How hot is the human body?

How hot is the human body? HOW MUCH BODY HEAT is gained or lost during exercise and/or environmental exposure? This seems like a simple question that should be easily quantified; however, this is an unresolved issue that likely has produced flawed deductions concerning thermoregulatory control and treatment affects on body heat content (3, 8, 13, 15). Body heat content is the product of mean body temperature (T b) and body heat capacity (body mass ϫ tissue specific heat), with the latter being constant for any given body composition (1, 9). Body heat content changes are usually estimated by measuring body temperature changes (thermometry), and values are rarely measured directly by calorimetry, as the latter approach is complicated and requires extensive sophisticated instrumentation (5). In addition, there is a paucity of data comparing T b change values measured by calorimetry with values simultaneously estimated by thermometry. For calorim-etry, T b change is calculated from the difference between measured metabolic heat production (indirect calorimetry) and measured (direct calorimetry) heat exchange with the environment (11). For thermometry, T b changes are estimated from core and shell (skin and superficial tissues) temperature measurements (2). Core temperature and shell temperature measures are weighted by their anticipated size, which varies reciprocally with cutaneous vasodilation and cutaneous vasoconstriction (2). Traditionally, core/shell weightings of 0.9/0.1, 0.79/0.21, and 0.67/0.33 are used in hot, warm/temperature, and cool conditions, respectively. There is no one uniform " core " temperature , and relationships between internal tissue (e.g., organs and skeletal muscle) temperatures can change as a function of physical exercise and environmental exposure (2, 10). It is known that muscle, brain, and other tissue temperatures are often higher than core temperature during physical exercise and environmental heat exposure, but current thermometry equations do not make adjustments for these underestimated core temperatures. Likewise, there is a paucity of data regarding shell temperature gradients during exercise, heat, or cold exposure. Jay and colleagues (6) recently determined that a three-compartment (muscle, core, and skin temperatures) ther-mometry equation predicted T b changes better than the traditional two-compartment (core and skin temperatures) ther-mometry equation during exercise in temperate and warm conditions; but even when including invasive muscle temperature measurements, this three-compartment thermometry equation accounted for only ϳ50% of the variance of T b from calorimetry measurements. In the present study in the Journal of Applied Physiology, Jay and colleagues (7) constructed a two-compartment ther-mometry model that employed …