The effect of different doses of nitrate from beetroot juice on exercise performance and cognitive function in healthy female and male recreational exercisers : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Nutrition and Dietetics at Massey University, Auckland, New Zealand

Abstract

Dietary nitrate supplementation has been shown to improve endurance exercise following acute and long-term supplementation periods as the bioactive form of nitrate, nitric oxide, works within the body to create greater blood flow and therefore oxygen delivery to working muscles through enhanced vasodilation. Limited studies have researched the effects of long-term nitrate supplementation through beetroot juice with female and male recreational athletes. This study investigated if intake of varying doses of dietary nitrate from beetroot juice affect cognitive and exercise performance in recreational female and male athletes due to their reduced cardiovascular systems following a 7-day supplementation period. In a double-blind randomised cross over study, 16 participants (7 female; 9 male) aged 28.6 ± 12.1 years underwent three 7-day supplementation periods, with a daily dose of 140 ml high-nitrate beetroot juice (HBRJ) containing 20.6 mmol NO₃-, 200 ml moderate-nitrate (MBRJ) containing 18.06 mmol NO₃- or 200 ml placebo (Pla) containing 1.66 mmol NO₃-. On day 7, participants completed baseline testing, cognitive tests of inhibition and working memory (including Stroop and Corsi blocks), and blood samples were collected for plasma nitrate and nitrite concentration analyses. Following supplementation, participants completed a 120-minute absorption period, baseline tests were repeated, and then completed a 120-minute cycle at 60% peak power output followed immediately by a 4-km cycle time-trial. Perceptual measures were collected during the cycle and baseline tests were completed again post exercise. Results showed that plasma nitrate was highest with the HBRJ and lowest with the Pla (p < 0.001) and plasma nitrite was highest in HBRJ at baseline and pre-exercise and MBRJ at post-exercise (p = 0.003). A large effect size (η² = 0.32), suggested a substantial impact of beetroot juice on 4-km cycle time trial performance in comparison to Pla, however, due to interindividual variation or a lack of statistical power, there was no significant difference (p = 0.15). A small sample size may have limited the studies ability to detect differences despite the observed effect size. Cognitive test results showed no variance between conditions at any time point (p > 0.05), however reaction time improved from baseline to post-exercise in all conditions (p = 0.008). Overall, future research should look to recruit more participants for a larger sample size to obtain a greater statistical significance, as this study showed potential to have.