Effects of dietary nitrate on respiratory physiology at high altitude - Results from the Xtreme Alps study

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 (E_NO) 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 4559 m for 1 week; 14 receiving a beetroot-based high-nitrate supplement and 14 receiving a low-nitrate 'placebo' of matching appearance/taste. E_NO, 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 E_NO levels increased from 1.5/3.0 mPa at SL, to 3.5/7.4 mPa 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), 1.7-5.1 μM at SL and 1.6-6.3 μM at D5, and this rise appeared to be associated with increased levels of E_NO. However, no significant changes occurred to levels of EBC nitrate or nitrosation products (RXNO). Median salivary nitrite/nitrate concentrations increased from 56.5/786 μM to 333/5,194 μM with nitrate supplementation at SL, and changed to 85.6/641 μM and 341/4,553 μM on D5. Salivary RXNO rose markedly with treatment at SL from 0.55 μM to 5.70 μM. At D5 placebo salivary RXNO had increased to 1.90 μM whilst treatment RXNO decreased to 3.26 μM. 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.

Keywords: Altitude; Hypoxaemia; Hypoxia; Nitrate; Nitric oxide; Nitrite.

Fig. 1

Changes in resting physiological variables during the Xtreme Alps expedition. All values are presented as means ± SEM, displayed separately for Trek 1 and Trek 2 (solid and dotted lines, respectively) as Trek 1 ascended one day faster from the Gnifetti Hut (3611 m) to the Margherita Hut (4559 m) due to inclement weather conditions. Each arrow represents the administration of 3 x doses of either treatment or placebo supplement (1 dose before every meal each day), starting 3 days prior to each testing period. SL = sea level testing, D1 – D5 = altitude testing days 1–5. Panel 1A - ascent profile, 1B - resting oxygen saturation per expedition day, 1C – resting respiration rate, 1D – resting heart rate, and 1E – resting blood pressure (systolic over diastolic).

Fig. 2

Representative results of the daily diary morning routine recording resting vital signs and symptoms of Acute Mountain Sickness (AMS). No significant difference was seen between the low nitrate group (placebo) and the high nitrate group (intervention) in peripheral oxygen saturation (SpO₂), respiratory rate, heart rate, systolic blood pressure, diastolic blood pressure or Lake Louise Score (for AMS) on testing at sea level in London (75 m), on participants' first day at 3611 m or on the first testing day at 4559 m (D1 in Fig. 1).

Fig. 3

Graphs showing mean exhaled NO and breath condensate values for control (low nitrate placebo) and supplement (high nitrate intervention) groups at sea level (SL) and days 1–5 (D1 - D5) at 4559 m altitude. Where available, corresponding values for untreated laboratory staff members is also presented. Significant rises were seen in exhaled NO and breath condensate nitrite in response to dietary nitrate treatment, but not breath condensate nitrate or RxNO. A: Partial pressure of exhaled NO (Pe_NO) over time, B: Breath condensate nitrate concentration over time, C: Breath condensate nitrite concentration over time, D: Breath condensate nitrosation products (RxNO) concentration over time. There were moderate correlations between Fractional exhaled NO (Fe_NO) and Exhaled Breath Condensate nitrite concentrations at sea level and at altitude in both the placebo (graph E) and the treatment (graph F) groups. However, this analysis is limited by low n values at sea level, particularly in the treatment group. r_S - correlation coefficient of Spearman Rank test (no adjustment for repeated measures).

Fig. 4

Graphs to values for placebo (low nitrate) and intervention (high nitrate) groups at sea level (day 0) and days 1, 3 & 5 at 4559 m altitude. All values showed significant increases in response to treatment with dietary nitrate. A: Salivary nitrate over time, B: Salivary nitrite over time, C: Salivary RxNO over time.