The Effect Of Inorganic Nitrate On Muscle Function In Healthy Females Across The Menstrual Cycle

Abstract

PURPOSE: Inorganic nitrate (NO3-) supplementation increases nitric oxide (NO) bioavailability and may improve exercise performance. In females, estrogen increases endogenous eNOS production of NO, but estrogen fluctuates throughout the menstrual cycle (MC). It is unknown if different phases of the MC or exogenous NO3- supplementation impact maximal voluntary isometric contraction (MVIC), isokinetic torque, or power in apparently healthy young females. The purpose of this study was to examine potential differences in MVIC, isokinetic torque, and power across the early (EF) and late follicular (LF) phase of the MC, and after either beetroot juice (NO3-containing ~13 mmol NO3-) or identical placebo (PL) supplementation. METHODS: Seven recreationally active women (age: 24.7 ± 4 yrs, VO2peak: 34.4 ± 8 mL/kg/min) with normal MC who were not using contraceptives were recruited in this double-blinded crossover study. Subjects were randomized to consume NO3 or PL, twice daily, for at least 2 days prior to testing that was conducted during EF and again during LF (1-5 and 11-14 days after menses onset, respectively). Subjects also consumed the supplement 2 hours prior to the testing, and a 14-day washout period was utilized between treatments. A linear mixed effects model was used to determine differences between MC phase, across supplements, and interaction effects. MVIC (at 90°), peak torque (PT), PT normalized to bodyweight, time to PT, average PT, and average power were acquired via a Biodex isokinetic dynamometer during knee extension when measured at 180°, 270°, and 360°/second. Data are mean ± SD and significance was determined a priori at p ≤ 0.05. RESULTS: Mixed effects models revealed no significant differences in MVIC, power, or any measures of torque either across the MC, or between the PL and NO3 supplementations (all p ≥ 0.05). There were also no significant interaction effects for MVIC (PL EF: 1.12 ± 0.3; PL LF: 1.10 ± 0.3; NO3 EF: 1.18 ± 0.3; NO3 LF: 1.14 ± 0.3 Hz; p = 0.93) or any other outcomes. CONCLUSIONS: This preliminary data suggests that neither the follicular phase of the menstrual cycle, nor NO3- supplementation, impacts MVIC or outcomes such as peak torque or average power during isokinetic knee extension in healthy young females.