Heat tolerance and acclimation in female athletes

Moderate ambient temperatures are known to present sufficient heat stress that result in an elevated core temperature and reduce endurance capacity. Changes in core temperature represent the rate of change in body heat storage, which in turn reflects the balance between metabolic heat production, heat absorbed from the environment and total body heat loss. Work by Nielsen in 1996 provided data to suggest an endurance athlete may experience up to a 1°C rise in core temperature every nine minutes when racing in high ambient conditions. This rate of rise in core temperature would result in a runner reaching a core temperature of 40°C within 25–30 min, with the immediate dangers of heat exhaustion.

Heat tolerance tests are currently used to evaluate military personal that have had a previous experience of exertional heat illness, to determine whether they are able to return to duty. In spite of the significant role of the Israeli Defence Force heat tolerance test to screen military personnel for heat intolerance, the application has been questioned in reference to endurance athletes due to differences in exercise intensity and duration.

Endurance runners typically race for a shorter duration and at a higher exercise intensity compared with military personnel. The incidence of exertional heat illness in endurance athletes raises the importance of developing a heat tolerance screening procedure that informs medical personal about an endurance runners current heat tolerance state, and is capable of monitoring changes in heat tolerance.

Repeated heat exposure to a stressful thermal environment initiates a phenotypic adaptation known as heat acclimation, an element of which has been identified as thermotolerance. Heat acclimation improves thermal comfort, submaximal exercise performance, and increases maximal aerobic capacity in the heat.

The benefits arise from enhanced sudomotor and skin blood flow responses, plasma volume expansion, cardiovascular stability and an improved fluid balance. Thermotolerance or acquired cellular thermotolerance describes the cellular adaptation accompanying systemic changes induced by successful heat acclimation. As a result, heat acclimation is the consensus recommendation strategy to attenuate the physiological strain associated with training and competing in the heat. However, heat acclimation it is not routinely adopted by endurance athletes and there is little information published on females’ responses to heat acclimation.

Sex differences in temperature regulation during exercise have been reported, irrespective of confounding differences in metabolic heat production and physical characteristics.

Interestingly, these differences only become evident above a certain requirement for heat loss and are solely attributed to peripherally mediated control of the sudomotor function. Furthermore, fluctuations in hormonal concentrations associated with the menstrual cycle have a known effect on thermoregulation in females. During the luteal phase of the menstrual cycle there is an elevation in progesterone concentration which modifies central regulatory mechanisms resulting in alterations in basal core temperature, the body temperature threshold for sweating and cutaneous vasodilation. These sex differences in thermoregulatory responses to acute exercise heat stress raise the question of whether females adapt to chronic heat exposure differently to males. Thus, research investigating females’ phenotypic and cellular responses to controlled hyperthermia heat acclimation over short-term and long-term timescales is required.

At the time the project began, novel heat acclimation strategies had been investigated in male participants. These methods involved permissive dehydration, sauna bathing, and hot water immersion. These methods aimed to establish an accelerated adaptation via targeting adaptation pathways including, plasma volume expansion, increased sudomotor function, and altered cutaneous vascular function. Once the temporal patterning to controlled hyperthermia in females has been determined, further research would be required to establish novel heat acclimation strategies that accelerate adaption responses in females.

Project aims
A heat tolerance test using a running mode of exercise, data on females’ phenotypic and cellular responses to controlled hyperthermia heat acclimation and establishing optimised heat acclimation strategies for females are required.

This research project consists of a series of experiments which firstly assessed the repeatability and sensitivity of a running heat tolerance test (RHTT). The phenotypic and cellular adaptation responses to controlled hyperthermia heat acclimation in males and females were then assessed. A further experiment was performed to evaluate a novel heat acclimation strategy combining controlled hyperthermia heat acclimation with a sauna-like exposure in females across a short-term time scale.

The running heat tolerance test demonstrated good agreement, strong correlations and small differences between repeated trials (Figure 1). The typical error of measure values suggested low within-participant variability. The running heat tolerance test was sensitive to changes in heat tolerance and sensitive to the magnitude of changes in heat tolerance between individuals; supporting the use of the running heat tolerance test in future investigations.

Short-term heat acclimation was effective in achieving partial adaptation in males and females. However, females required long-term heat acclimation to establish reductions in thermoregulatory and cardiovascular strain (Figure 2). However, there are no differences in heat shock protein 72 (Hsp72) mRNA increases during heat acclimation between sexes when controlled hyperthermia heat acclimation is utilised (Figure 3). Suggesting that there are no differences in the endogenous criteria to transcribe Hsp72 mRNA via the heat shock response between males and females despite differences in phenotypic adaptations.

Preceding short-term heat acclimation with a passive heat exposure resulted in reductions in thermoregulatory, cardiovascular and perceptual strain. The adaptation pathway was likely mediated in part, by plasma volume expansion and an improved thermoeffector and thermosensitivity response of the sudomotor function (Figure 4).

Together, this evidence supports the notion that special considerations need to be taken when using heat acclimation to attenuate thermoregulatory strain in female athletes prior to training and competing in the heat.

Publications

Mee, J.A. Gibson, O.R. Tuttle, J.A. Taylor, L. Watt, P. Doust, J. and Maxwell, N.S.(2016) Leukocyte Hsp72 mRNA transcription does not differ between males and females during heat acclimation. Temperature. DOI: 10.1080/23328940.2016.1214336

Mee, J.A. Gibson, O.R. Doust, J.D. Maxwell, N.S. (2015) Sex differences in adaptation to short and long term heat acclimation. Scandinavian Journal of Medicine and Science in Sports, 25 (Suppl. 1), 250-258.

Mee, J.A. Doust, J. and Maxwell, N.S. (2015) Repeatability of a running heat tolerance test. Journal of Thermal Biology 49-50, 91-97.

Conference proceedings

Mee, J.A., Doust, J. and Maxwell, N.S. (2016) The running heat tolerance test is a suitable protocol for between-group comparisons of heat tolerance in females. 6th International Conference on the Physiology and Pharmacology of Temperature Regulation, Slovenia, 5–9 December.

Mee, J.A. Peter, S. Doust, J. Maxwell, N.S. (2015). Restricted sweat evaporation preceding short term heat acclimation accelerates adaptation in females. International Conference of Environmental Ergonomics (ICEE), Portsmouth, UK, 28 June - 3 July 2015.

Mee, J.A.Gibson, O.R. Taylor, L. Tuttle, J.A.Watt, P.W. Doust, J. Maxwell, N.S.(2015) Sex comparison of leukocyte Hsp72 mRNA up regulation during heat acclimation. European College of Sport Science (ECSS) Malmo, Sweden, 24 – 27 June 2015.

Mee, J.A. Gibson, O.R. Doust, J. Maxwell, N.S. (2014) Sex differences in adaptation to short and long term isothermic heat acclimation. Training and competing in the heat conference, Aspetar, Doha, Qatar, 23 - 24 March 2014.

Mee, J.A. Doust, J. Maxwell, N.S. (2013). Reliability and validity of a short duration running heat tolerance test. British Association of Sport and Exercise Science (BASES) student conference, Cardiff, UK, 26 – 27 March 2013.