Beetroot juice supplementation speeds O2 uptake kinetics and improves exercise tolerance during severe-intensity exercise initiated from an elevated metabolic rate

Recent research has suggested that dietary nitrate (NO3 ) supplementation might alter the physiological responses to exercise via specific effects on type II muscle. Severe-intensity exercise initiated from an elevated metabolic rate would be expected to enhance the proportional activation of higher-order (type II) muscle fibers. The purpose of this study was, therefore, to test the hypothesis that, compared with placebo (PL), NO3--rich beetroot juice (BR) supplementation would speed the phase II (V) over dotO(2) kinetics (tau(p)) and enhance exercise tolerance during severe-intensity exercise initiated from a baseline of moderate-intensity exercise. Nine healthy, physically active subjects were assigned in a randomized, double-blind, crossover design to receive BR (140 ml/day, containing similar to 8 mmol of NO3-) and PL (140 ml/day, containing similar to 0.003 mmol of NO3-) for 6 days. On days 4, 5, and 6 of the supplementation periods, subjects completed a double-step exercise protocol that included transitions from unloaded to moderate-intensity exercise (U -> M) followed immediately by moderate to severe-intensity exercise (M -> S). Compared with PL, BR elevated resting plasma nitrite concentration (PL: 65 +/- 32 vs. BR: 348 +/- 170 nM, P < 0.01) and reduced the (V) over dotO(2) tau(p) in M -> S (PL: 46 +/- 13 vs. BR: 36 +/- 10 s, P < 0.05) but not U -> M (PL: 25 +/- 4 vs. BR: 27 +/- 6 s, P > 0.05). During M -> S exercise, the faster (V) over dotO(2) kinetics coincided with faster near-infrared spectroscopy-derived muscle [deoxyhemoglobin] kinetics (tau; PL: 20 +/- 9 vs. BR: 10 +/- 3 s, P < 0.05) and a 22% greater time-to-task failure (PL: 521 +/- 158 vs. BR: 635 +/- 258 s, P < 0.05). Dietary supplementation with NO3--rich BR juice speeds (V) over dotO(2) kinetics and enhances exercise tolerance during severe-intensity exercise when initiated from an elevated metabolic rate.