Beetroot juice supplementation speeds O₂uptake kinetics and improves exercise tolerance during severe-intensity exercise initiated from an elevated metabolic rate

<jats:p>Recent research has suggested that dietary nitrate (NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) 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), NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>-rich beetroot juice (BR) supplementation would speed the phase II V̇o<jats:sub>2</jats:sub>kinetics (τ<jats:sub>p</jats:sub>) 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 ∼8 mmol of NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) and PL (140 ml/day, containing ∼0.003 mmol of NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) 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 V̇o<jats:sub>2</jats:sub>τ<jats:sub>p</jats:sub>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 V̇o<jats:sub>2</jats:sub>kinetics coincided with faster near-infrared spectroscopy-derived muscle [deoxyhemoglobin] kinetics (τ; 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 NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>-rich BR juice speeds V̇o<jats:sub>2</jats:sub>kinetics and enhances exercise tolerance during severe-intensity exercise when initiated from an elevated metabolic rate.</jats:p>