Dietary nitrate improves muscle but not cerebral oxygenation status during exercise in hypoxia

Abstract

Exercise tolerance is impaired in hypoxia, and it has recently been shown that dietary nitrate supplementation can reduce the oxygen (O(2)) cost of muscle contractions. Therefore, we investigated the effect of dietary nitrate supplementation on arterial, muscle, and cerebral oxygenation status, symptoms of acute mountain sickness (AMS), and exercise tolerance at simulated 5,000 m altitude. Fifteen young, healthy volunteers participated in three experimental sessions according to a crossover study design. From 6 days prior to each session, subjects received either beetroot (BR) juice delivering 0.07 mmol nitrate/kg body wt/day or a control drink (CON). One session was in normoxia with CON (NOR(CON)); the two other sessions were in hypoxia (11% O(2)), with either CON (HYP(CON)) or BR (HYP(BR)). Subjects first cycled for 20 min at 45% of peak O(2) consumption (VO(2)peak; EX(45%)) and thereafter, performed a maximal incremental exercise test (EX(max)). Whole-body VO(2), arterial O(2) saturation (%SpO(2)) via pulsoximetry, and tissue oxygenation index of both muscle (TOI(M)) and cerebral (TOI(C)) tissue by near-infrared spectroscopy were measured. Hypoxia per se substantially reduced VO(2)peak, %SpO(2), TOI(M), and TOI(C) (NOR(CON) vs. HYP(CON), P < 0.05). Compared with HYP(CON), VO(2) at rest and during EX(45%) was lower in HYP(BR) (P < 0.05), whereas %SpO(2) was higher (P < 0.05). TOI(M) was ~4-5% higher in HYP(BR) than in HYP(CON) both at rest and during EX(45%) and EX(max) (P < 0.05). TOI(C) as well as the incidence of AMS symptoms were similar between HYP(CON) and HYP(BR) at any time. Hypoxia reduced time to exhaustion in EX(max) by 36% (P < 0.05), but this ergolytic effect was partly negated by BR (+5%, P < 0.05). Short-term dietary nitrate supplementation improves arterial and muscle oxygenation status but not cerebral oxygenation status during exercise in severe hypoxia. This is associated with improved exercise tolerance against the background of a similar incidence of AMS.

Dietary nitrate supplementation enhances high-intensity running performance in moderate normobaric hypoxia, independent of aerobic fitness

“…These findings are similar to those of Masschelein and colleagues [37] who reported a significant increase in SaO2 during cycle ergometry exercise in severe hypoxia (FIO2 11 %) following 6 days nitrate supplementation (~ 5 mmol·d -1 ). Tissue oxygenation in the vastus lateralis was also elevated following nitrate supplementation in that study.…”

“…These findings are in line with most previous investigations [4,5,22,37], and similar to the results of a recent meta-analysis [6]. The physiological mechanisms underlying the decreased V O2 subsequent to nitrate supplementation have not been fully elucidated, but may be accounted for by: a) an improvement in the efficiency of mitochondrial respiration [35] and/or b) enhanced efficiency of muscle force production [7].…”

“…This is supported by the findings of a recent study, in which nitrate supplementation improved tolerance to severe intensity cycling in hypoxia (FIO2 13.1 %) but not normoxia [36]. However, in other studies, nitrate supplementation has been shown both to improve [8,22,37] and have no effect [38][39][40] on hypoxic exercise performance. These equivocal findings may be attributed to differences in the training status of participants, exercise protocol, supplementation protocol, and hypoxic stimuli applied, and warrant further exploration.…”

“…In normoxia, Murphy and colleagues [63] reported lower RPE during the first mile of a 5 km running TT, although others have reported no effect of nitrate supplementation on RPE during sub-maximal and maximal exercise in hypoxia following nitrate supplementation [37,38].…”

Effects of dietary nitrate supplementation on symptoms of acute mountain sickness and basic physiological responses in a group of male adolescents during ascent to Mount Everest Base Camp

“…normobaric hypoxic exposure, nitrate supplementation has also been shown to: increase both peripheral and muscle oxygen saturations during rest and exercise (11) , and improve exercise muscle energetics (12) and performance (11; 12; 13) . Conversely, during more sustained hypoxic exposure (a six week normobaric hypoxic training regime), exercise performance was not altered by nitrate supplementation (14) .…”