Dietary nitrate supplementation improves time to fatigue in maximal cycling performance

Abstract

Introduction: Previous research has shown improved exercise economy via reduced oxygen (O2) cost and increased performance during exercise with supplemental dietary nitrate intake. This supplementation has been shown to increase exercise time at VO2max power in cyclists as well as improved cycling time trial performance. Nitrate intake is believed to exert its effect on oxygen cost and performance by increasing nitric oxide bioavailability. This investigation examined the effect of varied nitrate concentration, contained in beetroot juice, on sub-maximal and maximal performance during cycling exercise. Methods: Eleven trained subjects [age 31.5+/-9 years, VO2max 61.8+/-10.3 ml/kg/min] participated in the study, which had a randomised, double-blind, placebo controlled, crossover design. Subjects completed three regimes of testing and received nitrate doses of approximately 9.2 mmol/day, 3.8 mmol/day or placebo in the form of beetroot stamina shots. Beetroot supplementation was consumed once per day for 3 days prior to the exercise testing. Each supplementation period was separated by a 10-day washout period. The performance test consisted of both a submaximal and maximal test carried out on an electronic cycle ergometer (WattBike). The submaximal test involved four intervals of five minutes duration at workloads of 70, 110, 150 and 200Wfor females and 90, 150, 200 and 250Wfor males. After a rest interval of 10 minsubjects exercised at their pre-determined VO2max power until volitional fatigue. Power output, oxygen consumption, carbon dioxide output, pulmonary ventilation, respiratory rate, heart rate and lactate were measured and recorded throughout the sessions. Results: A 16.6% increase in exercise time at maximal work levels following the 9.2mmol nitrate concentration supplementation compared with placebo supplementation (p = 0.0387) was found. The results from the 3.8mmol nitrate supplementation were not statistically different from the placebo dose. No significant difference in oxygen cost was found at submaximal or maximal work levels and no significant difference in cardiovascular or respiratory measures was detected between each of the three regimes. Discussion: In conclusion, whilst there was no significant difference in oxygen cost a significant improvement in performance was found following the higher dose of nitrate supplementation. The results from this study indicate a significant increase in maximal exercise performance with supplementation of 9.2 mmol/day nitrate whereas the 3.8 mmol/day dose was not found to bring about a significant improvement in performance. This indicates a threshold dosage may need to be administered for a performance benefit to be found.