Regular physical activity enhances immune function (Gleeson, 2007; Petersen and Pedersen, 2005). However, acute bouts of intense endurance exercise or periods of intensified training can have an immunodepressive effect, subsequently resulting in an increased likelihood of illness and infection (Gleeson, 2007). This may be of particular consequence for athletes, as even minor infections may result in decrements to exercise performance and a reduced capacity for heavy training (Roberts, 1986). Heat exposure is another factor that can compromise immune function via an increase in pro-inflammatory cytokine concentrations (Bouchama and Knochel, 2002; Sawka et al., 2011), and there is some evidence to suggest that performing exercise in a hot environment can exacerbate the immune response (Peake et al., 2008). Exercising in the heat also stimulates the production of the stress hormone cortisol (Brenner et al., 1997; Niess et al., 2003), which has a known immunomodulatory effect (Walsh and Whitham, 2006). If prolonged or severe, these endocrine and immune disturbances can contribute to the pathology of heat stroke (Lim and Mackinnon, 2006).
Sex hormones also modulate the immune system (Schuurs and Verheul, 1990), and it is well established that oral contraceptives (OC) change the hormonal milieu (Wiegratz et al., 2003). Given the relationship between sex hormones and immunity, it is perhaps surprising that relatively little research has investigated the role of exogenous female sex steroids on the modulation of immune function. Giraldo et al. (2008) previously demonstrated that OC use improves inflammatory status in untrained women (i.e., by lowering neutrophil activation and interleukin (IL)-8 concentrations and increasing the concentration of IL-13), and stated that this greater anti-inflammatory environment is likely related to the lower concentration of endogenous estrogen when compared to normally-menstruating women. Conversely, Timmons et al. (2005) observed increases in resting leukocyte counts in OC users compared to non-users, which could be indicative of inflammation. The same study also observed ~80% greater exercise-induced IL-6 concentrations in normally-menstruating women during the follicular phase when compared to women on OC. While IL-6 can have proinflammatory properties, increased IL-6 concentrations after exercise induce an anti-inflammatory environment by inhibiting pro-inflammatory (i.e., TNF-[alpha]) cytokines and facilitating anti-inflammatory cytokine (i.e., IL-1ra, IL-10) production (Petersen and Pedersen, 2005). Thus, it is possible that the blunted IL-6 response among OC users in the Timmons et al. (2005) study reflects a disruption to the normal immune response to exercise. Oral contraceptive use is also associated with increased C-reactive protein (CRP; a biomarker of inflammation and tissue damage) concentrations in the general population (Cauci et al., 2008; S0rensen et al., 2014; van Rooijen et al., 2006) and in athletes (Cauci et al., 2017), which could predispose users to a higher inflammatory response to physical stress and elevate cardiovascular risk (Ridker et al., 2000). It is clear, then, that OC use has the potential to influence various aspects of the immune response, although further research is required to confirm the direction, magnitude, and implications of these changes. Importantly, no previous study has investigated the combined effect of heat and exercise on the immune response of women taking OC.
Previous studies investigating the stress response (i.e., cortisol) during exercise in the heat are limited by the exclusive use of male subjects (e.g., Brenner et al., 1997; Hoffman et al., 1997; Niess et al., 2003), or the use of female subjects on and off OC but without the potentially additive effect of heat exposure (e.g., Boisseau et al., 2013; Bonen et al., 1991; Kirschbaum et al., 1996). Even in this literature the data are equivocal; one study observed increases in free cortisol at rest in OC users compared to non-users (Boisseau et al., 2013), while two reported no differences in pre-exercise cortisol concentrations (Bonen et al., 1991; Kirschbaum et al., 1996). Furthermore, while these three studies agree that the cortisol response to exercise is blunted in women on OC, Kirschbaum et al. (1996) nonetheless reported an increase in free cortisol in response to exhaustive cycling exercise in OC users (although to a lesser degree than non-users), whereas Boisseau et al. (2013) and Bonen et al. (1991) observed no exercise-induced changes in cortisol concentration in women taking OC. This may be important, as cortisol acts as a powerful natural immunosuppressant (Petrovsky, 2001). Thus, considerably more research is required before firm conclusions can be drawn in respect to OC and the stress response, and in particular, the cortisol response elicited during exercise in the heat.
This exploratory study investigated the immunoendocrine response to exercise in the heat in women taking combined monophasic OC when compared to normally-menstruating women. This an important consideration, as exercising in hot conditions is commonplace for both recreational and competitive athletes (e.g., the 2020 Olympic Games have been scheduled for the peak of the Tokyo summer). Specifically, CRP and immune cell counts were measured at rest, and pro- and anti-inflammatory cytokines (IL-1P, IL-1RA, IL-6, IL-8, IL-10, and TNF-[alpha]) and cortisol were evaluated in OC users and non-users before and after exercise in both temperature and hot conditions.
Eighteen recreationally-active women (performing 300500 min/wk of moderate-intensity exercise) voluntarily participated in this study. Nine of the women were normally-menstruating (i.e., every 25-32 days) for >12 months (WomenNM), while the remaining nine women were taking a low-dose combined monophasic OC for >12 months (WomenOC). WomenOC continued their OC use throughout the experimental period. None of the participants had ever knowingly been pregnant, and they had no documented history of pulmonary, cardiovascular, or metabolic disorders. Written informed consent was provided by participants prior to data collection, and ethical approval was granted by the Griffith University Human Ethics Committee.
WomenNM were tested in the follicular phase of their cycle (day 2-6) when endogenous estrogen is low (Charkoudian and Johnson, 2000), whilst WomenOC were tested during the active phase of the OC pill (day 2-21; when exogenous estrogen is administered). As women on OC have suppressed endogenous hormone concentrations (Burrows and Peters, 2007), these phases were selected to isolate the effects (if any) of the synthetic hormones provided by the OC pill (Cherney et al., 2007). The follicular phase was identified by self-report (the participants alerted researchers at the onset of menses) and was confirmed via blood concentrations of the female sex hormones (see Results).
Participants completed three exercise tests, the first of which was an incremental cycling test to exhaustion to determine the workloads for the subsequent two trials (based on lactate threshold 1 [LT1]). Two 3-stage cycling trials were then performed, one in a temperate environment (TEMP; 22[degrees]C) and the other under hot ambient conditions (HEAT; 35[degrees]C). Room temperature and relative humidity were measured continuously throughout all trials (Vantage Vue, Davis Instruments, Hayward, USA). The incremental cycling test was performed 4 wk before the 3-stage cycling trials, and the 3-stage cycling trials were then separated by 48 h to ensure adequate recovery time. The exercise tests were performed at the same time each morning (between 5:00-9:00 am), and trial order for the 3-stage cycling trials was randomised. Participants were instructed to refrain from alcohol, caffeine, and vigorous exercise in the 24 h prior to testing.
Incremental cycling test to exhaustion
All cycling tests were performed on an electronically-braked cycle ergometer (Sport Excalibur, Lode B.V. Medical Technology, Groningen, The Netherlands). Heart rate was monitored continuously throughout the incremental test (RS300X, Polar, Finland), and blood was sampled from the earlobe for the determination of blood [La-] (Lactate Pro, ARKRAY Inc., Japan). Gas exchange variables were measured breath-by-breath using a calibrated metabolic measurement system (CosMed, Pulmonary Function Equpiment, Quark CPET, Italy). Participants began the protocol by cycling at 15 W at a pedal cadence of 70-80 rpm. Power output was increased by 15 W every 4 min until blood [La-] reached a value of >4.0 mmol/L, after which the power output was increased by 15 W every 30 s until volitional exhaustion. When the required cadence could not be maintained despite strong verbal encouragement, the test was terminated. Peak HR and power output were recorded at this time. LT1 was calculated using the modified Dmax method (Cheng et al., 1992), and the final 30 s of oxygen consumption ([VO.sub.2]) data were averaged to determine peak [VO.sub.2].
3-stage cycling trials
Participants ingested a core temperature ([T.sub.c]) sensor (CorTemp COR-100 Wireless Ingestible Temperature Sensor, HQ Inc., FL) ~8 h before each trial to allow adequate time for the pill to transit from the stomach to the small intestines (Lee et al., 2000). Participants arrived to the laboratory in a fasted state and emptied their bladder before the measurement of body mass. Baseline [T.sub.c] was recorded at this time. Participants then moved into the environmental chamber where they remained for 1 h to allow for passive acclimation. They were...