The effects of hot ambient temperatures on human energetics

Abstract

Environmental conditions have played a huge role in humans’ ability to survive. Humans have evolved to maintain energy balance with a substantial bias towards preserving energy because of the challenge and great energy cost of finding food, shelter and fending off predators. Maintaining a core temperature (Tc) of ~37℃ is central to homeostasis and if humans are exposed to external temperatures that cause Tc to vary, this triggers energy-costly thermoregulatory processes, increases metabolic rate and uses more calories. A central component of thermoregulation, the cardiovascular system is subjected to strain when defending core temperature. This poses a major threat to older populations, particularly females, as their cardiovasculature declines with age. The range of ambient temperatures within which no additional energy is expended to defend Tc is known as the thermoneutral zone (TNZ). Relatively little research has been done into ambient temperatures above the TNZ compared to below it. This thesis investigated whether hot ambient temperatures influences energy expenditure, increases metabolic rate and how age and sex moderates heat- induced responses in cardiac function. Using real- world data, it was found that daily total and basal energy expenditure increases with falling ambient temperatures (mean: ~14℃, range: - 9.3-30.2℃) in all countries, whilst in developed countries total energy expenditure rises with increasing temperatures (mean: 8.78 ℃, range: -9.3-24.1℃). In the lab, resting metabolic rate increased at temperatures above the human thermoneutral zone (at least as low as 40℃ 25% relative humidity). Older adults were found to have a muted, delayed response to heat exposure compared to the young. Females had a greater response to heat exposure and exhibited greater systolic strain. This thesis is the first to provide evidence that human metabolic rate increases above the TNZ and asserts evidence that ambient temperatures influence energy expenditure in real-world conditions. Finally, the findings suggest that
cardiac response to heat exposure differs with sex as well as age, warranting further research.

Date of Award	10 Sept 2024
Original language	English
Awarding Institution	University of Roehampton
Sponsors	University of Roehampton Vice-Chancellor’s Scholarship
Supervisor	Lewis Halsey (Director of Studies) & Lewis Halsey (Co-Supervisor)