Abstract
Environmental hypoxia poses substantial physiological challenges to oxidative metabolism, skeletal muscle perfusion, and exercise capacity in endurance athletes. Dietary nitrate (NO3-) supplementation has emerged as an ergogenic aid capable of augmenting nitric oxide (NO) bioavailability via the non-enzymatic nitrate-nitrite-NO pathway, a mechanism potentiated in acidic and hypoxic environments. This study evaluated the effects of acute dietary nitrate supplementation on skeletal muscle microvascular oxygenation and 16.1-km cycling time-trial (TT) performance in elite male cyclists under normobaric hypoxia (fraction of inspired oxygen [FiO2] = 0.150; ~2,750 m equivalent). In a randomized, double-blind, crossover design, 14 elite male cyclists (VO2max: 71.4 ± 4.2 mL·kg-1·min-1) consumed either nitrate-rich beetroot juice (BR; ~12.8 mmol NO3-) or nitrate-depleted placebo (PL; ~0.08 mmol NO3-) 2.5 hours prior to testing. Muscle microvascular oxygenation of the vastus lateralis was continuously monitored using near-infrared spectroscopy (NIRS) to quantify tissue saturation index (TSI) and deoxygenated hemoglobin ([HHb]). Nitrate ingestion significantly reduced 16.1-km TT completion time compared to placebo (24.82 ± 0.94 min vs. 25.31 ± 1.02 min; p = 0.014; d = 0.50), accompanied by a higher mean power output (342 ± 28 W vs. 331 ± 31 W; p = 0.011). Furthermore, BR attenuated the decline in microvascular TSI (58.4 ± 3.9% vs. 54.7 ± 4.6%; p = 0.008) and reduced [HHb] accumulation throughout the sustained TT protocol. These findings demonstrate that acute nitrate supplementation enhances microvascular oxygen delivery and distribution within working skeletal muscle, mitigating hypoxia-induced performance decrements in elite endurance athletes.
