Exploring the human thermoneutral zone - A dynamic approach

To date, the position and shape of the human thermoneutral zone (TNZ) remain uncertain. Indications exist that the individual TNZ might be influenced by age, body composition and level of acclimatisation. The objective of the present study was to explore the individual metabolic TNZ, using dynamic thermal conditions to assess both metabolic lower and upper critical temperatures (LCT and UCT) and, secondly, to test the effect of passive mild heat acclimation on the human metabolic TNZ. A dynamic protocol consisting of two experimental conditions was designed: starting from a thermoneutral condition (28.8 ± 0.3 °C), temperature gradually increased to 37.5 ± 0.6 °C during warming (UP) or decreased to 17.8 ± 0.6 °C during cooling (DOWN). For six participants, temperature increased further to 41.6 ± 1.0 °C during UP. Eleven healthy men (19-31 y) underwent UP and DOWN twice, i.e. before and after passive mild heat acclimation (PMHA, 7 days at ~33 °C for 6 h/day). Energy expenditure, body temperatures and heart rate were measured during UP and DOWN. We show that the generally assumed LCT of approximately 28 °C for an average male person does not match the dynamically assessed LCTs in this study, as those were considerably lower in most cases (23.3 ± 3.2 °C pre-acclimation; 23.4 ± 2.0 °C post-acclimation). Distinct inter-individual variation of the dynamic LCT was evident (range pre-PMHA:9.7 °C; post-PMHA:5.4 °C). Regarding the metabolic response to increasing temperatures, only minor or no increases in energy metabolism occurred. PMHA did not significantly change the positioning of the LCTs, but lowered Tcore (pre-PMHA: -0.13 ± 0.13 °C, P = 0.011; post-PMHA: -0.14 ± 0.15 °C, P = 0.026) and affected skin temperature distribution. The applied method allowed for the determination of individual dynamic LCTs, however, distinct metabolic UCTs were not evident in humans. For a better understanding of the human UCT, future studies should incorporate individualised temperature ranges and also a measurement of evaporative heat loss, to allow for a two-factor analysis of both metabolic and evaporative human UCT.