Dietary nitrate supplementation: physiological responses during prolonged exercise and optimizing nitric oxide bioavailability

Abstract

Dietary nitrate supplementation has been evidenced to lower the oxygen cost of submaximal exercise and, in some circumstances, to be ergogenic. Recent advances indicate that the mechanistic bases involve the independent or combined effects of nitric oxide-mediated improvements of contractile efficiency, mitochondrial efficiency, tissue perfusion, and/or redox signalling with effects perhaps being greater in type II muscle fibres. The aims of this thesis were: 1) to explore the potential effects of dietary nitrate supplementation on physiological responses in prolonged endurance exercise; and 2) to investigate potential supplementation strategies to enhance nitric oxide bioavailability during such exercise. In all four original investigations of the present thesis, the subjects consisted of healthy recreationally active adults who volunteered to participate. The subjects underwent various nitrate supplementation regimens, invasive and non-invasive physiological measurements, and a battery of exercise tests to assess the influence of dietary nitrate supplementation on attenuating fatigue during prolonged type exercise and enhancing subsequent exercise performance. The original findings from the present thesis indicate that the favourable effects of dietary nitrate supplementation are likely ascribed to its nitrate content and that nitrate supplementation may be a strategy to attenuate the decline in physiological function (i.e. intramuscular glycogen depletion, phosphocreatine depletion, and decline in muscle excitability) during prolonged cycling exercise lasting 1 to 2 h. In addition, results also showed that nitric oxide bioavailability can be influenced by ingesting an additional dose of nitrate during exercise and/or co-ingesting nitrate with a reduced thiol donor (i.e. N- acetylcysteine), which may subsequently influence the efficacy of nitrate. Together, the findings from the present thesis extend knowledge by indicating that dietary nitrate supplementation holds potential for aiding individuals in performing endurance exercise lasting ≥ 1 h and that altering the supplementation procedure has potential to increase nitric oxide bioavailability. These novel contributions have important implications for the application of dietary nitrate supplementation, particularly in endurance events.