Factors that influence dietary nitrate metabolism and its effects on cardiovascular health and exercise performance

Nitric oxide (NO) is an important and multifunctional physiological signalling molecule. Although NO was traditionally considered to be synthesised exclusively through the NO synthase (NOS) enzymes, it is now appreciated that NO can be generated through the serial reduction of nitrate (NO3-) to nitrite (NO2-) and then NO2- to NO via the NO3--NO2--NO pathway. This pathway recycles NO3- and NO2- synthesised endogenously from the oxidation of NOS-derived NO and from exogenous NO3- consumed via the diet to generate NO endogenously. Both the delivery of NO3- to the oral cavity through the enterosalivary circulation and second-pass metabolism of NO3- by the oral microbiome are critical for NO generation via this pathway. While dietary NO3- supplementation has been reported to improve aspects or cardiovascular health and exercise performance, there is significant inter-individual variability in NO3- metabolism and the efficacy of NO3- supplementation to elicit cardioprotective and ergogenic effects. Therefore, further research is required to better understand the factors that can influence dietary NO3- metabolism and its efficacy to improve cardiovascular function and exercise performance. Accordingly, the experimental studies that comprise this thesis explored how manipulating environmental temperature and the timing of NO3- supplementation administration, factors purported to influence salivary flow rate, influenced NO3- metabolism and its effects on cardiovascular function and exercise performance. In addition, the efficacy of NO3- supplementation to improve cardiovascular function and exercise performance in females taking oral contraceptive pills (OCP), which is purported to alter NOS derived NO, was assessed.

In Chapter 2, markers of NO3- metabolism and systolic blood pressure (SBP) were measured pe and post consumption of NO3--rich beetroot juice in an environmental chamber. Greater increases in plasma [NO2-] (592 ± 239 nM vs. 410 ± 195 nM) and reductions in brachial SBP were observed in cool compared with normothermic conditions following dietary NO3- supplementation. Brachial SBP was not significantly reduced following NO3--rich beetroot juice consumption in normothermia (-4 mmHg), but NO3--rich beetroot juice was able to mitigate the cool induced elevations in BP observed at 1 h (+4 mmHg), 2 h (+7 mmHg) and 3 h (+9 mmHg) in the cool environment. Chapter 3 investigated whether the time-of-day NO3--rich beetroot juice is consumed influences NO3- metabolism, SBP and exercise tolerance. NO3--rich beetroot juice resulted in similar increases in plasma [NO2-] (642 ± 289 nM, 670 ± 314 nM, 675 ± 355 nM) and reductions in aortic SBP (-3 ± -3 mmHg, -4 ± 3 mmHg, -2 ± 3 mmHg) in the morning, afternoon, and evening, respectively, but did not improve cycling time-to-exhaustion performance at any timepoint. The final study (Chapter 4) investigated the effect of dietary NO3- supplementation on NO3- metabolism, SBP and exercise performance in female OCP users. NO3--rich beetroot juice increased plasma [NO2-] (535 ± 343 nM and 472 ± 216 nM) and lowered peripheral (-5 ± 4 mmHg and -4 ± 3 mmHg) and central SBP (-6 ± 4 mmHg and -4 ± 2 mmHg) but did not improve determinants of endurance performance (including peak aerobic power, gas exchange threshold, respiratory compensation point and peak aerobic capacity (V̇O2peak)) in this population on pill withdrawal and pill consumption days, respectively. The results from this thesis confirm previous findings that acute consumption of NO3--rich beetroot juice can elevate plasma [NO2-] and lower SBP in healthy adults. However, the principal novel observations of this body of work are that acute NO3- supplementation: 1) lowers SBP more during cool exposure compared to normothermic conditions; 2) is similarly effective at lowering central SBP across the day; 3) can lower SBP in female OCP users; but 4) does not improve endurance exercise performance in OCP users or across the day in males. Collectively, these observations improve understanding of the settings which can impact NO3- metabolism and its efficacy to improve cardiovascular function after NO3- supplementation.