Assessing the impact of dietary protein on recovery indices from resistance exercise when nutrient timing is controlled for

Previous research has demonstrated that peri-exercise protein ingestion is beneficial for adaptive gains following resistance training (Cribb and Hayes, 2006, Medicine and Science in Sports and Exercise, 38 (11), 1918-1925). High protein intakes (in excess of 2.0g.kg-1.d-1) have also been reported to enhance recovery indices (Hoffman et al., 2010, Amino acids, 38 (3), 771-778), contrary to evidence that no further training benefits are observed above 1.8g.kg-1.d-1 (Lemon et al., 1992, Journal of Applied Physiology, 73 (188), 767-775). This study investigated the effect of total dietary protein, whilst controlling for protein timing, on recovery indices from resistance exercise. Following University Faculty Ethics Committee approval, fourteen strength-trained individuals were randomised to two 10-day dietary regimes with a protein content of 1.8g.kg-1.d-1 (PROMOD) or 2.9g.kg-1.d-1 (PROHIGH) in a cross-over design. On three consecutive days at the end of each regime (days 8,9,10), participants performed 3 sets of squat, bench press and bent-over rows at 80% 1 repetition maximum until volitional exhaustion. A 0.4g.kg-1 whey protein concentrate/isolate beverage was provided 30minutes before and after training sessions to control for nutrient timing. Recovery was assessed across days 8-10 via assessment of perceived muscle soreness, bioelectrical impedance phase angle (as a proxy measure of muscle integrity), plasma creatine kinase (CK) and tumor necrosis factor-α (TNF-α) levels prior to daily exercise; as well as repetition performance count for each exercise. A two-way repeated measures ANOVA was performed for the main analyses, with Bonferonni post-hoc pairwise comparisons where applicable. For repetition count, no significant differences were reported between conditions (P>0.05). For squat performance, total repetition count was lower at day 10 (19.7±1.8) compared to day 8 (23.0±2.0; P=0.006, ƞp2 = 0.335; 95% CI for difference -5.65 to -0.92) within PROMOD only. Post-exercise CK concentrations significantly increased across test days (P<0.001, ƞp2 = 0.583) increasing from 293±48U.L-1 to 718±140U.L-1 within PROHIGH and from 281±45U.L-1 to 656±155U.L-1 within PROMOD, although no differences were reported between groups. No differences for TNF-α or muscle soreness were reported between groups. Phase angle was significantly greater at day 10 for PROHIGH (8.26 ± 0.82°) compared with PROMOD (8.08 ± 0.80°, P=0.012, ƞp2 = 0.237; 95% CI for difference -0.55 to -0.08). Protein intake at 2.9g.kg-1.d-1 did not enhance indices of recovery following resistance exercise when peri-exercise protein consumption was controlled for. This finding suggests that daily protein intakes of 1.8g.kg-1 may be sufficient for consecutive days of resistance exercise in habitually trained individuals.