Endothelial dysfunction contributes to the increased cardiovascular risk associated with high blood pressure levels (e.g., 120-139/80-89 mmHg and 140/90 mmHg or higher), and it is considered as an early event in the development of atherosclerotic vascular disease. Mechanistically, endothelial dysfunction is characterised by a reduced bioavailability of the endothelium-derived molecule nitric oxide (NO), which is reflected by impaired brachial artery flow-mediated dilation (FMD). However, endothelial dysfunction is largely preventable by its early detection, thus approaches that might restore the loss of NO in the body could have obvious therapeutic potential.

Consumption of the prebiotic fibre inulin has been shown to increase the fermentation of bacterial-derived metabolites such as acetate by our gut microbiome. While increased acetate has local benefits in the gut (i.e., anti-inflammatory, lower luminal pH), it also has the potential to improve endothelial health. Research has shown that acetate can increase NO bioavailability via the activation of the NO synthase pathway by increasing phosphorylation of endothelial NO synthase (eNOS), a key enzyme in the maintenance of endothelial homeostasis, following a 15-day supplementation with inulin. Similarly, NO is produced following the consumption of nitrate, with individual responses being dependent on the basal eNOS activity. Therefore, individuals with compromised eNOS activity (e.g., pre- and hypertensive) might show an augmented response to nitrate supplementation. Since both nitrate and inulin supplementations can increase NO production, it is possible that their combined ingestion might enhance endothelial function to a greater extent. When supplemented in isolation, nitrate proved an effective dietary strategy to ameliorate endothelial dysfunction in older and clinical populations. For example, a 4-week chronic dietary nitrate supplementation (400 mg nitrate/day) was associated with an increased peak brachial artery FMD of 1.0% (95% CI, 0.3-1.5; p \< 0.001), which was not evident in the placebo group. Similarly, following a 4-week dietary nitrate supplementation (400 mg nitrate/day), brachial artery FMD (%) values increased significantly in the nitrate group compared to the placebo group. Unlike nitrate, the effects of inulin on brachial artery FMD in humans are yet to be established. Preliminary findings demonstrated an enhancement of endothelial function after inulin supplementation in mice. These findings were further supported by another study which demonstrated a brachial artery FMD increase of \~1% following a 3-month prebiotic supplementation with 12 g fructo-oligosaccharides in clinical patients with less severe endothelial dysfunction.

To date, no studies that the investigators are aware of have explored the role of nitrate and inulin in combination; however, based on the above, these two nutrients have the potential to work synergistically. Therefore, the primary aim of this study is to assess whether adding nitrate to inulin during a 4-week chronic supplementation might lead to an increase in brachial artery FMD in middle-aged and youngest elderly adults (45-74 years old) with elevated blood pressure and compare this response to inulin supplemented in isolation. The secondary aims of this study include brachial artery FMD % changes at 4 h post-consumption of a single dose of 15 g inulin and 1300 mg potassium nitrate combined and 15 g inulin supplemented in isolation. This study will also explore whether changes in brachial artery FMD % following the two supplementations are mediated by potential changes in the gut microbiome composition and diversity that may occur following prebiotics supplementation. Further secondary outcomes of the study will investigate differences in plasma nitrate, nitrite, and short-chain fatty acids (SCFAs) following both types of supplementation. Additionally, the study will examine changes in the levels of S-nitrosothiols (RSNO) in red blood cells and whole blood. The study will also compare changes in resting blood pressure between the combined inulin and nitrate supplementation and the inulin-only supplementation.