Levelling the playing field: It’s time to advance women’s thermal health and safety

Do hormonal contraceptives compromise heat regulation for women, increasing heat strain by blunting the ability to lose heat during exercise? Nathalie Kirby dedicated her time to investigate this to answer the unknowns on the physiological influence of the menstrual cycle and hormonal contraceptives on thermoregulation.

How do hormones affect heat tolerance and thermal sensitivity

A whirring fills the room, coming from a human sized tin-can. The specialised cylindrical structure is engineered to measure whole body heat loss. Within its cramped, heated chamber a woman cycles on a customed-made bike fitted with a deck chair. She breathes into a large mask that looks like a piece of diving equipment to measure gas exchange, with wires taped at various points on her body to measure body temperatures. Dr Nathalie Kirby and her team monitor the woman on the bike using a little camera fitted on the inside of the chamber, their voices crackling through the microphone as they check in during the nearly two-hour long exercise test.

“It’s an uncomfortable set up,” admits Nathalie, as she describes what her study participants went through, stressing the unpleasantness of having an oesophageal probe, which extends through the nose down to the heart, plus a rectal probe inserted to measure body temperature. “I admire all the women who participated in our study and I am so grateful to them for being involved in this research,” praises Nathalie.

One essential part of the study was that the women participants carried out the cycling exercise test during their period. “Planning a study design around the menstrual cycles was tricky,” confesses Nathalie, recalling the days study participants rang her because they had started their period early, or hadn’t started it yet. “We needed to test them on exact days to collect blood level hormones,” says Nathalie, who rescheduled testing slots and adjusted them around the other participants.

A lot of time, care and precision were needed to carry out this women-only thermal physiology study. It is the first of its kind to assess the influence of hormonal contraceptives, and specifically intrauterine devices, on women’s thermoregulation at the whole-body level. Nathalie had the challenge of scheduling in exercise tests at two specific phases during the menstrual cycle for more than 30 women. “We followed strict study design recommendations and hormone verification protocols, alongside managing several experimental controls on top of any variables related to the menstrual cycle.”

The effort was “worth it,” exclaims Nathalie, deservedly proud of her study ‘Hormonal intrauterine devices and heat exchange during exercise’ published in The Journal of Physiology. Nathalie sought to advance the field of thermal physiology for women’s research and health. For too long women have been underrepresented from study designs and research data on exercise performance and heat stress. Nathalie suggests, “Our study and the evidence it offers is critical for informing participant inclusion in future research and serving women’s health.”

It is now more prevalent to understand the factors that could exacerbate heat strain in women. We are living in a rapidly warming world, where climate change is increasing the frequency and the intensity of heat exposure. Nathalie’s investigation into the effects of hormonal contraceptives on heat loss during exercise provides an evidence base for thermoregulation research questions across women’s lifestyle and work demands, whether they are an athlete, have a labour-intensive job, or live in a warm climate.

Women only studies: Historical rarities in thermal physiology

Women have long been underrepresented in sport, exercise, and health science research. “There’s a dearth in women-only studies and a lack of women participants in research studies across the fields of physiology,” says Nathalie. She reminds us that thermal physiology was primarily born out of contexts where the research wasn’t designed to serve women’s health and needs. The most urgent need for thermoregulation research was serving what were, at the time, male-dominated occupations.

The first heat acclimation studies took place in the late 1800s/early 1900s as mining companies in South Africa wanted to prevent heat casualties among their workers while maximising their gold production. Interest in heat tolerance rose during the 2nd World War to protect the health and welfare of soldiers. The war is considered to have triggered the first investigations in heat tolerance to limit and prevent heat-related injuries sustained by military units both in the field and in training camps.

Flash forward to the present day and the field is catching up. “Demand is now growing for research and data on elite female athletes. Previously, there was little attention and funding for international women’s sport. This has contributed to the underrepresentation of women-only studies”, explains Nathalie. Adding “it can also be largely attributed to hormonal fluctuations during the menstrual cycle and how sex hormones affect a host of physiological systems”.

The most well-known change across the menstrual cycle is resting body temperatures. Nathalie explains that body core and skin temperatures are typically the primary outcomes of interest in thermal physiology. “Fluctuations in these factors lead to additional variability in the data influencing our main research questions.” This has led to researchers historically excluding female participants. However, variability is inherent for physiologists studying humans. Day-to-day differences can occur in everyday behaviours and activities, from sleep, nutrition, mood, exercise recovery, etc, in both men and women.

“We can do our best to control for it. Researching the influence of the menstrual cycle requires a lot of planning to properly account for any potential variability,” says Nathalie. She adds, “there is an ongoing debate about how we should properly consider hormones and the menstrual cycle in our research. I wanted to address this long-deliberated question: do hormones and physiological changes across the menstrual cycle alter whole-body heat loss in women?”

The unknowns on contraceptive use and heat regulation during exercise

“About a third of all women under the age of 50 in North America and Europe use hormonal contraceptives,” says Nathalie. Hormonal intrauterine devices (IUDs) are implanted long-acting, reversible and highly effective contraceptives. Hormonal IUDs are commonly used by women aged 15−49 years old worldwide. The method has surpassed that of oral contraceptive pills since 2022. Nathalie points out, “research hasn’t been able to keep up. Despite the popular use of IUDs, there is no evidence on the influence of IUDs on heat regulation during exercise”. She stresses, “without this knowledge, women regularly living, working, or competing in the heat can’t make evidence-based decisions about their contraceptive choices”.

IUDs release progestin, a synthetic hormone that mimics the effect of progesterone. Progestins are considered thermogenic and assumed to elevate core body temperature. This is based on limited evidence on oral contraceptives, where levonorgestrel, the same progestin released by IUDs, has been reported to impair vascular function and blunt sweating during exercise.

“No one had tested whether IUDs affected women’s thermoregulation when they were exercising in the heat,” states Nathalie. “We had no evidence on whether IUDs altered heat tolerance or increased heat strain during exercise.” Nathalie’s primary research comparison was between groups, IUD users and non-IUD users, in two phases of their menstrual cycles. “This allowed us to isolate the effects of both the synthetic and the natural hormones on thermoregulation.” Heat exchange is a complex and dynamic process. Sweating and skin blood flow work in tandem to facilitate heat loss from the body. Nathalie was interested in how synthetic and natural hormones that can alter these underlying mechanisms of thermoregulation translated into net, whole-body heat loss effects.

A total of 36 women exercised inside the tin can-like temperature-controlled chamber, called the Snellen Direct Air Calorimeter, a sophisticated and powerful tool to measure whole body heat loss. “It’s the only one in the world,” states Nathalie, of the device that resides at Human and Environmental Physiology Research Unit (HEPRU) at the University of Ottawa (Canada).

All participants were physically active and they were divided into two groups. 19 were using IUDs, who were compared to 17 controls who weren’t using IUDs or any other form of hormonal contraception. Both groups carried out exercise-heat stress tests, completing two 45-minute bouts of semi-reclining cycling at two exercise intensities (light- and moderate-intensity) in 30°C air temperature at 21% relative humidity, which were separated by a 15-minute recovery period.

Participants were tested during both the follicular phase (the first phase of the menstrual cycle, before ovulation) and luteal phase (the second phase of the menstrual cycle, which takes place after ovulation) of their menstrual cycle. They had at home testing kits to mark the day they were ovulating, so Nathalie and her team would know exactly when to bring them in for the cycling exercise test.

Participants wore a respiratory mask that was connected to an open circuit gas analysis system to analyse the amount of oxygen and carbon dioxide they inspired and expired, “similar to the setup for a VO2 max test,” explains Nathalie. She adds, “Using the gas exchange measurements, we could calculate metabolic heat production. This indicates how much heat the body is producing”.

To effectively thermoregulate, the heat that the body produces needs to be dissipated. Nathalie could precisely measure the temperature and the moisture in the air flowing in and out of the Calorimeter. Small increases in air temperature and humidity indicate how much heat and moisture is being lost from the participant into their environment.

In fact, measurements were so sensitive that a tiny spike in the moisture reading could be detected just from a couple drops of sweat evaporating off a participant when they changed position in the chair. “You can see the data in real time. Even these tiny changes,” admires Nathalie. “This precision allowed us to exactly measure how much heat a participant’s body loses through both evaporative and dry heat loss. This includes heat lost through sweating, as well as convective and radiant heat loss from the skin, which combine to cover whole-body heat exchange.”

“This is where the uniqueness and power of the Calorimeter really shines,” states Nathalie. “It paints a full picture, allowing us to study and understand the net effects of millions of sweat glands and the blood flowing to the skin surface across the entire body.” She explains how this helps identify the mechanisms involved, “We can see whether there are net effects of any independent physiological minutiae at the whole-body level, helping advance our understanding of thermoregulation as a whole.”

Previous research samples and evidence have been inefficient to cover this complexity. Sweating and skin blood flow samples were usually taken from just one or two skin sites to evaluate these measures, which only equates to a couple centimetres squared overall. But sweating and blood flow can vary substantially across the body, as can how that pattern shows up person-to-person. This means that changes at one site don’t always reflect the rest of the body. To see how mechanisms of thermoregulation affected body temperatures, Nathalie and her team also measured skin temperature at eight sites, including the forehead, chest, upper back, bicep, forearm, hand, thigh and calf.

“Ours was the first study showing that hormonal IUDs do not appear to alter women’s thermoregulation during exercise, which we showed using the gold-standard measurement methods,” states Nathalie. She adds, “Women using hormonal IUDs lost heat just as effectively as women who were not using a hormonal IUD. In fact, their total whole body heat loss, rate of heat storage and their increases in body core temperature were all statistically equivalent during moderate- and high-intensity exercise in the heat.”

Previous research has identified sizeable menstrual cycle differences in individual thermoregulatory responses. For example, some studies have a reported 30–40% increases in local or maximal skin blood flow in the luteal phase when both oestrogen and progesterone hormone levels are elevated. However, that does not mean women lose 30–40% more heat during the luteal versus the follicular phase. “Previous studies have given us valuable information about individual mechanisms, but we did not know whether those changes were enough to affect the body’s overall ability to lose heat,” says Nathalie. “That is what direct calorimetry allowed us to test.”

Skin blood flow carries warm blood from the body’s core towards the skin, but the heat still has to pass from the skin into the surrounding air. That depends on factors such as the difference between skin and air temperature, airflow, and sweat evaporation. Increases in blood flow may therefore produce only a very small additional rise in skin temperature, such as the ~0.1°C difference in skin temperature recorded in this study. That widened skin-to-air gradient still causes only a minor increase in heat lost through convection and radiation – the “dry” heat loss measured using the calorimeter.

“The variations are so tiny. We found a 3 W/m² increase in dry heat loss in the luteal phase, which would equate to less than a 0.1°C difference in body core temperature when exercising for over an hour, a difference often too small to reliably measure,” clarifies Nathalie. “We saw this translate to a 1% higher relative contribution of dry heat loss to total heat loss in participants, but overall, total whole-body heat loss was equivalent between phases. This is consistent with previous findings that we don’t typically see much of a difference in body temperatures during exercise-heat stress between one phase of the menstrual cycle versus another.”

Total whole-body heat loss and evaporative heat loss remained equivalent between menstrual-cycle phases across both groups of women. “This shows why a noticeable change in one physiological mechanism does not necessarily produce a meaningful change in whole-body heat loss,” says Nathalie. “Dry heat loss is only one part of the system, and during exercise most heat is generally lost through the evaporation of sweat. The whole-body measurement tells us whether all of those processes combined ultimately change how much heat the body can dissipate.”

Removing barriers for women’s health and the research participation obstacle

Nathalie’s study could put women at ease around concerns they might have about their contraceptive choices and heat stress. “It’s unlikely that hormonal IUDs put women at greater risk of overheating, or of a heat-related illness either during exercise or when they carry out any physical labour in the heat,” assures Nathalie.

Her mechanistic insights on the effects of hormones on thermoregulation also have important practical implications for researchers designing thermal physiology and exercise studies. New methodological guidance for considering female participants in exercise study designs had been released prior to Nathalie and the team carrying their study. “It was a good first step forward for including women in exercise research, but it was limited to recommendations related to women who were either not using any form of contraception or women who are using oral contraceptive pills,” states Nathalie. “There were no criteria for including hormonal IUD users. If these women are missing from the research, it’s impossible to make evidence-based health recommendations for them,” she argues.

“Our findings indicate that including hormonal IUD users shouldn’t introduce any major biases or additional variability when it comes to exercise and thermoregulation research. This supports that IUD users can and should be considered during participant recruitment, removing what is currently a major barrier for including female participants in exercise performance and thermoregulation research,” encourages Nathalie.

With the growing number of women using hormonal IUDs, broadening the participant pool and widening the representation of women in exercise and environmental studies could further advance women’s research, health, and well-being. It’s time to level the playing field for women’s heat health and safety, let’s hope Nathalie’s research keeps the ball in play.

Read the full research article by Nathalie Kirby et al., ‘Hormonal intrauterine devices and heat exchange during exercise’ in our Thermal Physiology in Health and Disease eBook.