Cumpstey, Andrew F., Hennis, Philip J., Gilbert-kawai, Edward T., Fernandez, Bernadette O., Poudevigne, Matthieu, Cobb, Alexandra, Meale, Paula, Mitchell, Kay, Moyses, Helen, Pöhnl, Helmut, Mythen, Monty G., Grocott, Michael P.w., Feelisch, Martin and Martin, Daniel S. (2017) Effects of dietary nitrate on respiratory physiology at high altitude: results from the Xtreme Alps study. Nitric Oxide, 71, 57-68. (doi:10.1016/j.niox.2017.10.005).
Abstract
Nitric oxide (NO) production plays a central role in conferring tolerance to hypoxia. Tibetan highlanders, successful high-altitude dwellers for millennia, have higher circulating nitrate and exhaled NO (ENO) levels than native lowlanders. Since nitrate itself can reduce the oxygen cost of exercise in normoxia it may confer additional benefits at high altitude. Xtreme Alps was a double-blinded randomised placebo-controlled trial to investigate how dietary nitrate supplementation affects physiological responses to hypoxia in 28 healthy adult volunteers resident at 4559m for 1 week; 14 receiving a beetroot-based high-nitrate supplement and 14 receiving a low-nitrate ‘placebo’ of matching appearance/taste. ENO, vital signs and acute mountain sickness (AMS) severity were recorded at sea level (SL) and daily at altitude. Moreover, standard spirometric values were recorded, and saliva and exhaled breath condensate (EBC) collected. There was no significant difference in resting cardiorespiratory variables, peripheral oxygen saturation or AMS score with nitrate supplementation at SL or altitude. Median ENO levels increased from 1.5/3.0mPa at SL, to 3.5/7.4mPa after 5 days at altitude (D5) in the low and high-nitrate groups, respectively (p=0.02). EBC nitrite also rose significantly with dietary nitrate (p=0.004), 1.7 to 5.1μM at SL and 1.6 to 6.3 μM at D5, and this rise correlated with increased levels of ENO (R=0.5, p<0.0001). However no significant changes occurred to levels of EBC nitrate or nitrosation products (RXNO). Median salivary nitrite/nitrate concentrations increased from 56.5/786µM to 333/5194µM at SL, to 85.6/641µM and 341/4553µM on D5. Salivary RXNO rose markedly with treatment at SL from 0.55µM to 5.70µM. At D5 placebo salivary RXNO had increased to 1.90µM whilst treatment RXNO decreased to 3.26µM. There was no association with changes in any observation variables or AMS score. In conclusion, dietary nitrate supplementation is well tolerated at altitude and significantly increases pulmonary NO availability and both salivary and EBC NO metabolite concentrations. Surprisingly, this is not associated with changes in hemodynamics, oxygen saturation or AMS development.
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