Comparison of a Novel Condensed Heat Acclimation Programme With a Traditional Longer-term Heat Acclimation Programme

Repeated exposure to heat in a laboratory setting (acclimation) elicits a range of adaptations, which reduce heat illness risk and increase work capacity in the heat. Traditional approaches to heat acclimation require daily heat exposures of 1 to 2 hours over \~7 to 10 consecutive days. Heat acclimation approaches which reduce the number of days to achieve acclimation may have utility. The primary purpose of the proposed research is to determine whether it is possible to achieve a similar degree of heat acclimation to that seen with a traditional longer-term heat acclimation approach by increasing the frequency of heat exposure, utilising multiple daily heat exposures over a smaller number of days. Secondary aims of the research are to examine whether heat acclimation provides cross-adaptation to a hypoxic stressor and whether heat acclimation improves aerobic fitness.

Individuals are often required to operate in adverse conditions and may be exposed to prolonged periods of high ambient heat and humidity. High environmental temperatures impair work capacity and increase heat-illness risk. Repeated exposure to heat in a laboratory setting (acclimation) elicits a range of adaptations, which reduce heat illness risk and increase work capacity in the heat. However, traditional approaches to heat acclimation are time consuming, typically requiring daily heat exposures of 1 to 2 hours over \~7 to 10 consecutive days. This can be logistically difficult to implement and impractical, particularly in situations where an individual must be rapidly deployed. Therefore, heat acclimation approaches which reduce the number of days to achieve acclimation may have utility. In addition, recent research studies have suggested that adaptation to heat may improve tolerance to hypoxia (cross-adaptation) and improve aerobic fitness; these effects may also be beneficial in a military context. Therefore, the primary purpose of the proposed research is to determine whether it is possible to achieve a similar degree of heat acclimation to that seen with a traditional longer-term heat acclimation approach by increasing the frequency of heat exposure, utilising multiple daily heat exposures over a smaller number of days. Secondary aims of the research are to examine whether heat acclimation provides cross-adaptation to a hypoxic stressor and whether heat acclimation improves aerobic fitness. Heat acclimation will be evaluated using a range of whole-body (cardiovascular, thermoregulatory, psychophysiological) and biochemical markers.

Eligibility Criteria:
Inclusion Criteria:

* Male
* Physically active and healthy as determined by pre-participation health screening questionnaire.
* Free of COVID-19 symptoms (loss or change in sense of smell or taste, fever and new, continuous cough) as determined by pre-screening questionnaire.
* Resting diastolic blood pressure between 60 and 90 mmHg; resting systolic blood pressure between 90 and 150 mmHg A resting ECG is required if: individuals are over 30 years of age; they are unfamiliar with exercise of a maximal nature; they have a family history of adverse cardiac events. The ECG is to be reviewed by the Independent Medical Officer.

Exclusion Criteria:

* Current smokers
* Recent (\< 3 months) participation in a formal heat acclimation regimen, or regular frequent heat exposures that may render the participant partially heat acclimated (other than regular exercise)
* Recent (\< 3 months) high altitude (\>1 500m ) sojourn that may render the participant partially altitude acclimated
* Diagnosed cardiovascular, metabolic or respiratory conditions (excluding asthma)
* Prior history of heat illness
* Prior history of collapse or intolerance with exposure to altitude
* Recent blood donation (within 3 months of commencing study)
* Baseline serum sodium level \<135 mmol.L-1
* Any other extant medical condition which may be exacerbated by participation
* Participants with inadequate understanding of English.
* Any volunteers who are currently participating in any other research studies which may influence their responses or the results obtained