Examining the Relationship Between Core Temperature, Heat Balance, and Energy Expenditure

Core body temperature reflects core heat content, which is determined by the balance of heat production and heat loss. Studies and interventions focusing on temperature rarely measure metabolic heat production and heat loss. This study tests whether net heat balance (NHB) in humans can predict core temperature changes and secondarily whether NHB combined with skin surface temperatures (ST) can estimate core temperature. We conducted a laboratory study of healthy volunteers cooled with gel-adhesive circulating water pads with or without treatment with a drug (dexmedetomidine) to prevent shivering. We measured heat flux on the forehead, deltoid, anterior abdominal wall, and above the knee. We measured energy expenditure (W) using indirect calorimetry and core temperature (T) using deep gastrointestinal temperature. Thirteen participants (age 20-51 years; height 160-188 cm; mass 61-101 kg) participated in 21 protocol days. Mean (standard deviation [SD]) NHB ranged from +14 (26) W at baseline to -56 (25) W with drug and cooling pads. NHB predicted change in core temperature 60 minutes later (lagged regression slope: 0.33°C/100W; 95% confidence interval [CI] [0.2, 0.5]) (pseudo r² = 12.81%). Forehead ST had the narrowest limits of agreement [-2.6°C, -2.4°C] for predicting core temperature with a mean bias of -2.5°C. In conclusion, NHB of -100W predicts a 0.33°C/60 minutes decrease in core temperature. Forehead temperature is the most consistent peripheral site to predict core temperature. While a cooling device increases heat loss, energy expenditure (EE) also rises with surface cooling, minimizing NHB, and core temperature change unless a drug is utilized to suppress the increase in EE.