Influence of dietary nitrate supplementation on high-intensity exercise performance: association with skeletal muscle oxidative phenotype
Creators: name: Robinson, George
Publisher: name: Loughborough University
Year: 2024
Subjects: classificationCode: 420799
subject: Sports science and exercise not elsewhere classified

description: Dietary nitrate (NO<sub>3</sub><sup>-</sup>) has emerged as ergogenic aid in recent years with the potential to enhance performance in various exercise settings. The ergogenic effects of dietary NO<sub>3</sub><sup>-</sup> supplementation have been attributed to its stepwise reduction to nitrite (NO<sub>2</sub><sup>-</sup>) and then nitric oxide (NO). Dietary NO<sub>3</sub><sup>-</sup> was initially recognised as an ergogenic aid for endurance exercise on the basis of initial studies reporting enhanced exercise economy and performance during continuous submaximal exercise. More recently, and inspired by murine models revealing enhanced physiological responses in fast-twitch (type II) skeletal muscle compared to slow-twitch (type I skeletal muscle), there has been interest in exploring the ergogenic potential of NO<sub>3</sub><sup>-</sup> supplementation to improve performance in higher intensities of exercise that mandate greater type II muscle fibre recruitment. Moreover, since muscle pH and oxygenation decline to a greater extent in contracting type II skeletal muscle and at higher exercise intensities, and since the stepwise reduction of NO<sub>3</sub><sup>-</sup> to NO<sub>2</sub><sup>-</sup> and then NO<sub>2</sub><sup>-</sup> to NO is enhanced in acidosis and hypoxia, NO<sub>3</sub><sup>-</sup> may be more effectively metabolised to NO after NO<sub>3</sub><sup>-</sup> supplementation in such settings. However, the effect of NO<sub>3</sub><sup>-</sup> supplementation on performance in such settings, and to what extent any ergogenic effect of NO<sub>3</sub><sup>-</sup> supplementation was related to skeletal muscle oxidative phenotype was unclear. Therefore, the overarching purpose of the current thesis was to assess the effect of NO<sub>3</sub><sup>-</sup> supplementation on performance during short-duration, high-intensity exercise modes, and to assess the degree to which this was influenced by markers of skeletal muscle oxidative phenotype. In Study 1, the effect of short-term supplementation with NO<sub>3</sub><sup>-</sup>-rich beetroot juice (BR) on performance during high-intensity intermittent running completed in normoxia and two doses of normobaric hypoxia, to reflect terrestrial altitudes of ~1200 m and 2400 m, was assessed in endurance trained participants. Compared to a NO<sub>3</sub><sup>-</sup>-depleted beetroot juice (PL), BR increased plasma NO<sub>2</sub><sup>-</sup> concentration but did not improve high-intensity intermittent running performance in normoxia or normobaric hypoxia to reflect low-to-moderate altitude exposure. Chapter 3 investigated the effect of short-term BR supplementation on 40 m sprint running performance and how this related to % type II muscle fibres, mitochondrial respiration and citrate synthase activity as a marker of mitochondrial content. Although BR supplementation did not improve 40 m running performance, instantaneous velocity (IV) over the final 10 m of the sprint was higher after BR compared to PL (7.81 ± 0.81 vs. 7.75 ± 0.87 m·s<sup>-1</sup>; P&lt;0.05). The increase in IV during 30-40 m after BR supplementation was positively correlated with type II muscle fibre % (r = 0.57, P&lt;0.05), inversely correlated with mitochondrial respiration (r = -0.59, P&lt;0.05), but not correlated with mitochondrial content (r = 0.22, P&lt;0.05). Chapter 4 investigated the effect of short-term BR supplementation on single, dynamic, maximal contractions, specifically concentric and eccentric torque and counter movement (CMJ) performance, and to what extent any BR-induced improvement in these assessments was related to skeletal muscle oxidative phenotype variables and relative NO<sub>3</sub><sup>-</sup> dose. Rate of force development (RFD) during the CMJ was lower after BR compared to PL supplementation in females (6108 ± 1886 vs. 7038 ± 2271 N·s<sup>-1</sup>, P=0.004), but otherwise, BR supplementation did not alter concentric and eccentric torque or CMJ performance in males or males and females combined (P&gt;0.05). However, the degree to which CMJ RFD was increased by BR supplementation was inversely related to mitochondrial respiration (r =-0.464, P=0.039) and NO<sub>3</sub><sup>-</sup> dose relative to fat free mass (r =-0.640, P&lt;0.01).In summary, whilst BR supplementation was largely ineffective at improving performance in the short-duration, high-intensity exercise modes assessed in the current thesis, and could compromise aspects of CMJ performance in females, it was effective at increasing IV at 30-40 m of a 40 m sprint run in a mixed sex group of participants. There was also some evidence in the thesis to suggest that the potential for BR supplementation to improve short-duration, high-intensity exercise performance may be inversely linked to markers of skeletal muscle oxidative phenotype and relative NO<sub>3</sub><sup>-</sup> dose. The original findings presented in this thesis improve our understanding of physiological correlates that could impact the efficacy of BR supplementation to improve short-duration, high-intensity exercise performance.