The effect of dietary nitrate supplementation on the spatial heterogeneity of quadriceps deoxygenation during heavy-intensity cycling

This study investigated the influence of dietary inorganic nitrate (NO3-) supplementation on pulmonary O2 uptake ( o2) and muscle deoxyhemoglobin/myoglobin (i.e. deoxy[Hb+Mb]) kinetics during submaximal cycling exercise. In a randomized, placebo-controlled, cross-over study, eight healthy and physically active male subjects completed multiple step cycle tests at a work rate equivalent to 50% of the difference between the gas exchange threshold and peak o2 over separate 4-day supplementation periods with NO3--rich (BR; providing 8.4 mmol NO3-∙day-1) and NO3--depleted (placebo; PLA) beetroot juice. Pulmonary o2 was measured breath-by-breath and time-resolved near-infrared spectroscopy was utilized to quantify absolute deoxy[Hb+Mb] and total[Hb+Mb] within the rectus femoris, vastus lateralis, and vastus medialis. There were no significant differences (P > 0.05) in the primary deoxy[Hb+Mb] mean response time or amplitude between the PLA and BR trials at each muscle site. BR significantly increased the mean (three-site) end-exercise deoxy[Hb+Mb] (PLA: 91 ± 9 vs. BR: 95 ± 12 µM, P < 0.05), with a tendency to increase the mean (three-site) area under the curve for total(Hb+Mb) responses (PLA: 3650 ± 1188 vs. BR: 4467 ± 1315 µM·s-1, P = 0.08). The o2 slow component reduction after BR supplementation (PLA: 0.27 ± 0.07 vs. BR: 0.23 ± 0.08 L·min-1, P = 0.07) correlated inversely with the mean increases in deoxy[Hb+Mb] and total[Hb+Mb] across the three muscle regions (r2 = 0.62 and 0.66, P < 0.05). Dietary NO3- supplementation increased O2 diffusive conductance across locomotor muscles in association with improved o2 dynamics during heavy-intensity cycling transitions.