An acute dietary nitrate supplement has no effect of endothelial function but has a modest effect on systemic hemodynamics in healthy younger men
Open Access
- Author:
- Maurer, David Gregory
- Graduate Program:
- Kinesiology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 10, 2013
- Committee Members:
- David Nathan Proctor, Thesis Advisor/Co-Advisor
James Anthony Pawelczyk, Thesis Advisor/Co-Advisor
Lacy Marie Alexander, Thesis Advisor/Co-Advisor - Keywords:
- nitrate
dietary supplement
endothelial function
blood pressure
mental stress
men - Abstract:
- Epidemiological studies suggest that diets rich in fruits and vegetables are protective against cardiovascular disease. Recent pre-clinical (rodent) and human studies suggest that inorganic nitrate supplementation in the form nitrate salts, concentrated beetroot juice, or other nitrate-rich vegetables, may be a key mediator of this cardioprotection via the serial (L-arginine and O2-independent) reduction of ingested nitrate to nitrite and subsequently nitric oxide (NO). While nitrate supplementation has become established as a natural means of increasing NO, its influence on in vivo endothelial function in humans remains equivocal. The effects of nitrate supplementation on cardiovascular/mental stress reactivity, another in vivo/laboratory based prognostic indicator of cardiovascular disease risk (i.e., future risk of hypertension), are also not known. PURPOSE: The primary objective of the present investigation was to determine if acute dietary nitrate supplementation augments the dilation of the brachial artery during graded handgrip exercise, a test previously shown to be nitric oxide-mediated in healthy younger men (Wray et al J Appl Physiol H1101-H1107, 2011). A secondary objective was to determine if this supplement attenuates the rise in systemic blood pressures or favorably modifies conduit artery shear patterns during an acute mental stress challenge. METHODS: In a randomized, double-blind, placebo-controlled crossover study, thirteen young (22 ± 2 yrs) healthy men consumed a concentrated beetroot juice supplement (140 mL Beet-It Sport, James White Drinks Ltd.) or placebo (nitrate-depleted Beet-It Sport) on two study visits at least five days apart. Resting blood pressure (brachial auscultation) and venous nitrate/nitrite concentrations (ENO-20) were measured before and 2.5 hours post-consumption. Three hours post consumption, arterial pulse wave velocity was measured (Colin tonometry) followed by resting and handgrip exercise-induced changes in brachial artery diameter, blood flow, and blood velocity (Doppler Ultrasound). A sub-set of subjects (n=7) performed a serial subtraction test (SST) during which measures of superficial femoral artery diameter, flow, and blood velocity were assessed. RESULTS: Dietary nitrate supplementation raised plasma nitrate (17-fold) and nitrite (1.6-fold) concentrations, and lowered resting pulse wave velocity (Colin VP-2000) versus placebo (all p<0.05) indicating absorption, conversion, and a biological effect of this supplement. However, nitrate supplementation had no effect on brachial artery diameter, flow, or shear rates at rest (all p>0.28) or during any of the six exercise work rates (all p>0.18). The increase in brachial diameter per unit change in shear rate across exercise intensities also did not differ between visits (p=0.15). Nitrate supplementation did not affect resting or SST-induced increases in systolic BP, heart rate, or femoral hemodynamics, but it did attenuate the peak rise in diastolic BP (p=0.041 vs. placebo visit). CONCLUSION: These findings suggest that acute dietary nitrate supplementation favorably modifies arterial pulse wave velocity, but does not augment endothelial nitric oxide-mediated vasodilation in healthy younger men. The SST sub-study further suggests that acute nitrate supplementation may influence blood pressure responses to acute mental stress in healthy young men. The mechanisms and clinical significance of functional improvements in arterial compliance in healthy adults, independent of any measureable improvements in vascular function in the arm or leg conduit arteries, remains to be established.