Dietary nitrate supplementation: effects on plasma nitrite and pulmonary O₂ uptake dynamics during exercise in hypoxia and normoxia

Abstract

Ϫ ]), oxygen uptake (V O2) kinetics, and exercise tolerance in normoxia (N) and hypoxia (H). In a doubleblind, crossover study, 12 healthy subjects completed cycle exercise tests, twice in N (20.9% O2) and twice in H (13.1% O2). Subjects ingested either 140 ml/day of NO 3 Ϫ -rich beetroot juice (8.4 mmol NO3; BR) or NO 3 Ϫ -depleted beetroot juice (PL) for 3 days prior to moderate-intensity and severe-intensity exercise tests in H and N. Preexercise plasma [NO 2 Ϫ ] was significantly elevated in H-BR and N-BR compared with H-PL (P Ͻ 0.01) and N-PL (P Ͻ 0.01). The rate of decline in plasma [NO 2 Ϫ ] was greater during severe-intensity exercise in H-BR [Ϫ30 Ϯ 22 nM/min, 95% confidence interval (CI); Ϫ44, Ϫ16] compared with H-PL (Ϫ7 Ϯ 10 nM/min, 95% CI; Ϫ13, Ϫ1; P Ͻ 0.01) and in N-BR (Ϫ26 Ϯ 19 nM/min, 95% CI; Ϫ38, Ϫ14) compared with N-PL (Ϫ1 Ϯ 6 nM/min, 95% CI; Ϫ5, 2; P Ͻ 0.01). During moderate-intensity exercise, steady-state pulmonary V O2 was lower in H-BR (1.91 Ϯ 0.28 l/min, 95% CI; 1.77, 2.13) compared with H-PL (2.05 Ϯ 0.25 l/min, 95% CI; 1.93, 2.26; P ϭ 0.02), and V O2 kinetics was faster in H-BR (: 24 Ϯ 13 s, 95% CI; 15, 32) compared with H-PL (31 Ϯ 11 s, 95% CI; 23, 38; P ϭ 0.04). NO 3 Ϫ supplementation had no significant effect on V O2 kinetics during severe-intensity exercise in hypoxia, or during moderate-intensity or severe-intensity exercise in normoxia. Tolerance to severe-intensity exercise was improved by NO 3 Ϫ in hypoxia (H-PL: 197 Ϯ 28; 95% CI; 173, 220 vs. H-BR: 214 Ϯ 43 s, 95% CI; 177, 249; P ϭ 0.04) but not normoxia. The metabolism of NO 2 Ϫ during exercise is altered by NO 3 Ϫ supplementation, exercise, and to a lesser extent, hypoxia. In hypoxia, NO 3 Ϫ supplementation enhances V O 2 kinetics during moderate-intensity exercise and improves severe-intensity exercise tolerance. These findings may have important implications for individuals exercising at altitude. hypoxia; beetroot juice; nitric oxide; efficiency; performance NITRIC OXIDE (NO) IS A UBIQUITOUS, water-soluble, free radical gas that plays a crucial role in many biological processes. Effective NO production is important in normal physiological functioning, from the regulation of blood flow, muscle contractility, and mitochondrial respiration to host defense, neurotransmission, and glucose and calcium homeostasis (11,17,60). NO production via the oxidation of L-arginine, in a process catalyzed by nitric oxide synthase (NOS), may be blunted in conditions of reduced O 2 availability (52). It is now widely accepted that NO can also be generated via an alternative pathway, whereby inorganic nitrate (NO 3 Ϫ ) is reduced to nitrite (NO 2 Ϫ ) and further to NO. This NOS-and O 2 -independent NO 3 Ϫ -NO 2 Ϫ -NO pathway represents a complementary system for NO synthesis spanning a broad range of redox states (49). In addition to being produced endogenously, the body's NO 3 Ϫ stores can be increased via the diet, with green leafy vegetables and beetroot being particularly rich in NO 3 Ϫ . Upon ingestion, inorganic NO 3 Ϫ is absorbed from the ...

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

“…A number of previous investigations have confirmed the beneficial effect of nitrate supplementation on TTE [8,24,37] and TT performance [22] in hypoxia. Conversely, studies in well-trained individuals exercising in hypoxia have typically found no effect of nitrate supplementation [38][39][40].…”

“…Increasing NO bioavailability via nitrate supplementation has been reported to improve exercise time to exhaustion (TTE) or time-trial (TT) performance in the majority [5,[16][17][18], but not all [23,24] previous investigations in untrained and moderately-trained individuals. Conversely, well-trained participants (V O2max > 60 ml·kg -1 ·min -1 ) are typically less responsive to dietary nitrate supplementation [17,[19][20][21][25][26][27], even when very high nitrate doses (~ 19.5 mmol) are administered [28].…”

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

“…Accordingly, we are unable to confirm that the dietary nitrate supplementation regime was successful in enhancing NO availability. Previous studies using a similar protocol (2 x 70 ml/day of concentrated beetroot juice produced by the same manufacturer), did find ingestion to increase plasma nitrate and nitrite levels (22) , however in these, (22) ; however in these, the final dose of beetroot juice was consumed 2.5 hours before blood was taken and physiological variables were measured, potentially ensuring the acute effects of the supplementation were present. In our study, the second beetroot juice of the day was consumed in the evening, with measurements undertaken the following morning approximately twelve hours later.…”

“…Kelly et al (22) demonstrated that in the absence of beetroot supplementation, plasma nitrite levels were largely unaltered during hypoxic exposure of less than two hours, furthermore, the pulmonary bioavailability of NO has been shown to be reduced 3 hours and 20 hours after arrival at 4559 meters due to an elevation of oxidative stress (5) . Conversely, two studies using very similar ascent profiles to the our current study, found elevated plasma NO metabolite concentrations (including nitrite) after 3-4 days at altitude -values that remained higher for the duration of the study (15 days) (4; 24) .…”

Effect of Dietary Nitrate Supplementation on Swimming Performance in Trained Swimmers

“…Indeed, recent evidence indicates that nitrate supplementation is more effective at enhancing exercise performance in situations where oxygen availability is decreased (Kelly et al, 2014), possibly due to greater nitrite reduction into NO (Castello et al, 2006). Engan et al (2012) also recently demonstrated a beneficial effect of nitrate supplementation on apnea performance in trained free divers, which might suggest a possible benefit of nitrate supplementation in improving underwater performance in swimming.…”