Effects of dietary nitrate, caffeine, and their combination on 20 km cycling time-trial performance.

The aim of this study was to examine the acute supplementation effects of dietary nitrate, caffeine, and their combination on 20 km cycling time-trial performance. Using a randomized, counterbalanced, double-blind, Latin-square design, 14 competitive, female cyclists (age: 31 +/- 7 years; height: 1.69 +/- 0.07 m; body mass: 61.6 +/- 6.0 kg) completed four 20 km time-trials on a racing bicycle fitted to a turbo-trainer. 2.5 hours before each trial, subjects consumed a 70 ml dose of concentrated beetroot juice containing either 0.45 g of dietary nitrate, or with the nitrate content removed (placebo). 1 hour before each trial, subjects consumed a capsule containing either 5 mg[dot operator]kg-1 of caffeine or maltodextrin (placebo). There was a significant effect of supplementation on power output (p = 0.001), with post hoc tests revealing higher power outputs in caffeine (205 +/- 21 W) versus nitrate (194 +/- 22 W) and placebo (194 +/- 25 W) trials only. Caffeine-induced improvements in power output corresponded with significantly higher measures of heart rate (caffeine: 166 +/- 12 vs. placebo: 159 +/- 15 b[middle dot]min-1; p = 0.02), blood lactate (caffeine: 6.54 +/- 2.40 vs. placebo: 4.50 +/- 2.11 mmol[middle dot]L-1; p < 0.001), and respiratory exchange ratio (caffeine: 0.95 +/- 0.04 vs. placebo: 0.91 +/- 0.05; p = 0.03). There were no effects (p >= 0.05) of supplementation on cycling cadence, ratings of perceived exertion, V[Combining Dot Above]O2, or integrated electromyographic activity. The results of this study support the well-established beneficial effects of caffeine supplementation on endurance performance. In contrast, acute supplementation with dietary nitrate appears to have no effect on endurance performance and adds nothing to the benefits afforded by caffeine supplementation. Copyright (C) 2014 by the National Strength & Conditioning Association.