Effect of dietary nitrate supplementation on conduit artery blood flow, muscle oxygenation, and metabolic rate during handgrip exercise

Abstract

Dietary nitrate supplementation has positive effects on mitochondrial and muscle contractile efficiency during large muscle mass exercise in humans and on skeletal muscle blood flow (Q ) in rats. However, concurrent measurement of these effects has not been performed in humans. Therefore, we assessed the influence of nitrate supplementation on Q and muscle oxygenation characteristics during moderate- (40 %peak) and severe-intensity(85% peak) handgrip exercise in a randomized, double-blind, crossover design. Nine healthy men (age: 25 2 yr) completed four constant-power exercise tests (2/intensity) randomly assigned to condition [nitrate-rich (nitrate) or nitrate-poor (placebo) beetroot supplementation] and intensity (40 or 85% peak). Resting mean arterial pressure was lower after nitrate compared with placebo (84 4 vs. 89 4 mmHg, P 0.01). All subjects were able to sustain 10 min of exercise at 40% peak in both conditions. Nitrate had no effect on exercise tolerance during 85% peak (nitrate: 358 29; placebo: 341 34 s; P 0.3). Brachial artery Q was not different after nitrate at rest or any time during exercise. Deoxygenated [hemoglobin myoglobin] was not different for 40% peak (P 0.05) but was elevated throughout 85% peak (P 0.05) after nitrate. The metabolic cost (V O2) was not different at the end of exercise; however, the V O2 primary amplitude at the onset of exercise was elevated after nitrate for the 85% peak work rate (96 20 vs. 72 12 ml/min, P 0.05) and had a faster response. These findings suggest that an acute dose of nitrate reduces resting blood pressure and speeds V O2 kinetics in young adults but does not augment Q or reduce steady-state V O2 during small muscle mass handgrip exercise. NEW & NOTEWORTHY We show that acute dietary nitrate supplementation via beetroot juice increases the amplitude and speed of local muscle V O2 on kinetics parameters during severe- but not moderate-intensity handgrip exercise. These changes were found in the absence of an increased blood flow response, suggesting that the increased V O2 was attained via improvements in fractional O2 extraction and/or spatial distribution of blood flow within the exercising muscle.