Influence of acute nitrate-rich beetroot supplementation on cardiovascular function

Dietary supplementation with inorganic nitrate (NO3-) has emerged, over several previous decades, as a nutritional intervention to improve cardiovascular health. These effects have been principally attributed to potential for NO3- to function as a precursor to multi-functional physiological signalling molecule, nitric oxide (NO). Indeed, exogenous NO3- can undergo a stepwise reduction to nitrite (NO2-) and then NO via the NO3--NO2--NO pathway to complement NO generated through the NO synthase enzymes. Whilst the downstream effects of NO on the cardiovascular system are mediated by cyclic guanosine monophosphate (cGMP) linked signalling and/or S-nitrosylation, it is currently unclear which of these processes underpins the beneficial effects of dietary NO3- supplementation. The most well assessed aspect of cardiovascular function after NO3- supplementation is brachial artery blood pressure (BP). Although NO3- supplementation has also been reported to improve other aspects of cardiovascular function, including endothelial function, assessed through flow mediated dilation (FMD); the pulse wave variables, augmentation pressure (AP) and augmentation index (AIx) as markers of arterial stiffness; and central BP, the dose-response effects of NO3- supplementation on these variables is unclear. There is evidence to suggest that NO3- supplementation may be more effective at improving cardiovascular function as baseline cardiovascular function is increasingly compromised, but the extent to which NO3- supplementation can mitigate against established cardiovascular stressors, such as acute caffeine ingestion and local cold exposure during a cold pressor test (CPT), is unclear. Therefore, this thesis presents data from three original research studies completed to assess the effects of acutely ingesting NO3--rich beetroot powder (BR) on brachial and central BP, FMD and pulse wave variables in healthy young adults. Specifically, these variables were assessed: study 1) following ingestion of three different NO3- doses (200 mg, 400 mg and 800 mg) compared to a control condition without dietary NO3- ingestion; study 2) prior to and following independent and combined acute ingestion of BR and caffeine; and study 3) prior to and following acute BR ingestion as well as during a subsequent 3 min CPT. In all these studies, plasma [NO3-] and [NO2-] were measured to assess NO3- metabolism and cGMP was assessed to provide insight into NO-mediated signalling. In study 1, there were no between-condition differences for brachial or central BP. On the other hand, AP and AIx were lower than control following all NO3- doses, with no between-dose differences. Absolute FMD was increased from control (0.23 ± 0.09 mm) after 400 mg NO3- (0.34 ± 0.23 mm; P = 0.037), but not 200 mg or 800 mg NO3-. Compared to control, plasma [NO3-] increased dose-dependently, plasma [NO2-] increased after ingesting 400 and 800 mg NO3-, but not 200 mg NO3-, whereas there were no between-condition differences for [cGMP]. In study 2, AP and AIx were higher after caffeine alone (3 ± 2 mmHg and 10 ± 9%) than a NO3--depleted beetroot placebo (PL; 1 ± 3 mmHg and 4 ± 11%) and BR (0 ± 3 mmHg and 1 ± 11%), with no difference between combined caffeine and BR (1 ± 3 mmHg and 3 ± 10%) and PLA or BR. Compared to a NO3--depleted beetroot placebo, brachial BP, central BP and FMD were not altered with independent BR or caffeine ingestion. In study 3, compared to PL, BR lowered central pulse pressure (systolic BP ' diastolic BP) (27 ± 5, 26 ± 5 mmHg,) and AP (1 ± 3, 0 ± 3 mmHg), but not brachial or aortic BP, nor AIx. Whilst CPT increased brachial and central BP and pulse wave variables in both BR and PL, there were no differences in these responses between BR and PL. In both studies 2 and 3, BR provided 428 mg NO3- with PL providing 92 mg NO3-, with BR increasing plasma [NO3-] and [NO2-], but not [cGMP], compared to PL. The principal original findings from this thesis are that acute BR ingestion: 1) did not alter brachial nor central BP, but improved endothelial function and arterial stiffness markers, albeit with different NO3- dose response profiles; 2) attenuated caffeine induced increases in arterial stiffness markers, but did not alter brachial BP, central BP and FMD which were not compromised by caffeine consumption; 3) did not attenuate CPT-induced increases in brachial and central BP and arterial stiffness markers; 4) did not increase plasma [cGMP] in any of the three experimental studies. Collectively, these findings improve our understanding of the effects of dietary NO3- supplementation on cardiovascular health variables by providing novel insight into the NO3- dose required to improve different aspects of cardiovascular function, and the extent to which NO3- supplementation can mitigate against cardiovascular strain elicited by acute cold exposure and caffeine consumption.