Acute dietary nitrate supplementation improves cycling time trial performance


Journal Title (Medline/Pubmed accepted abbreviation): Med. Sci. Sports. Exerc.
Year: 2011
Volume: 43
Number: 6
doi (if applicable): 10.1249/MSS.0b013e31821597b4

Summary of background and research design

Background: Nitrate supplementation is thought to increase sports performance due to its conversion to nitric oxide in the body, which may reduce muscle fatigue. Beetroot juice is a natural source of nitrate. Many exercise studies have measured exercise performance with a time-to-exhaustion assessment. The authors argue that a time trial is a more practical test, in that it can be more easily translated to athletic races. The authors wished to test the effects of beetroot juice on performance in a time trial environment.

Hypothesis: Drinking 500 mL of beetroot juice before cycling will increase athletes’ speed in a 4.0 and 16.1 km time trial.

Subjects: Nine competitive male cyclists, age 21 ± 4 yrs old

Experimental design: randomized, double-blind, cross-over

Treatments: 1) Organic beetroot juice (500 mL) containing about 6.2 mmol of nitrate/serving
2) Placebo: nitrate-depleted beetroot juice, i.e. the same juice as above with the nitrate removed.

The participants completed 4 cycling trials: 2 × 4.0 km and 2 × 16.1 km time trials with the beetroot juice and the placebo (all possible combinations)


Protocol:Before the treatment sessions, the athletes were evaluated for VO2max and were familiarized with the time trial protocol. Times were noted for the time trials during familiarization. They then reported to the laboratory on 4 occasions, separated by at least 2-3 days to complete the 4 time trials. They arrived in the morning fed but not having eaten for at least 1 hr. Blood pressure was measured and a blood sample was collected and later analyzed for nitrite. Subjects ingested 500 mL of either the beetroot juice or the placebo. After 2 hrs, blood pressure and blood sample acquisition was repeated. About 2 hrs, 45 min after consuming the beverage, the 16.1 or 4 km time trial was completed on a stationary bicycle. Power output and respiratory gases were monitored.
 
Summary of research findings
  • Beetroot juice increased plasma nitrate concentration from 293 ± 133 nM to 575 ± 199 nM (p < 0.05).  The placebo did not show an effect.
  • Beetroot juice significantly lowered systolic blood pressure (placebo = 131 ± 8 mmHg, beetroot juice = 125 ± 5 mmHg, p < 0.05).
  • Beetroot juice reduced the time it took for all subjects to complete the 4 km trial.  Average times were 6.43 ± 0.42 min at the familiarization trials and 6.27 ± 0.35 min with supplementation (p < 0.05).  The placebo did not have an effect (6.45 ± 0.42 min, p > 0.05).
  • Times were reduced for all subjects in the 16.1 km time trial with beetroot juice supplementation from 28.6 ± 2.4 min in the familiarization trial to 26.9 ± 1.8 min (p < 0.01).  The placebo had no effect (27.7 ± 2.1 min, p > 0.05).  This effect (mean time decrease of 2.8% in the 4-km trial and 2.7% in the 16-km trial) is much greater that the “smallest worthwhile change” (0.6%) proposed by another group of exercise scientists.
  • Beetroot juice increased power output 5% in the 4 km time trial and 6% in the 16.1 km time trial.
  • The power output:VO2 ratio was significantly elevated at the 1, 1.5, 2, 2.5, 3, 3.5, and 4 km distances for the beetroot juice vs. placebo in the 4-km time trial. This ratio was also elevated above placebo (albeit not significantly) at distance measuremen (taken every 2 km) during the 16-km time trial.
  • No difference was noted in oxygen/carbon dioxide breathing rates.

Interpretation of findings/Key practice applications

Two key advantages to this study are: 1) the crossover design to reduce inter-individual variability; and 2) the use of a nitrate-depleted beetroot juice as the placebo to isolate nitrate content as the active component. Beetroot juice appears to increase performance in cyclists when performing sprints (about 6-7 min runs) or longer distances (about 30 min). Because the beetroot juice lowered systolic blood pressure, it is thought that the nitrates are being converted to nitric oxide, which leads to vasodilation, contributing to the mechanism of action. Further, the increased power output:VO2 ratios indicate that the athletes could produce more power per unit of oxygen consumed when on beetroot juice vs. placebo. This finding suggests that not only is there the potential for improved blood flow (oxygen delivery) with nitrate, but also more efficient ATP production at a given level of oxygen consumption. Finally, this study represents the results of just a single administration of beetroot juice before exercise. It is not clear if this result would hold true with more extended use of beetroot juice or what the effects of different dosages would be.

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