4 ng.mL(-1), P < 0.001) but not in RT (22.4 +/- 92.6 ng.mL(-1), P = 0.67; group-time interaction, P < 0.01). The change in RMR was not associated with the change in FFM (r = -0.11, P = 0.36) or irisin (r = -0.004, P = 0.97). Conclusions: Preventive resistance training elicits an increase in RMR. However, in contrast to currently discussed hypotheses, this increase does not seem to be mediated by training-induced changes in FFM or circulating irisin concentration, which casts doubt in the meaning of irisin for human energy balance.