Dietary nitrate and muscle contractile function in women: effect of menstrual cycle phase

Dietary nitrate ([Formula: see text]), a source of nitric oxide (NO), enhances muscle contractility in numerous populations, but it is still unclear whether young women also benefit. The efficacy of [Formula: see text] supplementation might vary with menstrual cycle phase, due to lower endogenous NO bioavailability when estradiol (E₂) is low. Using a double-blind, placebo-controlled, crossover design, we determined the effects of acute ingestion of 200 μmol/kg of [Formula: see text] (from concentrated beetroot juice) or placebo on muscle function in 12 normally menstruating women during the early follicular (EF) and late follicular (LF) phases of their cycle. Muscle function was determined through maximal knee extensions on an isokinetic dynamometer and electrical stimulation of the quadriceps, with menstrual phase confirmed through plasma hormone measurement. E₂ concentrations were significantly lower in EF versus LF (220 ± 90 vs. 583 ± 260 pM; <i>P</i> < 0.001), whereas progesterone levels did not differ. Despite this, dietary [Formula: see text] had no effect on maximal muscle power or velocity during either phase. Dietary [Formula: see text] also had no effect on unpotentiated or potentiated peak twitch torque, rate of torque development (RTD), rate of relaxation (RR), or the torque frequency relationship during either phase. However, the RTD was 2%-8% greater in the LF versus EF phase, especially in the unpotentiated state (<i>P</i> < 0.01). The RR was also 9% slower in the LF versus EF phase (<i>P</i> < 0.05). Variations in E₂ during the menstrual cycle seem to subtly influence muscle contractile function. Acute [Formula: see text] supplementation, on the contrary, has no apparent effect on muscle contractility in young women.<b>NEW & NOTEWORTHY</b> We determined the effects of dietary [Formula: see text] on muscle contractile function during the early follicular (EF) and late follicular (LF) phases of the menstrual cycle. Although [Formula: see text] supplementation had no impact on muscle contractility, during electrically evoked twitches, the rate of torque development was greater, and the rate of relaxation was slower, in the LF phase. These findings enhance our understanding of how dietary [Formula: see text] and female sex hormones impact muscle contractile function.