The thermoneutral zone in women takes an “arctic” shift compared to men
Robert J. Brychta, Suzanne McGehee, Shan Huang, Brooks P. Leitner, Courtney J. Duckworth, Laura A. Fletcher, Katherine Kim, Thomas M. Cassimatis, Nikita S. Israni, Hannah J. Lea, Taylor N. Lentz, Anne E. Pierce, Alex Jiang, Samuel R. LaMunion, R Jørgensen Thomas, Asuka Ishihara, Amber B. Courville, Shanna Yang, Marc L. Reitman, Aaron M. Cypess, Kong Y. Chen
Abstract
Conventionally, women are perceived to feel colder than men, but controlled comparisons are sparse. We measured the response of healthy, lean, young women and men to a range of ambient temperatures typical of the daily environment (17 to 31 °C). The Scholander model of thermoregulation defines the lower critical temperature as threshold of the thermoneutral zone, below which additional heat production is required to defend core body temperature. This parameter can be used to characterize the thermoregulatory phenotypes of endotherms on a spectrum from “arctic” to “tropical.” We found that women had a cooler lower critical temperature (mean ± SD: 21.9 ± 1.3 °C vs. 22.9 ± 1.2 °C, P = 0.047), resembling an “arctic” shift compared to men. The more arctic profile of women was predominantly driven by higher insulation associated with more body fat compared to men, countering the lower basal metabolic rate associated with their smaller body size, which typically favors a “tropical” shift. We did not detect sex-based differences in secondary measures of thermoregulation including brown adipose tissue glucose uptake, muscle electrical activity, skin temperatures, cold-induced thermogenesis, or self-reported thermal comfort. In conclusion, the principal contributors to individual differences in human thermoregulation are physical attributes, including body size and composition, which may be partly mediated by sex.