The effect of dietary nitrate supplementation on physical and cognitive performance during load carriage in military cadets

Purpose: The purpose of this study was to determine the effects of dietary nitrate supplementation on physical and cognitive performance during exercise conducted under heavy pack load in military cadets. Methods: Ten college-aged males (VO2max 56.2 ± 3.7 ml/kg/min) consumed 140mL/d of concentrated beetroot juice (BRJ; containing 12.8 mmol of dietary nitrate) or a placebo (PL; flavor, color, energy, and texture-matched with no dietary nitrate) for six days preceding an exercise trial. The trial consisted of 45 min of constant-load exercise on a treadmill at 4.83 km/h and 1.5% grade, followed immediately by a 1.6-km time-trial completed at maximal effort on a treadmill at 4% grade. Exercise was completed while carrying a weighted backpack (55% of body mass). Physiological responses to exercise (VO2, ventilation, RER, heart rate, and RPE) were assessed at 5, 25 and 45 min of constant-load exercise, and during the time-trial. Exercise performance was assessed via time to complete the 1-mile time-trial. Cognitive function was assessed immediately prior to exercise, during exercise (at 30 min), and immediately post-exercise, using the psychomotor vigilance test (response time and lapse count were recorded). Following a 7-day washout period, participants completed a second exercise trial after consuming the opposing supplement (BRJ or PL) for six days. Supplements were double-blinded, and treatment effects were assessed using dependent t-tests (measurements with a single time-point) and two-way repeated-measures ANOVAs (measurements with multiple timepoints), with the alpha level for statistical significance set at p \u3c .05. Results: All physiological responses (VO2, ventilation, heart rate, RPE, and RER) changed significantly over time during exercise, but there were no significant effects of supplementation (BRJ vs PL) on any responses during constant-load exercise, or the time-trial. Exercise-related changes in cognitive response times (PL -19.2 ± 37.2; BRJ -1.0 ± 52.4 ms) and lapse counts (PL -3.1 ± 6.8; BRJ +0.2 ± 8.3) were not significantly different between PL and BRJ (p = .137; p = .186), despite moderate-large treatment-effects for these changes (ES = 0.45 and 0.72 SD units, respectively). Time trial performance was significantly faster in the BRJ trial (770.9 ± 78.2 s) versus the PL trial (809.8 ± 61.4 s). Conclusion: Dietary nitrate supplementation improved time-trial performance during weight carriage in a military population. However, the mechanisms for this outcome are not clear, due to the absence of changes in oxygen uptake or cognitive performance between treatments