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3-day oral N-acetyl-cysteine supplementation alters metabolism but not performance of high intensity aerobic exercise in trained cyclists

A. Trewin,1 F. Billaut,1,2 A. Petersen,1,2 B.D. Perry,1 E. Goff,1 T. Atanasovska1 and N.K. Stepto,1,2 1School of Sport and Exercise Science, Victoria University, Melbourne, VIC 8001, Australia and 2Institute of Sport Exercise and Active Living, Victoria University, Melbourne, VIC 8001, Australia.

Redox homeostasis is essential for proper functioning of biological systems. Oxidative stress impairs contractile activity in skeletal muscle, and contributes to muscular fatigue during heavy exercise (Barclay & Hansel, 1991; Reid et al., 1992). Accordingly, antioxidant supplements may assist endogenous antioxidants to prevent deleterious effects associated with oxidative stress (Medved et al., 2004; Kelly et al., 2009). In this study we investigated the effect of oral N-acetyl-cysteine (NAC) supplementation on metabolism and high intensity cycling performance. Nine well-trained male cyclists (mean ± SD; 27 ± 6 years of age, VO2peak 69.4 ± 5.8−1.min−1) provided written informed consent. In a randomized, double-blind crossover design, subjects performed a 6 × 5 min High Intensity-Interval Training (HIT) cycling session at 82.5% of peak sustained power output, followed by a 10 minute self-paced Time Trial (TT) on two occasions 7 d apart. Prior to one session subjects consumed 5 × 750ml doses (2 × 2 d, 2 × 1 d, 1 × 1 hr pre-trial) of sports drink each containing−1 NAC, which was repeated for the other session, but without NAC. Metabolic, electromyographic (EMG), performance data, and blood/plasma samples were collected for analysis before, during, and after the 6 × 5 min HIT bouts and subsequent TT. Respiratory Exchange Ratio (RER) was decreased in the NAC condition throughout HIT exercise, and was significant at bouts 1 and 5 (p < 0.05) as shown in The Figure. Compared to placebo, NAC decreased blood lactate during TT and recovery (p < 0.05). Both pH (p < 0.01), and HCO3 (p < 0.05) were reduced throughout exercise and recovery with NAC. In contrast NAC resulted in higher blood glucose concentration during HIT (p < 0.05). EMG median frequency of the vastus lateralis decreased in HIT bout 6 in the NAC condition (p < 0.05). No significant difference was observed in the total work performed in the 10-min TT (p = 0.16). These data indicate that NAC does not change performance in a self-paced 10-min TT, but induces a shift in muscle fibre-type recruitment and alters metabolism during high intensity interval exercise, which may provide a glycogen-sparing effect during prolonged exercise.

Figure 1

Barclay, J. & Hansel, M. (1991) Free radicals may contribute to oxidative skeletal muscle fatigue. Canadian Journal of Physiology and Pharmacology, 69, 279-284.

Kelly, M., Wicker, R., Barstow, T. & Harms, C. (2009) Effects of N-acetylcysteine on respiratory muscle fatigue during heavy exercise. Respiratory Physiology & Neurobiology, 165, 67-72.

Medved, I., Brown, M.J., Bjorksten, A.R., Murphy, K.T., Petersen, A.C., Sostaric, S., Gong, X. & McKenna, M.J. (2004) N-acetylcysteine enhances muscle cysteine and glutathione availability and attenuates fatigue during prolonged exercise in endurance-trained individuals. Journal of Applied Physiology, 97, 1477-1485.

Reid, M., Haack, K., Franchek, K., Valberg, P., Kobzik, L. & West, M. (1992) Reactive oxygen in skeletal muscle. I. Intracellular oxidant kinetics and fatigue in vitro. Journal of Applied Physiology, 73, 1797.