Preparing for Heat Waves - Enhancing Human Thermophysiological Resilience

As the ongoing progression of climate change exposes individuals to elevated temperatures and an escalating frequency of extreme heat events, the risk of more intense and prolonged heat waves raises significant concerns for public health, particularly among vulnerable populations. The physiological response to acute heat stress involves involuntary thermolytic reactions that may strain the cardiovascular system, especially in individuals with pre-existing vulnerabilities. Heat acclimation has been identified as a potential strategy to enhance thermoregulation and mitigate the adverse effects of heat stress. While existing research primarily focuses on athletes and military, this study aims to investigate the impact of a practical heat acclimation strategy, combining passive and active heat exposure, on thermophysiological, cardiovascular and metabolic parameters in healthy overweight adults. The study targets a population at increased risk for heat-related complications, seeking to provide realistic guidelines for broader application when a heat wave appears on the weather forecast.

A non-randomized, within-subject experimental trial will be conducted. The complete study will consist of a screening day, a control week, a heat acclimation week, 3 heat stress test (HST) and meal-test (MT) days (t = -7, 0, and 8 (days)). To avoid a confounding effect of seasonal thermal differences and achieve maximal methodological quality, the study will be performed in the shortest possible timeframe for each participant. Given the seasonal fluctuations in NL, transitioning from April (\~7oC) to June (\~20oC), a crossover design is not applicable as a washout period of approximately 2 months would be needed and the measurements would likely take place in different climatic outdoor conditions. In total, 12 healthy male and female participants will be included in the study. During the no-intervention week (week 1), participants will be asked to continue their normal life at home, without being present at the university. In addition, during week 1, participants will be asked to wear a temperature sensor on their clothing to measure the environment temperature they are exposed to at home. Thereafter, on the intervention week (week 2), participants will be exposed to increasing elevated temperatures and undergo 30 min of low-to-moderate intensity cycling in the heated laboratory room for 7 days. The temperature settings are derived from past heat waves in the Netherlands, ensuring a realistic representation. The low exercise intensity is selected to be accessible to the majority of individuals within the target population. The HST-MT will be conducted on the first visit after the screening to serve as baseline measurements. The same tests will also be repeated before and after the heat acclimation week to determine the physiological response to increasing ambient temperatures. All measurements will take place in the Metabolic Research Unit of Maastricht University (MRUM), the Netherlands.

Eligibility Criteria:
Inclusion Criteria:

* Healthy males and females
* 60 to 80 years
* BMI 25-30 kg/m2
* Sedentary (\<2h exercise/week)

Exclusion Criteria:

* Presence of chronic cardiovascular disease, as determined by medical history and physical examination
* Participating in a structured exercise program
* Taking hot baths/saunas regularly
* Travelled for a prolonged time to destinations with elevated temperatures the last 3 months
* Pre-existing T2DM
* Blood donation within a month of study initiation
* People with low haemoglobin concentration (males: Hb = 13.5-17.5 g/dL, females: Hb = 11.5-15.5 g/dL)
* Recent participation in biomedical study (less than 1 month)
* Using any medication that may influence glucose or lipid metabolism (beta-blockers, thyroid medications, stimulants, antidepressants, anticoagulants)
* Smoking or abuse of alcohol