Effect of dietary nitrate supplementation on high-intensity exercise performance and skeletal muscle calcium handling

Supplementation with inorganic nitrate (NO3−) has emerged as a precursor of nitric oxide (NO) and a promising nutritional strategy to enhance aspects of health and exercise performance. Initial studies reported enhanced performance during continuous submaximal endurance exercise after NO3− supplementation, but its effects on single and repeated bouts of short-duration high-intensity exercise are equivocal. Moreover, despite its purported ergogenic effects, the underlying mechanisms for enhanced exercise performance after NO3− supplementation are not fully understood. Accordingly, this thesis explored the potential for NO3− supplementation to improve performance during single and repeated bouts of short-duration high-intensity exercise, high-intensity intermittent exercise (HIIE) and severe-intensity continuous cycling and some of the possible intramuscular mechanisms that could underpin ergogenic effects of NO3− supplementation.

In Chapter 2, a systematic review and meta-analysis was conducted to assess the effects of dietary NO3− supplementation on single and repeated bouts of short-duration high-intensity exercise. NO3− supplementation had small positive effects on some, but not all, performance aspects during single and repeated bouts of high-intensity exercise. Specifically, time to reach peak power (SMD: 0.75, P < 0.05), mean power output (SMD: 0.20, P < 0.05) and total distance covered in the Yo-Yo intermittent recovery level 1 test (SMD: 0.17, P < 0.05) were improved following NO3− supplementation while peak power output (SMD: 0.01, P > 0.05) and total work done (SMD: 0.06, P > 0.05) remained unchanged. Chapter 3 investigated the effects of short-term NO3– supplementation on muscle sarcoplasmic reticulum (SR) vesicle calcium (Ca2+) handling and the number of repetitions completed during HIIE. Compared to NO3--depleted beetroot juice (PL) supplementation, NO3--rich beetroot juice (BR) did not alter SR Ca2+ release rate pre-HIIE or post-HIIE (P > 0.05). While BR supplementation did not alter SR Ca2+ reuptake tau pre-HIIE (P > 0.05), SR Ca2+ reuptake tau was lower (SR Ca2+ reuptake rate was faster) post-HIIE in BR compared to PL (30.4 ± 6.9 vs.35.3 ± 6.5 s, P < 0.05). No changes in the expressions of Ca2+ handling proteins (SERCA1 & 2, PLN, P-PLN, CSQ 1 & 2) were observed (P > 0.05). The total number of repetitions completed in the HIIE test continued to exhaustion were greater in BR compared to PL (13 ± 5 vs 12 ± 4, P < 0.05). Chapter 4 assessed the effect of short-term dietary NO3– supplementation on skeletal muscle mitochondrial function in permeabilised muscle fibres and SR vesicle Ca2+ release and reuptake rates at rest and following severe-intensity cycling. Mitochondrial respiration analysis revealed no significant alterations in complex I leak, ADP-stimulated respiration through complexes I-II, or maximal electron transfer system activity through complexes I-IV following BR supplementation at rest or after severe-intensity cycling (all P > 0.05). Similarly, SR vesicle Ca2+ release and reuptake rates were not different between BR and PL pre- or post-exercise. There was no difference in severe-intensity exercise time to exhaustion following BR compared to PL supplementation (453 ± 156 s vs. 414 ± 147 s, P > 0.05). The principal novel findings from this thesis are that NO3– supplementation: 1) improves some performance aspects of single and repeated bouts of short-duration high-intensity exercise; 2) attenuates the HIIE-induced slowing of SR Ca2+ reuptake without changing muscle Ca2+ handling proteins; 3) does not alter skeletal muscle mitochondrial respiration or Ca2+ release and reuptake rates at rest or following severe-intensity cycling. Collectively, these original findings enhance understanding of the exercise settings and potential mechanisms by which NO3– supplementation can improve exercise performance in healthy humans.