Increased VO2max Does Not Influence Nitric Oxide Availability or the Response to a Nitrate Dose

Abstract

Dietary nitrate (NIT) supplementation increases nitric oxide (NO) metabolites and may improve exercise performance. However, NIT supplementation seems to be less efficacious in highly trained athletes. Despite this, no study has directly investigated the effects of a change in aerobic capacity (VO2max) on baseline plasma NO metabolites or the response to a NIT dose within the same cohort. PURPOSE: To determine whether an increase in VO2max influences plasma nitrate (NO3-), nitrite (NO2-) or the plasma NO2- response to an acute NIT dose. METHODS: Twenty seven untrained males were assessed for baseline measures of plasma NO3- and NO2- and for determination of VO2max. In a separate trial, the change in plasma NO2- following ingestion of two NIT gels (∼8.1 mmol NIT) was measured. Participants were subsequently assigned to either 3-weeks sprint interval training (SIT; n=19) three times per week or a control condition (CON; n=8). SIT consisted of 4-6 repeated 15 s all out sprints on a cycle ergometer, interspersed with 4 min active recovery. The CON group were asked to maintain their normal lifestyle throughout the 3-weeks training. Participants then repeated the same experimental trials as at baseline. Differences between groups, time point and their interaction were established by two-way repeated measures ANOVA. RESULTS: VO2max, plasma NO3- and NO2- were not different between groups at baseline (SIT: VO2max 41 ± 7 ml·kg·min-1, NO2- 182 ± 65 nM, NO3- 30 ± 16 μM; CON: VO2max 45 ± 8 ml·kg·min-1, NO2-, 170 ± 32 nM, NO3-, 41 ± 22 μM; all P>0.15) and plasma NO2- increased to the same extent following NIT (SIT: ΔNO2- 224 ± 143 nM; CON: ΔNO2- = 182 ± 146 nM; both P<0.01). Following SIT, VO2max increased by 5.4% (P=0.03) but did not change in CON (P=0.74). There were no changes in plasma NO3- and NO2- within or between groups (SIT: NO2-, 192 ± 61 nM, NO3-, 37 ± 22 μM; CON: NO2-, 200 ± 75 nM, NO3-, 45 ± 29 μM; all P>0.15). Ingestion of NIT increased plasma NO2- post-training, however, the change was not different from pre-training (SIT: ΔNO2-, 225 ± 196 nM; CON: ΔNO2-, 167 ± 102 nM; both P<0.01). CONCLUSIONS: A moderate improvement in aerobic fitness following 3-weeks SIT did not alter baseline levels of plasma NO3- or NO2-, or the response to a NIT dose. Therefore, training status per se, may not be the primary factor underlying the lack of response to NIT in highly trained cohorts.