Effects of dietary nitrate on the O2 cost of submaximal exercise: Accounting for "noise" in pulmonary gas exchange measurements

Abstract

Dietary nitrate (NO₃^-) supplementation can reduce the oxygen cost of submaximal exercise, but this has not been reported consistently. We hypothesised that the number of step transitions to moderate-intensity exercise, and corresponding effects on the signal-to-noise ratio for pulmonary $\dot{V}$ O₂, may be important in this regard. Twelve recreationally active participants were assigned in a randomised, double-blind, crossover design to supplement for 4 days in three conditions: 1) control (CON; water); 2); PL (NO₃^--depleted beetroot juice); and 3) BR (NO₃^--rich beetroot juice). On days 3 and 4, participants completed two 6-min step transitions to moderate-intensity cycle exercise. Breath-by-breath $\dot{V}$ O₂ data were collected and $\dot{V}$ O₂ kinetic responses were determined for a single transition and when the responses to 2, 3 and 4 transitions were ensemble-averaged. Steady-state $\dot{V}$ O₂ was not different between PL and BR when the $\dot{V}$ O₂ response to one-, two- or three-step transition was compared but was significantly lower in BR compared to PL when four-step transitions was considered (PL: 1.33 ± 0.34 vs. BR: 1.31 ± 0.34 L·min^-1, P < 0.05). There were no differences in pulmonary $\dot{V}$ O₂ responses between CON and PL (P > 0.05). Multiple step transitions may be required to detect the influence of NO₃^- supplementation on steady-state $\dot{V}$ O₂.

Keywords: Nitric oxide; exercise economy; nitrite; placebo; polyphenols.