(CN) — How hot is too hot for the human body? The question has been asked and fathomed several times over, but one researcher believes previous studies have overlooked the role metabolism plays in heating the human body in extreme temperatures.
Lewis Halsey from England’s University of Roehampton presented his team’s ongoing research Thursday at a conference for the Society of Experimental Biology, revealing how an “upper critical temperature” exists for humans and likely occurs between 104 and 122 degrees Fahrenheit.
The findings, Halsey believes, could have implications for working conditions, sports, medicine and international travel, particularly as several regions of the world are now hotter than ever.
The upper critical temperature represents the uppermost threshold of temperatures along the human metabolic “thermoneutral zone,” a range of temperatures in which one’s resting metabolic rate is minimal and the body doesn’t need to defend its core temperature.
Previously, studies have established humans can withstand higher core body temperatures without adverse health effects and that human metabolic rates increase when core temperatures dip below the lower critical temperature — about 95 degrees Fahrenheit. However, Halsey’s work in a 2022 analysis reveals inconsistencies amongst research as to whether metabolic rates increase above upper critical temperatures or whether an upper critical temperature even exists.
“Surprisingly limited research has been carried out on the most tractable of species — the human being,” Halsey wrote in his 2022 analysis. “While the lower critical temperature of the human thermoneutral zone is well characterized, a putative metabolic upper critical temperature, beyond which energy expenditure at rest increases, has been far less explored.”
Halsey noted that while there have been numerous studies on the effects of heat on human physiology at rest and in the context of ergonomics and sports performance, none of these studies measured energy expenditure in semi-nude participants at rest in high temperatures. Most studies, he added, assume that a metabolic upper critical temperature has already been established.
To understand whether there’s a temperature at which the body’s metabolic rate increases as a response to physiological upregulation, Halsey’s research team is conducting ongoing experiments to compare the baseline physiology of supine, minimally clothed participants in comfortable room temperatures versus a temperature-controlled environment chamber.
Halsey explained that when researchers bring participants into the environment chamber — which is also controlled for humidity — subjects rest for an hour while researchers measure their heart rate, blood pressure, internal body temperature, sweat rate and, most importantly, metabolic rate. The team found some individuals experienced a noteworthy increase in metabolic rates at relatively high ambient temperatures, for some starting around 104 degrees Fahrenheit.
In terms of how much participants’ metabolism increased, Halsey said his team has documented rate increases anywhere between zero and 28%.
“A lot of people are presenting around 20% and some people less,” Halsey said, adding that compared to his first study — which studied subjects at 122 degrees Fahrenheit and 50% humidity — his current study is looking at subjects at the same temperature with 25% humidity. This time, the increase in metabolic rates is less.
“If you’re not presenting with an increase in metabolic rate, at least you’re not increasing heat production in your body just at the moment when you’re in a high temperature, because that wouldn’t be very good,” Halsey said.
In addition to observing metabolic rate, Halsey’s lab is also focusing on how heat affects heart function above the upper critical temperature and how the effects vary between people with different characteristics like age and physical fitness. Using a state-of-the-art echocardiogram, he said the team has already found considerable changes in heart function responses, particularly between men and women.
“We are steadily building a picture about how the body responds to heat stress, how adaptable it can be, the limits to those adaptations, and — crucially — how varied responses are between individuals,” Halsey said in a previous statement. “In a warming world, this knowledge becomes ever more valuable.”
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