Dietary nitrate as an ergogenic aid in sprint exercise performance and as an adjunct to sprint training

Abstract

In some circumstances, dietary nitrate (NO3-) supplementation can favourably alter the physiological response to exercise. Recent research highlights a potential preferential effect of NO3- supplementation on type II muscle fibre function. The purpose of this thesis was to investigate the practical applications of NO3- supplementation in sport and exercise settings requiring a substantial contribution of type II muscle fibre recruitment such as team and sprint sports. Specific questions addressed in this thesis include whether NO3--rich beetroot juice (BR) supplementation can influence the performance of sprint and high-intensity intermittent exercise and if BR supplementation, when combined with sprint interval training (SIT), can enhance performance and muscle metabolic adaptations in competitive team sport players. In addition, since cognitive performance is a key aspect of team sport performance, the effect of BR supplementation on decision-making during high-intensity intermittent exercise was also assessed. Chapter 4: Total work done during a prolonged intermittent sprint test (IST) designed to mimic the metabolic demands of team sport play was 3.5% greater following short-term BR supplementation (123 ± 19 kJ) compared to short-term NO3--depleted beetroot juice (PL) supplementation (119 ± 17 kJ) (P<0.05). Decision-making reaction time was shorter in the second half of the IST in BR (817 ± 86 ms) compared to PL (847 ± 118 ms) (P<0.05). Chapter 5: Short-term BR supplementation improved the performance of sprint running compared to PL over distances of 20 m (1.2% improvement; BR: 3.98 ± 0.18 vs. PL: 4.03 ± 0.19 s; P<0.05), 10 m (1.6% improvement; BR: 2.53 ± 0.12 vs. PL 2.57 ± 0.19 s; P<0.05) and 5 m (2.3% improvement; BR: 1.73 ± 0.09 vs. PL: 1.77 ± 0.09 s; P<0.05). Relative to PL, BR supplementation also increased the distance covered in the Yo-Yo intermittent recovery test level 1 by 3.9% (BR: 1422 ± 502 vs. PL: 1369 ± 505 m; P<0.05). Decision-making reaction time was shorter in BR (615 ± 98 ms) compared to PL (645 ± 120 ms) (P<0.05) at rest but not during the Yo-Yo IR1 test. Chapter 6: 4 weeks BR ingestion significantly reduced the O2 cost of moderate intensity exercise (by 5%; P<0.05) and significantly increased the peak work rate (WR) attained during incremental exercise (P<0.05). When 4 weeks SIT was combined with 4 weeks of BR ingestion, peak WR increased more than when SIT was combined with PL supplementation (SIT+BR: 24 ± 8 W increase vs. SIT+PL: 16 ± 15 W increase; P<0.05). Although the change in time to task failure during severe-intensity exercise was not statistically different between SIT+BR (69% increase) and SIT+PL (55% increase), there was evidence of a more favourable muscle metabolic response to severe-intensity exercise in SIT+BR. Specifically, at the 3-min iso-time during severe-intensity exercise, muscle pH was higher and muscle (and blood) [lactate] was lower in SIT+BR only (P<0.05). These findings demonstrate that BR supplementation may be ergogenic in sprint cycling, sprint running and high-intensity intermittent exercise and that when combined with SIT, BR supplementation may enhance the resultant exercise performance adaptations and some muscular adaptations to exercise. The results of this thesis also suggest that BR supplementation may improve cognitive performance, specifically decision-making reaction time, during team sport exercise.