Dietary nitrate supplementation reduces the O2 cost of walking and running: a placebo-controlled study


Journal Title (Medline/Pubmed accepted abbreviation): J Appl Physiol
Year: 2011
Volume: 110
Number:
Page numbers:591-600
doi (if applicable): 10.1152/japplphysiol.01070.2010

Summary of Background and Research Design

Background:Supplementation with beetroot juice has been shown to lower resting blood pressure and the oxygen cost of submaximal exercise. It has been suggested that beetroot juice effectiveness is attributed to the high nitrate (NO3–) concentration, which can be converted to bioactive NO2– and nitric oxide. However, there are multiple bioactive components in beetroot juice that are potential sources of these beneficial effects.

Hypothesis/purpose of study:The physiologic effects of beetroot supplementation on oxygen cost and blood pressure is attributed to high NO3– content, and not to other potentially bioactive compounds.

Subjects:Nine healthy, physically active men (mean age, 22 ± 4 yr; body mass, 69.3 ± 7.2 kg; height, 1.77 ± 0.06 m; maximal oxygen uptake, 55 ± 7 mL/kg•min) participated in the study.

Experimental design:Randomized, double-blind, crossover

Treatments and protocol:All subjects performed a pre-intervention control during which they performed incremental treadmill and “step” running tests and were assessed for blood pressure, blood lactate, pulmonary oxygen consumption (VO2) dynamics, and time to task failure. The treadmill test consisted of an initial incremental ramp test to determine maximal VO2 (VO2max) and gas exchange threshold (GET). Treadmill speeds were calculated based on individual subject’s GET. The step tests consisted of one exhaustive bout of high-intensity running, followed the next day by two 6-minute bouts of moderate-intensity running plus 6 minutes of severe-intensity running. After the pre-intervention meetings, the subjects were randomized to consume either 0.5 L/day NO3–-rich beetroot juice or NO3–-depleted beetroot juice (placebo) daily for 6 days. The NO3–-depleted beetroot juice was produced by passing beetroot juice through an ion-exchange resin that selectively removes the nitrate ion. Subjects returned to repeat the treadmill test and step test on Days 4 and 5. On Day 6, subjects completed an incremental, single-leg knee extension exercise while prone in an MR scanner for Qmax estimation. Pulmonary gas exchange and ventilation were measured continuously during all tests

Summary of research findings:
  • Supplementation with untreated beetroot juice elevated plasma NO2– levels vs treated beetroot juice (373 nM vs 183 nM, respectively; P < .05).
        - There was no significant main effect for time (P = .315).
  • Supplementation with untreated beetroot juice decreased systolic blood pressure vs treated beetroot juice (124 mmHg vs 129 mmHg; P < .01).
  • There was a significant main effect for treatment across the VO2 response.
        - The O2 cost of exertion was reduced with treated beetroot juice supplementation vs  untreated.
    • Walking (0.77 L/min and 0.87 L/min, respectively; P < .01)
    • Moderate-intensity running (2.10 L/min and 2.26 L/min, respectively; P < .01)
    • Severe-intensity running (end-exercise O2 uptake: 3.50 L/min and 3.77 L/min, respectively; P < .01)
  • Active beetroot juice supplementation increased time to exhaustion during severe-intensity running 15% vs placebo (8.7 min and 7.6 min, respectively; P < .01).
  • There was no difference in Qmax >between the 2 supplementation protocols as assessed by knee extension exercises.

Interpretation of findings/Key practice applications:

These results confirm previous reports on the benefits of beetroot juice supplementation and further indicate that the physiologic effects are attributed to the high NO3– content. In addition to physiologic markers, all 9 subjects experienced an improvement in time-to-task failure following active beetroot supplementation vs placebo. Subjects receiving the active beetroot supplementation demonstrated a 15% improvement in time-to-task failure, which is approximately equivalent to a 1% reduction in the time necessary to cover a set distance. This is a clinically meaningful benefit in that it would be notably beneficial in subjects participating in elite sporting events.
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