Introduction:
Nitric oxide (NO) is a vasodilator that increases blood flow by promoting relaxation of endothelium. Dietary nitrate supplementation increases plasma nitrite, a marker of overall NO bioavailability. Previously, acute dietary nitrate supplementation has been shown to reduce oxygen consumption and improve tolerance during submaximal exercise in healthy populations. Less is known about the effect of dietary nitrate on oxygen consumption at rest.
Hypothesis:
We hypothesized that dietary nitrate supplementation would reduce resting metabolic rate (RMR) and oxidative stress (8-isoprostane) at rest, via enhanced NO bioavailability via the oxygen independent Nitrate-Nitrite-Nitric Oxide pathway in healthy, young males.
Methods:
In a randomized, double-blind, cross-over study, ten healthy, young males (21 ± 2 years) visited the laboratory on 5 separate occasions. Participants completed informed consent paperwork and underwent protocol familiarization during visit 1. Prior to visits 2 and 4, participants fasted for 12 hours and adhered to an NIH-approved low-nitrate diet for 48 hours. During visits 2 and 4, an initial blood draw was performed to analyze baseline plasma nitrite and 8-isoprostane. Participants then completed a 30-minute resting metabolic rate (RMR) test. Two hours prior to visits 3 and 5, participants consumed either a placebo or dietary nitrate supplement (negligible and 6.2 mmol nitrate, respectively). During visits 3 and 5, participants also had blood drawn for analysis of the previously stated measurements, and completed an RMR test. Visits 2 and 3 were on consecutive days, followed by a week-long washout period between visit 3 and visit 4, while visit 4 and 5 also occurred on consecutive days.
Results:
Plasma nitrite significantly increased following dietary nitrate consumption compared to baseline values (27.56 ± 7.58 and 1.25 ± 1.51 arbitrary units, respectively). No difference was present between nitrate and baseline measurements for 8-isoprostane (155.75 ± 57.01 and 198.42 ± 66.44 pg/mL, respectively; p=0.55) and RMR (2086.60 ± 202.23 and 2050.00 ± 209.23 kcal/day, respectively; p=0.13).
Conclusion:
Dietary nitrate supplementation increases plasma nitrite, but does not change resting metabolic rate following an acute dose of dietary nitrate in healthy males. Individuals consuming dietary nitrate as an ergogenic aid during exercise may not, however, experience similar changes in their resting metabolism. The lack of change in oxidative stress may have been associated with the overall health of the cohort examined. Future research should investigate the clinical implications of dietary nitrate in populations with decreased NO bioavailability and associated endothelial disfunction (and elevated oxidative stress). In such populations, dietary nitrate may provide benefit. However, in a healthy cohort, dietary nitrate exerts minimal effects.
The effects of a thermogenic supplement on resting metabolic rate in healthy males: preliminary results