No Apparent Effect Of Dietary Nitrate On Muscle Contractility During Different Menstrual Cycle Phases

Numerous studies have demonstrated that dietary nitrate (NO3-), a source of nitric oxide (NO), enhances muscle contractility in young and older men and older women, but due to a paucity of research it is unclear whether this is also true in young women. It has been hypothesized that the efficacy of NO3- supplementation might vary with menstrual cycle phase, due to lower NO bioavailability when estradiol is low. PURPOSE: To examine the effect of dietary nitrate on voluntary muscle contractile function during the early follicular (EF) vs. late follicular (LF) phases of the menstrual cycle. METHODS: Using a double-blind placebo-controlled crossover design we determined the effects of acute ingestion of 200 μmol/kg of NO3- (from concentrated beetroot juice (BRJ)) or placebo (NO3- -free BRJ) on muscle function in 12 normally menstruating untrained young women during their EF and LF phases. Voluntary muscle contractile properties were determined via maximal knee extensions on an isokinetic dynamometer, with menstrual phase determined based on cycle tracking and urinary testing and confirmed via measurement of plasma hormone levels. RESULTS: Estradiol concentrations were significantly lower in EF vs. LF (216 ± 109 vs. 552 ± 353 pM; p < 0.01), whereas progesterone levels did not differ (3.14 ± 1.92 vs. 4.98 ± 3.73 nM; p = 0.09). Despite this, dietary NO3- supplementation had no effect on maximal muscle power during the EF (6.25 ± 1.28 vs. 6.21 ± 1.36 W/kg; p = 0.84) or LF (6.33 ± 1.42 vs. 5.96 ± 0.99 W/kg; p = 0.15) phases. Acute NO3- intake also did not alter maximal velocity during the EF (12.4 ± 2.4 vs. 12.1 ± 2.1 rad/s; p = 0.66) or the LF (12.8 ± 2.1 vs. 12.1 ± 1.1 rad/s; p = 0.36) phases. CONCLUSION: Acute dietary NO3- supplementation has no apparent effect on maximal muscle speed or power in normally menstruating untrained young women, regardless of estradiol levels.