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

Abstract

Dietary nitrate (NO₃^-), 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 NO₃^- 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 NO₃^- (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 via maximal knee extensions on an isokinetic dynamometer and electrical stimulation of the quadriceps, with menstrual phase confirmed via plasma hormone measurement. E₂ concentrations were significantly lower in EF vs. LF (220 ± 90 vs. 583 ± 260 pM; p<0.001), whereas progesterone levels did not differ. Despite this, dietary NO₃^- had no effect on maximal muscle power or velocity during either phase. Dietary NO₃^- 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 vs. EF phase, especially in the unpotentiated state (p<0.01). The RR was also 9% slower in the LF vs. EF phase (p<0.05). Variations in E₂ during the menstrual cycle seem to subtly influence muscle contractile function. Acute NO₃^- supplementation, on the other hand, has no apparent effect on muscle contractility in young women.