High-Dose Nitrate Supplementation Attenuates the Increased Blood Pressure Responses to Isometric Blood Flow Restriction Exercise in Healthy Males

The effect of nitrate (NO3−) supplementation on blood pressure (BP) responses during large muscle mass isometric and ischaemic exercise in healthy young adults is unclear. The aim of the present study was to assess the effect of 5-day supplementation of NO3− on BP responses during a short isometric contraction and a sustained ischaemic contraction. In a randomised, double-blinded, crossover design, 14 healthy active young adults underwent BP measurements after 5 days of either NO3− (NIT) or placebo (PLA) supplementation. Beat-by-beat BP was measured at pre- and post-exercise rest, and during a short (20 s) isometric contraction at 25% maximal strength and throughout a sustained ischaemic contraction. Plasma nitrite (NO2−) concentration increased significantly after NO3− supplementation compared to placebo (475 ± 93 nmol·L−1 vs. 198 ± 46 nmol·L−1, p < 0.001, d = 3.37). Systolic BP was significantly lower at pre- (p = 0.051) and post-exercise rest (p = 0.006), during a short isometric contraction (p = 0.030), and throughout a sustained ischaemic contraction (p = 0.040) after NO3− supplementation. Mean arterial pressure was significantly lower at pre- (p = 0.004) and post-exercise rest (p = 0.043), during a short isometric contraction (p = 0.041), and throughout a sustained ischaemic contraction (p = 0.021) after NO3− supplementation. Diastolic BP was lower at pre-exercise rest (p = 0.032), but not at post-exercise rest, during a short isometric contraction, and during a sustained ischaemic contraction (all p > 0.05). Five days of NO3− supplementation elevated plasma NO2− concentration and reduced BP during a short isometric contraction and a sustained ischaemic contraction in healthy adults. These observations indicate that multiple-day nitrate supplementation can decrease BP at rest and attenuate the increased BP response during isometric exercise. These findings support that NO3− supplementation is an effective nutritional intervention in reducing SBP and MAP in healthy young males during submaximal exercise.


Introduction
Beetroot juice supplementation has been used as a popular nutritional intervention, as its nitrate (NO 3 − )-rich content has documented ergogenic [1][2][3][4][5] and cardioprotective [6][7][8] effects. These effects of NO 3 − supplementation have been attributed to its capacity to increase the bioavailability of nitric oxide (NO). NO is a gaseous signalling molecule that serves as an essential regulator in numerous physiological functions, such as preservation of metabolic and cardiovascular integrity [9]. NO is mainly produced through the oxidation of the amino acid L-arginine (via NO synthase (NOS) enzyme), with NO 3 − and nitrite (NO 2 − ) being inert end products of endogenous NO generation [10][11][12]. It is now known that NO can be also produced through the sequential reduction of NO 3 − to NO 2 − , through anaerobic bacteria that reside in the oral cavity, and subsequently to NO [13]. − supplementations to modulate cardiovascular function during exercise. In addition, the effect of NO 3 − supplementation on BP responses was mainly assessed during arm muscle isometric exercise, but its effect during large muscle mass (e.g., vastus lateralis) isometric exercise has not been investigated. Thus, further research is warranted to examine the influence of NO 3 − supplementation on BP responses during large muscle mass contractions in humans.
Therefore, the aim of the present study was to assess the effect of 5-day supplementation of NO 3 − on BP responses at rest before and after exercise and during leg muscle isometric exercise in healthy humans. In an attempt to maximise the conditions for conversion of NO 2 − to NO [29,30], a sustained muscle contraction was completed with blood flow restriction which would exaggerate low PO 2 and pH during contractions. It was hypothesised that 5-day NO 3 − supplementation would: (1) increase plasma NO 2 − , (2) lower BP at rest pre-and post-exercise, and (3) attenuate the increased BP during isometric muscle contraction.

Participants
The sample size of this study was based on a prior calculation using G*Power software (version 3.1.9.4, Universität, Düsseldorf, Germany). A two-sided significance level of 0.05 and a power of 0.80 indicated that 10 participants would be sufficient to detect a difference in BP response, based on a small standardised effect size of 0.2 and the variance of the difference between PLA and NIT trials previously reported [31].
Fourteen males (mean ± SD age: 25 ± 6 years, stature: 174 ± 1 cm, mass: 78 ± 8 kg) volunteered in this study by providing written informed consent. Participants were injuryfree and non-smokers. All participants self-reported that they were involved in regular moderate-intensity exercise~3 days per week and muscle-strengthening activities~2 days per week. The study received institutional ethical approval from the Manchester Metropolitan University Research Ethics Committee (reference no: 5951) and was conducted according to all aspects of the Declaration of Helsinki. Participants were requested to record their dietary intake five days prior to the first trial (throughout the first 5-day supplementation period) and to repeat the same diet five days prior to the second trial (throughout the second 5-day supplementation period). Individuals were also asked to maintain habitual physical activity and diet, refrain from antibacterial mouthwash, and abstain from caffeine, nutritional supplements, alcohol, and vigorous exercise (for 24 h) before trials.

Study Design
In a randomised, double-blinded, crossover design, participants attended the laboratory at a similar time of day (±2) on two separate occasions to complete two experimental trials, one with 5-day supplementation of NO 3 − -rich beetroot juice (BRJ), and one with 5-day supplementation of NO 3 − -depleted BRJ. A washout period of 7 ± 1 days separated the supplementation periods. Randomisation was applied in counter-balanced fashion, in which the participant sample was divided in half, with one half completing the two conditions in one order and the other half completing the conditions in the reverse order. Randomisation and blinding were administered by an independent technician who did not take part in the assessments. Participants attended the testing facility after each 5-day supplementation period. In each visit, first a blood sample was collected, and resting BP was measured for 5 min. Then, maximum voluntary contractions (MVCs) of the knee extensors were performed by using a custom-built dynamometer [32] to determine individual 25% MVC for submaximal exercise protocol. The exercise protocol has been described elsewhere in detailed [33]. Briefly, a 20 s contraction at 25% MVC was performed firstly; then, a blood flow occlusion (BFO) period was applied at the proximal thigh (with 220 mmHg) for 8 min, with a sustained submaximal contraction at 25% MVC in the final 3 min of the BFO period. Restriction was then released, and the participant remained sitting for a further rest of 5 min.

Supplementation Procedures
During the two 5-day supplementation periods, participants ingested 2 × 70 mL/day of concentrated NO 3 − -rich (NIT:~12.8 mmol/day NO 3 − ) or NO 3 − -depleted BRJ as placebo (PLA:~0.08 mmol/day NO 3 − ) (Beet It, James White Drinks Ltd., Ipswich, UK). Participants were instructed to supplement with the 2 × 70 mL shots around the same times each day (one in the morning at~9 a.m. and one in the evening at~9 p.m.) for the first 4 days of supplementation. On day five of supplementation, participants were instructed to ingest 2 × 70 mL shots together 2.5 h before the trial [33]. This regimen for multiple-day supplementation of NO 3 − has been used as a standard approach in numerous previous studies (e.g., [1,2,20,[33][34][35][36]). Participants were instructed to set up a time reminder for each time of the beetroot juice shots during a day and asked to keep a record of any days when they missed taking the beetroot juice, in order to monitor supplement compliance. Participants were also sent emails twice a day as reminders for beetroot juice ingestion. Given that the current recommendation for NO 3 − supplementation is~6-29 mmol/day of NO 3 − [33,36,37], and the potential ergogenic effect of NO 3 − supplementation by elevating plasma NO 2 − concentration has been reported at the highest following dose of~12.8 mmol/day in healthy humans [38],~12.8 mmol/day of NO 3 − was used in the present study.

Measurement
Beat-by-beat arterial BP: Following 10 min of rest of comfortable upright sitting, beatby-beat BP was recorded by using finger photoplethysmography (Finometer, Finapres Medical Systems, Amsterdam, The Netherlands) [39]. BP was measured throughout the testing protocol (at pre-exercise rest, during short isometric contraction and the 3 min of sustained contraction with BFO, and at post-exercise rest). Systolic BP (SBP), mean arterial pressure (MAP), and diastolic BP (DBP) values were calculated as a mean of each phase using LabChart8 software (v8.1.13, Adintstruments Products, Oxford, UK).
Plasma NO 2 − : Venous blood samples were collected, at least 2.5 h after the last meal, to determine plasma NO 2 − concentration. A venous blood sample was drawn into a lithium heparin tube (5 mL, Vacutainer, Becton Dickinson) at rest on both trial days. Samples were then centrifuged at 4000 rpm at 4 • C for 10 min (hettich ® 320 centrifuge, Tuttlingen, Germany). Plasma was subsequently extracted and immediately frozen at −80 • C for later analysis of NO 2 − using a modification of the chemiluminescence technique as described elsewhere [16].

Discussion
The present study examined the influence of multiple-day supplementation of NO3on BP responses during short isometric knee extensor contraction and a 3 min sustained ischaemic contraction in healthy young males. The main findings of the present study were that 5 days of NO3 − supplementation (~12.8 mmol/d −1 of NO3 − ) (I) elevated plasma NO2 -concentration, (II) lowered BP at pre-and post-exercise rest, and (III) attenuated the rise in BP during short isometric contraction and a sustained ischaemic contraction in healthy adults. These findings have extended the knowledge regarding the BP-lowering impact of NO3 -with multiple-day supplementation during isometric and ischaemic exercise in healthy adults using a double-blind, placebo-controlled, crossover design.

Discussion
The present study examined the influence of multiple-day supplementation of NO 3 − on BP responses during short isometric knee extensor contraction and a 3 min sustained ischaemic contraction in healthy young males. The main findings of the present study were that 5 days of NO 3 − supplementation (~12.8 mmol/d −1 of NO 3 − ) (I) elevated plasma NO 2 − concentration, (II) lowered BP at pre-and post-exercise rest, and (III) attenuated the rise in BP during short isometric contraction and a sustained ischaemic contraction in healthy adults. These findings have extended the knowledge regarding the BP-lowering impact of NO 3 − with multiple-day supplementation during isometric and ischaemic exercise in healthy adults using a double-blind, placebo-controlled, crossover design.
BP was lower in the NIT trial compared with the PLA trial at rest and during short isometric muscle contraction. The findings of the present study on BP at rest are consistent with previous reports in healthy active individuals [16,[18][19][20]. However, the observed effect on BP during short muscle contraction is inconsistent with a previous study by de Vries et al. [43] that reported no effect of acute NO 3 − supplementation on a contracting large muscle mass in young healthy adults. Since we administered the dose of 12.8 mmol/day of NO 3 − over multiple days (5 days), it is possible that the duration of supplementation may have contributed to the effects observed. Indeed, a greater effect of NO 3 − supplementation has been reported following multiple-day compared to acute supplementation [34].
Although isometric knee extension exercise with BFO increased BP following both NO 3 − and placebo supplementation, there was a markedly lower increase in BP following NO 3 − supplementation. While this effect was observed during the short contraction, it became evident from the first 20 s of the sustained ischaemic contraction. These findings are consistent with previous observations reporting reduced BP during submaximal handgrip exercise after NO 3 − supplementation in hypertensive patients [23] and during dynamic (aerobic) exercise in healthy individuals [21,22]. It is important to highlight several important differences in the present study compared to previous investigations. While we applied 5-day high-dose supplementation in healthy participants, the BP-reducing effect of NO 3 − was reported in hypertensives after a single dose of dietary NO 3 − ingestion [23,44]. Interestingly, larger reductions in BP after dietary NO 3 − were reported in individuals with chronic disease than in healthy individuals [44], and these reductions were greater in individuals with higher resting BP values [45]. We also applied the isometric exercise to a large muscle mass (leg) while others used a small muscle mass [23,44,45]. Regarding muscle mass, Polito et al. [46] reported a greater reduction in SBP after 10 sets of 10 reps of leg extension exercise when compared to elbow flexion exercise in normotensive men, suggesting the amount of muscle mass activated during exercise has an impact on BP. Together, these findings suggest (I) that the supplementation regimen of nitrate needs to be considered and/or managed depending on an individual's health status, and (II) that the exercising muscle mass/group might have an impact on the efficacy of NO 3 − supplementation, at least for BP responses. As mentioned above, the effect of NO 3 − supplementation on BP is mostly related to improved systematic NO bioavailability [18,19,23,42], and BFO likely leads to exaggerated hypoxic conditions during muscle contraction, which would be expected to elicit the reduction of NO 2 − to NO [29,30]. Further, existing evidence shows that NO 3 − supplementation may enhance post-ischaemic vasodilation in healthy young adults after both acute and chronic administration [15,47,48], suggesting improved endothelial function, which may explain lower BP response after NO 3 − supplementation compared to placebo during post-exercise rest. Taken together, these findings might explain the positive influence of NO 3 − supplementation on BP during the sustained contraction with BFO in the present study. The effects of NO 3 − supplementation have been reported to be more apparent in less fit or recreationally active individuals [49], likely due to their lower baseline plasma NO 2 − levels. Since the participants in the present study were all recreationally active and since elevated plasma NO 2− levels were observed after NO 3 − supplementation, it is plausible that NO 3 − supplementation increased NO bioavailability at the vascular level and thus induced the reducing effect on BP.
Lowered BP is attributed to improved vascular tone (vasodilation) via increased cGMP after NO 3 − supplementation [14][15][16]. Elevated NO bioavailability via NO 3 − supplementation appears to increase cGMP [16], which might accelerate the removal of calcium in the vascular smooth muscle, leading to vasodilation [50]. In addition, since there is some evidence that NO may inhibit sympathetic vasoconstriction in resting and contracting skeletal muscle (sympatholysis) [25][26][27][28], lowered BP at rest and during short muscle contraction following NO 3 − supplementation might be linked to reduced sympathetic vasoconstriction and altered sympathetic control of BP. Given that the sympathetic nervous system can also influence pressure and endothelium-dependent vasodilation [51,52], reduced BP via increased NO bioavailability might be also linked to alterations in efferent sympathetic outflow [53,54]. This view is supported by studies reporting that NO 3 − supplementation has (I) lowered muscle sympathetic nerve activity during a 2 min isometric handgrip exercise [25], and (II) reduced sympathetic signalling during aerobic exercise [21].

Limitations
Several important points must be considered when interpreting the findings of this study. The current investigation did not include females. However, since different plasma NO 3 − and BP responses between males and females have been reported [19], the findings of the present study can only be interpreted for males, and future studies should include females to investigate if the potential effect of NO 3 − supplementation in altering BP is different compared with males, and to what extent. Though it remains to be elucidated fully, there are multiple proposed mechanisms for the BP-lowering effect of NO 3 − supplementation; however, we did not measure any mechanism of action-related parameters (e.g., blood flow and/or sympathetic nerve activity) in this investigation. Future studies should measure these parameters independently or in combination to improve understanding of the mechanisms of the BP-lowering effect of NO 3 − supplementation during exercise. It is also important to note that acute effects due to doubling the dose (2 × 70 mL shots together) on the testing day cannot be ruled out. Therefore, future protocols should consider a longer-term intervention (longer than 8 weeks), without the need to further supplement on the testing day, as the longer duration would have already allowed the supposed BP and associated circulatory adaptations. We could not control participants' dietary intake but relied instead on the participants recording their dietary intake for 5 days before (throughout the first 5-day supplementation period) the first trial and replicating this before the subsequent trial (throughout the second 5-day supplementation period). While the participants reported that they had complied with this requirement, since it was self-reported, monitoring of dietary intake can be considered a limitation. Thus, future studies might control pre-test diet more rigorously. Since it was self-reported, monitoring of supplement compliance can be also considered a limitation, although the rate of supplement compliance was 100%. There is plenty of evidence to suggest that nitrate can improve exercise and/or physical capacity, which can facilitate the regulation of BP itself [33,35,36]. Given that this study focused on BP responses during exercise, but not exercise or/and physical parameters, we cannot exclude the possibility that some effects could be due to supplement-induced improved exercise and/or physical capacity instead of the direct effect of supplementation on BP. Nevertheless, the findings of this study provide novel data which will be of interest to those wanting to know the effects of NO 3 − on blood pressure during exercise.

Conclusions
The findings reveal that short-term high-dose NO 3 − supplementation elevated plasma NO 2 − concentration and reduced BP during a short isometric contraction and a sustained ischaemic contraction in healthy males. These findings support that NO 3 − supplementation is an effective nutritional intervention in reducing SBP and MAP in healthy young males during submaximal exercise.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nu14173645/s1, Table S1: Systolic blood pressure (SBP), diastolic blood pressure (DBS), mean arterial pressure (MAP) responses at pre-and post-exercise rests, and during short isometric contraction and the sustained ischemic contraction.