Abstract
Nitric oxide (NO) is a lipid-soluble molecule found in various organ systems that assists in regulating blood pressure, vascular function, and exercise performance. Inorganic nitrate (NO3-) supplementation has been touted as both an exercise ergogenic as well as a clinical therapeutic, as NO3- ¬reduces to NO via the NO3--NO2--NO pathway. While NO3- has shown to improve exercise economy, time to exhaustion, and muscular power, these studies have predominately only studied healthy young males. Females are largely understudied, perhaps because of the intricacies of the hormonal fluctuations that occur throughout the menstrual cycle. Of these hormones, estrogen fluctuates cyclically in young healthy females ranging from lower levels during the early follicular (EF) phase of the menstrual cycle and peaking typically during the late follicular (LF) phase. This is important as estrogen is known to directly improve NO bioavailability, yet despite this, the interactions of NO3- supplementation with estrogen fluctuations in young healthy females has not been understudied. Further, as females undergo menopause, estrogen is largely reduced, as is the NO bioavailability afforded by estrogen. This results in a loss of vascular protection in post-menopausal (PM) females and an exacerbated cardiovascular disease (CVD) risk. While exercise training is known to improve vascular function, this does not occur in PM females, potentially due to a loss of bioavailable NO that comes with the loss of estrogen. Improving bioavailable NO either by increasing exercise intensity or by supplementing with NO3- supplementation may provide interventional options for PM females to prevent the elevated CVD risk.
Thus, the purpose of manuscript 1 of this dissertation was to determine the effects of NO3- supplementation on aerobic exercise economy and endurance capacity across follicular phases of the menstrual cycle. The results of this placebo-controlled randomized control trial (RCT) revealed that NO¬3- supplementation (BRJ) elevated plasma nitrite and nitrate, but plasma nitrate was higher in the LF phase of the menstrual cycle (MC). Exercise economy was unaltered by BRJ or the MC, however exercise endurance was significantly worsened by 40 seconds (~9%) after BRJ supplementation (p = 0.04) but was not different across the MC with no interaction effects.
The purpose of manuscript 2 of this dissertation was to determine the effects of NO3- supplementation on isokinetic peak muscular power, maximal voluntary isometric contraction (MVIC) force, muscular endurance, and recovery from fatigue across the follicular phases of the menstrual cycle. Isokinetic peak power was worsened by NO3- supplementation (p = 0.02), with maximal knee extensor power (Pmax) to being significantly worsened in the LF+BRJ condition compared to the EF+PL condition (p = 0.04). Maximal knee extensor velocity (Vmax) was also worsened by NO3- supplementation (p = 0.03). Muscular endurance, MVIC, and fatigue recovery was unaltered by BRJ or the MC (all p > 0.05).
Finally, manuscript 3 of this dissertation was to determine the effects of NO3- supplementation and different intensities of exercise on acute changes to vascular health in PM females. BRJ + high intensity exercise (HIE) improved Peak ∆ Flow-Mediated Dilation (FMD) compared to all control (CON) conditions (p < 0.05), while BRJ + moderate intensity exercise (MIE) improved Peak ∆ FMD compared only to BRJ+CON. Neither PL+HIE nor PL+MIE improved Peak ∆ FMD compared to PL+CON (p > 0.05). Plasma NO2- was positively correlated with Peak ∆ FMD, while body fat percentage with inversely correlated. Exercise prevented the increase in MAP and PWV over time independent of treatment.
These dissertation data taken together suggest a potential hormetic relationship with estrogen and NO in which NO¬3- supplementation may harm exercise performance in young healthy females who already have sufficient NO, whereby NO3- supplementation may be further elevating NO to an extent where it is no longer beneficial. Contrarily, in PM females where endogenous NO is lower due to the loss of estrogen, NO3- supplementation and higher intensity of exercise may bring NO bioavailability back into a healthy range, whereby NO3- supplementation is beneficial. These data have revealed a fascinating relationship between estrogen and its relationship on both endogenous and exogenous NO that had never been explored previously. Understanding these relationships allows for a precision approach such that NO3- supplementation can be implemented in the context of underlying low endogenous NO bioavailability. Hopefully, these data will provoke further study in this area and will invoke curiosity towards studying these relationships in a nuanced manner.
