Regional body composition and human core temperature responses to mild temperature water immersion in adults

This project studied the effects of regional body composition on the core temperature responses to water immersion. Forty‐six volunteers participated in the study, with subgroups of eighteen immersed from the neck down in 18, 22, and 26°C water for up to 10 h, respectively. Regional body composition was measured by Dual‐energy X‐ray Absorptiometry. Rectal temperature (Tc), and 10‐site mean skin temperatures (Tsk) were measured every minute. Immersion durations ranged from 0.83 to 10 h, and Tc ranged from 35.2 to 38.0°C at the end of immersion. Tc cooling rates were calculated over the first 0.83 h of immersion. Tc cooling rates varied widely, ranging from −0.37 to 0.93°C/h, −0.39 to 1.87°C/h, and −0.13 to 1.13°C/h at 18, 22 and 26°C water, respectively. The trunk fat mass was negatively and significantly correlated to Tc cooling rates (−0.58, −0.76, 0.60, −0.64, p ≤ 0.01) at 18, 22, 26°C water and across all temperatures combined. The arm fat mass, fat percentage and surface‐to‐mass ratio were negatively and significantly correlated with Tc cooling rates at most conditions, but not all. Individuals with high cooling rates (≥0.6°C/h) had on average half the trunk fat mass of those with low cooling rates (≤0.25°C/h), and those with low trunk fat mass are least capable of defending core body temperature.