Is dietary nitrate supplementation necessary to ensure proper endothelial function at altitude?

Introduction: At altitude the body is exposed to systemic hypobaric hypoxia and adapts in order to optimize oxygen delivery to the tissue. Maintaining peripheral vascular function is essential in this process, tightly regulated through complex pathways involving local endothelium derived factors. Nitric oxide (NO) is the most important player in regulation of endothelial tone via endothelium dependent vasodilation. Dietary nitrate (NO3-) supplementation has been shown to increase NO bioavailability and improve vascular functions during normoxia, however the effects during hypobaric hypoxia are unknown. Purpose: To study the endothelial function in lowlanders in response to 1) administration of NO3-supplementation via beetroot juice (BJ) at altitude, and 2) short term and 4-week altitude exposure. Methods: Endothelial function wasmeasured as flow-mediated dilation (FMD) using ultrasound and Doppler in the brachial artery of 11 healthy subjects (4 female, 24.7±5.0 years, outdoor management students) sojourning from sea level to altitude. In a randomized, double-blinded crossover design FMD measure was performed 3 hours after drinking BJ (5.0 mmol NO3-) and placebo (0.003 mmol NO3-) supplementation at 3700m (after 3 days above 2000m), with a 24-hour wash out period. FMD was also measured at low altitude (1370m), after 5 days at altitude (4200m) and upon return to 1370m after 4 weeks of altitude exposure (2825 -5330m). Results: The supplementation intervention was completed by 10 subjects, and the 4-week altitude stay by 8 subjects. FMD was 6.53±2.32% at low altitude (1370m) (mean±SD). At 3700m with PL supplementation FMD was 3.84±1.31% (p=0.004) and with BJ supplementation 5.77±1.14% (p=1.00). FMD was lower at 4200m (FMD 3.04±2.22%), and 1370m post-altitude exposure (FMD 3.91±2.58%), compared to baseline FMD at 1370m (mean±SD, p<0.05). Conclusion: Acute dietary nitrate supplementation may reverse the reduced endothelial function found in lowlanders at 3700m after 3 days of altitude exposure. FMD decreased 5 days into altitude exposure (at 4200m), and after a 4-week stay between 2825 -5330m compared to FMD baseline