Beetroot Increases Muscle Performance and Oxygenation During Sustained Isometric Exercise, but Does Not Alter Muscle Oxidative Efficiency and Microvascular Reactivity at Rest

Objective: To examine the effects of beetroot juice (BRJ) on (i) in vivo skeletal muscle O-2 consumption (mVO(2)) and microvascular reactivity at rest and (ii) muscle performance, muscle oxygenation, and mVO(2) during sustained isometric handgrip exercise (IHG).Methods: Sixteen young males consumed, randomly, a nitrate-rich (8.1mmol BRJnitrate) or nitrate-depleted (BRJplacebo) BRJ. After 2.5hours, they performed an occlusion-reperfusion maneuver at rest, a 3-minute sustained IHG, and a sustained IHG to exhaustion with arterial occlusion. Changes in muscle oxygenated hemoglobin (O(2)Hb), deoxygenated hemoglobin (HHb), microvascular red blood cell content (tHb), and mVO(2) were measured using near-infrared spectroscopy. Force output was recorded.Results: During occlusion, the O(2)Hb decline did not differ between BRJnitrate and BRJplacebo (magnitude: -30.3 1.6vs. -31.1 +/- 1.5 M; slope: -0.107 +/- 0.007vs. -0.111 +/- 0.007M second(-1)). During reperfusion, all microvascular reactivity indices were not altered after BRJnitrate (e.g., O(2)Hbslope: 1.584 +/- 0.093vs. 1.556 +/- 0.072M second(-1)). During the second and third minute of IHG, O(2)Hb and tHb were higher in BRJnitrate versus BRJplacebo (p < 0.05), and force output was higher during the third minute (10.8 +/- 0.7vs. 9.5 +/- 1.2kg; p < 0.05); HHb did not differ between trials. In IHG with arterial occlusion, BRJnitrate prolonged the time to fatigue (94.1 +/- 5.8vs. 80.1 +/- 3.3seconds; p < 0.01), with no effects on O(2)Hb decline (O(2)Hbslope: -0.226 +/- 0.015vs. -0.230 +/- 0.026M s(-1)) and mVO(2) (14.1 +/- 1.0vs. 14.3 +/- 1.6mol l(-1) minute(-1)).Conclusion: Acute BRJ ingestion in moderately trained individuals (i) did not alter in vivo skeletal muscle microvascular reactivity (index of microvascular function at rest) and basal oxidative efficiency, (ii) increased muscle oxygenation during IHG (possibly via enhanced O-2 delivery), and (iii) provided ergogenic benefits during sustained IHG with no effects on muscle oxidative efficiency. The ergogenic effects of BRJ appeared independent of its tissue perfusion benefits.