Influence of muscle oxygenation and nitrate-rich beetroot juice supplementation on O2 uptake kinetics and exercise tolerance

We tested the hypothesis that acute supplementation with nitrate (NO3-)-rich beetroot juice (BR) would improve quadriceps muscle oxygenation, pulmonary oxygen uptake (V) (over dot)O-2) kinetics and exercise tolerance (T-lim) in normoxia and that these improvements would be augmented in hypoxia and attenuated in hyperoxia. In a randomised, double-blind, cross-over study, ten healthy males completed two-step cycle tests to Tlim following acute consumption of 210 mL BR (18.6 mmol NO3-) or NO3--depleted beetroot juice placebo (PL; 0.12 mmol NO3-). These tests were completed in normobaric normoxia [fraction of inspired oxygen (FIO2): 21%], hypoxia (FIO2: 15%) and hyperoxia (FIO2: 40%). Pulmonary (V over dot)O-2 and quadriceps tissue oxygenation index (TOI), derived from multi-channel near-infrared spectroscopy, were measured during all trials. Plasma [nitrite] was higher in all BR compared to all PL trials (P < 0.05). Quadriceps TOI was higher in normoxia compared to hypoxia (P < 0.05) and higher in hyperoxia compared to hypoxia and normoxia (P < 0.05). Tlim was improved after BR compared to PL ingestion in the hypoxic trials (250 +/- 44 vs. 231 +/- 41 s; P = 0.006; d = 1.13), with the magnitude of improvement being negatively correlated with quadriceps TOI at Tlim (r = -0.78; P < 0.05). Tlim was not improved following BR ingestion in normoxia (BR: 364 +/- 98 vs. PL: 344 +/- 78 s; P = 0.087, d = 0.61) or hyperoxia (BR: 492 +/- 212 vs. PL: 472 +/- 196 s; P = 0.273, d = 0.37). BR ingestion increased peak (V) (over dot)O-2 in hypoxia (P < 0.05), but not normoxia or hyperoxia (P > 0.05). These findings indicate that BR supplementation is more likely to improve Tlim and peak (V) (over dot)O-2 in situations when skeletal muscle is more hypoxic.