The effects of inorganic nitrate supplementation on exercise economy and endurance capacity across the menstrual cycle

Abstract

Oral inorganic nitrate (NO₃^-) supplementation has been shown to increase bioavailable NO and provide potential ergogenic benefits in males; however, data in females is scarce. Estrogen is known to increase endogenous NO bioavailability and to fluctuate throughout the menstrual cycle (MC), being lowest in the early follicular (EF) phase and highest during the late follicular (LF) phase. This study examined the effects of oral NO₃^- supplementation on exercise economy, endurance capacity, and vascular health in young females across the MC. Ten normally menstruating females' MCs were tested in a double-blinded, randomized design during both the EF and LF phases of the MC. Participants consumed ∼13 mmol NO₃^-, in the form of 140 mL beetroot juice (BRJ) or an identical NO₃^--depleted placebo (PL) for ∼3 days before lab visits and 2 h before testing on lab visits. Plasma nitrate, nitrite, and estradiol were assessed, as was blood pressure and pulse wave velocity. Moderate-intensity exercise economy and severe intensity time to exhaustion (TTE) were tested on a cycle ergometer. As expected, plasma estradiol was elevated in the LF phase, and plasma nitrite and nitrate were elevated in the BRJ condition. Exercise economy was unaltered by BRJ or the MC, however TTE was significantly worsened by 48 s (∼10%) after BRJ supplementation (P = 0.04), but was not different across the MC with no interaction effects. In conclusion, NO₃^- supplementation did not affect exercise economy or vascular health and worsened aerobic endurance capacity (TTE), suggesting healthy females should proceed with caution when considering supplementation with BRJ.NEW & NOTEWORTHY Although inorganic nitrate (NO₃^-) supplementation has increased in popularity as a means of improving exercise performance, data in females at different phases of the menstrual cycle are lacking despite known interactions of estrogen with NO. This study revealed neither NO₃^- supplementation nor the menstrual cycle influenced exercise economy or vascular health in healthy young naturally menstruating females, while NO₃^- supplementation significantly worsened endurance capacity (10%) independent of the menstrual cycle phase.

Keywords: exercise economy; exercise endurance; females; inorganic nitrate; menstrual cycle.