Dietary nitrate supplementation reduces the O₂cost of low-intensity exercise and enhances tolerance to high-intensity exercise in humans

TLDR

Sodium nitrate supplementation lowers the oxygen cost of submaximal exercise, underscoring its influence on mitochondrial respiration and muscle energetics. The study aimed to determine whether beetroot‑juice–derived inorganic nitrate would reduce submaximal oxygen cost and improve tolerance to high‑intensity exercise. Eight men completed a double‑blind, placebo‑controlled crossover trial, ingesting 500 ml/day of beetroot juice (≈11 mM nitrate) or a nitrate‑free blackcurrant cordial for six days while performing stepwise moderate‑ and severe‑intensity exercise tests on days 4–6. Nitrate supplementation increased plasma nitrite, lowered systolic blood pressure, reduced muscle O₂ extraction and pulmonary O₂ uptake during moderate exercise, diminished the slow‑component O₂ demand during severe exercise, and extended time to exhaustion.

Abstract

Pharmacological sodium nitrate supplementation has been reported to reduce the O2 cost of submaximal exercise in humans. In this study, we hypothesized that dietary supplementation with inorganic nitrate in the form of beetroot juice (BR) would reduce the O2 cost of submaximal exercise and enhance the tolerance to high-intensity exercise. In a double-blind, placebo (PL)-controlled, crossover study, eight men (aged 19-38 yr) consumed 500 ml/day of either BR (containing 11.2 +/- 0.6 mM of nitrate) or blackcurrant cordial (as a PL, with negligible nitrate content) for 6 consecutive days and completed a series of "step" moderate-intensity and severe-intensity exercise tests on the last 3 days. On days 4-6, plasma nitrite concentration was significantly greater following dietary nitrate supplementation compared with PL (BR: 273 +/- 44 vs. PL: 140 +/- 50 nM; P < 0.05), and systolic blood pressure was significantly reduced (BR: 124 +/- 2 vs. PL: 132 +/- 5 mmHg; P < 0.01). During moderate exercise, nitrate supplementation reduced muscle fractional O2 extraction (as estimated using near-infrared spectroscopy). The gain of the increase in pulmonary O2 uptake following the onset of moderate exercise was reduced by 19% in the BR condition (BR: 8.6 +/- 0.7 vs. PL: 10.8 +/- 1.6 ml.min(-1).W(-1); P < 0.05). During severe exercise, the O2 uptake slow component was reduced (BR: 0.57 +/- 0.20 vs. PL: 0.74 +/- 0.24 l/min; P < 0.05), and the time-to-exhaustion was extended (BR: 675 +/- 203 vs. PL: 583 +/- 145 s; P < 0.05). The reduced O2 cost of exercise following increased dietary nitrate intake has important implications for our understanding of the factors that regulate mitochondrial respiration and muscle contractile energetics in humans.