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

Trunk fat mass was negatively and significantly correlated with core temperature cooling rates across all water temperatures tested.

• Correlation coefficients between trunk fat mass and Tc cooling rates were -0.58, -0.76, -0.60, and -0.64 at 18°C, 22°C, 26°C, and across all temperatures combined, respectively (p ≤ 0.01).
• 46 volunteers participated, with subgroups of 18 participants immersed at each water temperature (18, 22, and 26°C).
• Regional body composition was measured by Dual-energy X-ray Absorptiometry (DXA).
• Tc cooling rates were calculated over the first 0.83 h of immersion.

Core temperature cooling rates varied widely across participants and water temperatures during water immersion.

• Tc cooling rates ranged from -0.37 to 0.93°C/h at 18°C water, -0.39 to 1.87°C/h at 22°C water, and -0.13 to 1.13°C/h at 26°C water.
• Immersion durations ranged from 0.83 to 10 h across all conditions.
• Rectal temperature (Tc) at the end of immersion ranged from 35.2 to 38.0°C.
• Participants were immersed from the neck down for up to 10 h.

Individuals with high cooling rates had on average half the trunk fat mass of those with low cooling rates.

• High cooling rate group was defined as those with Tc cooling rates ≥ 0.6°C/h.
• Low cooling rate group was defined as those with Tc cooling rates ≤ 0.25°C/h.
• Those with low trunk fat mass were identified as least capable of defending core body temperature.
• This finding held across the range of mild cold water temperatures studied (18, 22, and 26°C).

Arm fat mass, fat percentage, and surface-to-mass ratio were also negatively and significantly correlated with core temperature cooling rates under most, but not all, conditions.

• These variables were significantly correlated with Tc cooling rates at most water temperature conditions but did not reach significance in all conditions.
• The correlations were negative, indicating that higher arm fat mass, fat percentage, and surface-to-mass ratio were associated with lower cooling rates.
• Trunk fat mass showed the most consistent significant correlations across all three water temperatures and the combined dataset compared to these other measures.
• Surface-to-mass ratio is a geometric body characteristic reflecting the ratio of body surface area to body mass.

The study used a within-subject design with rectal temperature and 10-site mean skin temperature measured continuously during neck-down water immersion.

• Rectal temperature (Tc) and 10-site mean skin temperatures (Tsk) were measured every minute throughout immersion.
• Three separate subgroups of 18 volunteers each were immersed at 18°C, 22°C, and 26°C water temperatures.
• Immersion was conducted from the neck down for up to 10 h.
• Total sample size was 46 volunteers.