Nitrate-rich beetroot juice ingestion reduces skeletal muscle O₂ uptake and blood flow during exercise in sedentary men

Dietary nitrate supplementation has been shown to reduce pulmonary O₂ uptake during submaximal exercise and enhance exercise performance. However, the effects of nitrate supplementation on local metabolic and hemodynamic regulation in contracting human skeletal muscle remain unclear. To address this, eight healthy young male sedentary subjects were assigned in a randomized, double-blind, crossover design to receive nitrate-rich beetroot juice (NO3, 9 mmol) and placebo (PLA) 2.5 h prior to the completion of a double-step knee-extensor exercise protocol that included a transition from unloaded to moderate-intensity exercise (MOD) followed immediately by a transition to intense exercise (HIGH). Compared with PLA, NO3 increased plasma levels of nitrate and nitrite. During MOD, leg VO₂ and leg blood flow (LBF) were reduced to a similar extent (∼9-15 %) in NO3. During HIGH, leg VO₂ was reduced by ∼6-10 % and LBF by ∼5-9 % (did not reach significance) in NO3. Leg VO₂ kinetics was markedly faster in the transition from passive to MOD compared with the transition from MOD to HIGH both in NO3 and PLA with no difference between PLA and NO3. In NO3, a reduction in nitrate and nitrite concentration was detected between arterial and venous samples. No difference in the time to exhaustion was observed between conditions. In conclusion, elevation of plasma nitrate and nitrate reduces leg skeletal muscle VO₂ and blood flow during exercise. However, nitrate supplementation does not enhance muscle VO₂ kinetics during exercise, nor does it improve time to exhaustion when exercising with a small muscle mass.