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

Abstract

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 haemodynamic 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, 9mmol) and placebo (PLA) 2.5h 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 and leg blood flow (LBF) were reduced to a similar extent (∼9%-15%) in NO3. During HIGH, leg was reduced by ∼6%-10% and LBF by ∼5%-9% (did not reach significance) in NO3. Leg 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 and blood flow during exercise. However, nitrate supplementation does not enhance muscle kinetics during exercise, nor does it improve time to exhaustion when exercising with a small muscle mass. KEY POINTS: Dietary nitrate supplementation has been shown to reduce systemic O2 uptake during exercise and improve exercise performance. The effects of nitrate supplementation on local metabolism and blood flow regulation in contracting human skeletal muscle remain unclear. By using leg exercise engaging a small muscle mass, we show that O2 uptake and blood flow are similarly reduced in contracting skeletal muscle of humans during exercise. Despite slower kinetics in the transition from moderate to intense exercise, no effects of nitrate supplementation were observed for kinetics and time to exhaustion. Nitrate and nitrite concentrations are reduced across the exercising leg, suggesting that these ions are extracted from the arterial blood by contracting skeletal muscle.