Effect of Dietary Nitrate Supplementation on Nitric Oxide-Mediated Vascular Responses

Open Access
Author:
Bundschuh, Mark Austin
Area of Honors:
Kinesiology
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • David Nathan Proctor, Thesis Supervisor
  • Steriani Elavsky, Honors Advisor
Keywords:
  • Key Words: brachial artery
  • dietary nitrate
  • enterosalivary circulation
  • handgrip exercise
  • nitric oxide
Abstract:
Objective: To assess the effect of dietary nitrate supplementation on nitric oxide (NO)-mediated vascular responses in the brachial artery. Methods: Five healthy, young men (22 ± 2 yrs) consumed a standard dose (140 mL, equivalent to the nitrate content of 5 servings of spinach) of nitrate-rich beetroot juice (active) or a similar quantity of nitrate-free beetroot juice (placebo) on two separate days, approximately one week apart. Three hours following beetroot juice consumption, Doppler ultrasound was used measure brachial artery diameter and mean blood velocity in response to a discontinuous protocol of six stages of handgrip exercise (200 g, 400 g, 600 g, 800 g, 1000 g, 1200 g) separated by 1 minute rest periods. Participants were paced via a metronome to contract 30 times per minute. Results: Resting supine blood pressures taken ~ 3 hours following beetroot juice consumption were similar (p > 0.05) during the nitrate (mean = 120/65 mmHg) and placebo (mean = 118/64 mmHg) visits. Nitrate supplementation did not alter the overall dilator response of the brachial artery to graded handgrip exercise (rest to peak; 6.9% placebo vs. 5.2% nitrate, p > 0.50), but it did delay this response, possibly due to the elevated baseline artery diameter observed during the nitrate visit (4.15 vs. 3.85 mm, p = 0.12). Nitrate supplementation also modified the rise in arm blood flow across increasing handgrip work rates (treatment by work rate interaction; p = 0.19). Conclusions: Acute dietary nitrate supplementation causes vasodilation of forearm resistance vessels, but does not appear to impact the shear-induced, nitric-oxide mediated dilation of the brachial artery in healthy, young men. The exercise intensity-dependent influence of dietary nitrate supplementation on forearm hyperemia has not previously been reported, and adds further insight into the physiological conditions under which this dietary supplement may have its largest vascular effects (i.e., when tissue PO2 and pH are low).