Resting metabolic responses during passive heat stress in healthy humans

In context of global warming, investigating the impact of passive heating on temperature regulation and metabolism is essential to understanding the influence of heat exposure on human health. We aimed to develop and characterize a non-invasive method to assess metabolic responses during acute passive heating in young healthy participants. We hypothesized that passive heating would increase metabolic rate, as reflected by increased oxygen consumption ([Formula: see text]O 2 ), with associated changes in respiratory exchange ratio (RER). Seventeen healthy participants (7M/11F; 21±2yrs, 23±2kg/m 2 ) underwent a 40-minute protocol consisting of baseline (BL; 10-min), passive heat stress using electric heating blankets (20-min), and cooling (10-min). Temperature was measured at the tympanic membrane (ear), axilla, forehead, and expired air. Metabolic responses, including [Formula: see text]O 2 , carbon dioxide production (V·CO 2 ), and RER, were quantified during each stage. Significant rises from BL in axilla and expired temperatures occurred during heating (P=0.0004 and P=0.0005, respectively), while tympanic temperature showed no change (P=0.9418). Cooling led to significant decreases from BL in tympanic, forehead, and expired temperatures (p<0.0001). [Formula: see text]O 2 increased during heating (P=0.0185), with no significant change in V·CO 2 or RER (P=0.5253 and P=0.0554, respectively). Passive cooling led to subsequent increases in V·CO 2 and RER (P=0.0056 and P=0.0123, respectively). Despite minimal changes in measures of core temperature, metabolic responses to passive heating and cooling were observed, including a significant rise in [Formula: see text]O 2 during heating. This increase in [Formula: see text]O 2 during passive heating without a corresponding rise in V·CO 2 suggests a shift toward fatty acid oxidation, minimizing metabolic heat production. Department of Biology, Faculty of Science and Technology, Mount Royal University, Calgary, Alberta, Canada This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.