Dietary Nitrate Supplementation: Dose-Response Relationships and Effects on Intermittent Exercise Performance

Dietary supplementation with inorganic nitrate has been shown to lower the oxygen cost of submaximal exercise and to improve performance during continuous endurance exercise. The objectives of this thesis were: 1) to improve understanding of the supplementation procedures in which nitrate is most likely to benefit exercise physiology and performance, and 2) to explore the ergogenic potential of dietary nitrate in intermittent exercise. Recreationally-active adult participants volunteered to participate in the original investigations presented in this thesis. These participants underwent various oral nitrate supplementation regimes and subsequently provided venous blood samples for the determination of plasma nitrate concentration and plasma nitrite concentration, and completed different exercise tests to assess the potential for nitrate supplementation to improve physiological responses during exercise and exercise performance. Chapter 4: Following the ingestion of 70, 140 and 280 mL concentrated nitrate-rich beetroot juice (containing 4.2, 8.4 and 16.8 mmol nitrate, respectively) plasma nitrite concentration increased dose-dependently, with peak changes occurring at ~2-3 h post ingestion. The oxygen cost of submaximal exercise and exercise tolerance were also influenced dose-dependently 2.5 h post acute BR ingestion when compared to a nitrate-depleted placebo. Specifically, 8.4 mmol and 16.8 mmol lowered the oxygen cost of moderate-intensity exercise by 1.7% (P = 0.06) and 3.0% (P < 0.05), respectively. Exercise tolerance during severe-intensity exercise was significantly improved after ingestion of 8.4 mmol (+14%; P < 0.05), with no further improvement evident following ingestion of 16.8 mmol (+12%; P < 0.05). 4.2 mmol nitrate did not significantly lower the oxygen cost of submaximal exercise or improve exercise tolerance. Chapter 5: Plasma nitrite concentration was dose-dependently elevated 2 h and after 7 days and ~4 weeks of supplementation with 3 mmol and 6 mmol nitrate. Compared to pre-supplementation baseline, and placebo, moderate-intensity exercise oxygen uptake was not lowered with 3 mmol nitrate acutely (2 h) or after ingesting it daily up to ~4 weeks. In contrast, ingestion of 6 mmol nitrate significantly lowered submaximal exercise oxygen uptake by 3% after 2 h (P = 0.06), 7 days and ~4 weeks (both P < 0.05) of supplementation. Another interesting observation was that the reduction in submaximal exercise oxygen uptake after ~4 weeks of supplementation with 6 mmol nitrate was preserved 24 h after consumption of the final nitrate dose despite plasma nitrite concentration having returned to baseline. Chapter 6: In recreational team sport players, 28 mmol dietary nitrate administered over ~30 h improved performance in the Yo-Yo Intermittent Recovery Level 1 (Yo-Yo IR1) test (a test that mimics the high-intensity intermittent exercise bouts typical of team sport games) by 4.2%, compared to placebo (nitrate: 1704 m vs. placebo: 1636 m). The decline (or ‘utilisation’) of plasma nitrite from pre-exercise to post-exercise was greater with BR compared to placebo, and the magnitude of this decline in plasma nitrite was positively correlated with the improvement in Yo-Yo IR1 performance. Chapter 7: Supplementation with dietary nitrate for 3-5 days (8.2 mmol nitrate per day) significantly improved performance during 24 x 6-s all-out cycling sprints interspersed with 24 s recovery, but not during 7 x 30-s all-out sprints interspersed with 240 s of recovery or 6 x 60-s self-paced maximal efforts interspersed with 60 s of recovery, in a group of recreational team sport players. The novel findings presented in this thesis suggest that the supplementation procedure, in particular the nitrate dose administered, should be carefully considered by individuals wishing to elicit ergogenic effects following dietary nitrate supplementation. Specifically, the findings suggest that a low dose (≤4.2 mmol) of nitrate is not sufficient to acutely improve exercise tolerance, and that submaximal exercise oxygen uptake is not lowered following acute or prolonged (~30 day) supplementation with a low nitrate dose. In contrast, results show that higher doses (6-8.4 mmol) of nitrate might be sufficient to acutely lower the oxygen cost of submaximal exercise and improve exercise tolerance. The present thesis also provides evidence to indicate that dietary nitrate might hold potential as an ergogenic aid for individuals participating in intermittent exercise involving brief bouts of high-intensity exercise interspersed with short recovery periods, such as team sport players. These findings are of importance as they will help inform supplementation procedures in future studies assessing the ergogenic efficacy of nitrate supplementation. Moreover, they suggest that nitrate supplementation has the potential to improve performance during intermittent exercise as well as continuous endurance exercise