Submitted on July 6, 2009
Revised on July 27, 2009
Accepted on August 3, 2009
Dietary nitrate supplementation reduces the O2 cost of low-intensity exercise and enhances tolerance to high-intensity exercise in humans
Stephen J. Bailey1, Paul Winyard1, Anni Vanhatalo2, Jamie R. Blackwell1, Fred J. DiMenna1, Daryl P. Wilkerson2, Joanna Tarr1, Nigel Benjamin3, and Andrew M. Jones1*
1 Exeter University
2 University of Exeter
3 University of Aberdeen Medical School
* To whom correspondence should be addressed. E-mail: a.m.jones{at}exeter.ac.uk.
Pharmacological sodium nitrate supplementation has been reported to reduce the O2 cost of sub-maximal exercise in humans. In this study, we hypothesised that dietary supplementation with inorganic nitrate in the form of beetroot juice (BR) would reduce the O2 cost of sub-maximal exercise and enhance the tolerance to high-intensity exercise. In a double-blind, placebo-controlled, crossover study, eight males (aged 19-38 yr) consumed 500 mL per day of either beetroot juice (BR, containing 11.2 ± 0.6 mM of nitrate) or blackcurrant cordial (as a placebo, PL, with negligible nitrate content) for six 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] was significantly greater following dietary nitrate supplementation compared to placebo (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 infra-red spectroscopy). The gain of the increase in pulmonary VO2 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 VO2 slow component was reduced (BR: 0.57 ± 0.20 vs. PL: 0.74 ± 0.24 L.min-1; 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 which regulate mitochondrial respiration and muscle contractile energetics in humans.
