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

ABSTRACT

Objective: To examine the effects of beetroot juice (BRJ) on (i) in vivo skeletal muscle O₂ consumption (mVO₂) and microvascular reactivity at rest and (ii) muscle performance, muscle oxygenation, and mVO₂ during sustained isometric handgrip exercise (IHG).

Methods: Sixteen young males consumed, randomly, a nitrate-rich (8.1 mmol BRJnitrate) or nitrate-depleted (BRJplacebo) BRJ. After 2.5 hours, 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 (O₂Hb), deoxygenated hemoglobin (HHb), microvascular red blood cell content (tHb), and mVO₂ were measured using near-infrared spectroscopy. Force output was recorded.

Results: During occlusion, the O₂Hb decline did not differ between BRJnitrate and BRJplacebo (magnitude: −30.3 ± 1.6 vs. −31.1 ± 1.5 ΔμΜ; slope: −0.107 ± 0.007 vs. −0.111 ± 0.007 μΜ second⁻¹). During reperfusion, all microvascular reactivity indices were not altered after BRJnitrate (e.g., O₂Hbslope: 1.584 ± 0.093 vs. 1.556 ± 0.072 μΜ second⁻¹). During the second and third minute of IHG, O₂Hb and tHb were higher in BRJnitrate versus BRJplacebo (p < 0.05), and force output was higher during the third minute (10.8 ± 0.7 vs. 9.5 ± 1.2 kg; p < 0.05); HHb did not differ between trials. In IHG with arterial occlusion, BRJnitrate prolonged the time to fatigue (94.1 ± 5.8 vs. 80.1 ± 3.3 seconds; p < 0.01), with no effects on O₂Hb decline (O₂Hbslope: −0.226 ± 0.015 vs. −0.230 ± 0.026 μΜ s⁻¹) and mVO₂ (14.1 ± 1.0 vs. 14.3 ± 1.6 μmol 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 O₂ 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.

KEYWORDS:

• Beetroot
• dietary nitrate
• isometric
• exercise
• muscle oxygenation
• near-infrared spectroscopy
• oxygen consumption
• microcirculation
• microvascular reactivity
• efficiency

Conflict of interest

The authors declare no conflict of interest associated with this article.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.