Abstract
Biological organisms are open thermodynamic systems in which internal structure and function arise from a network of heat- and entropy-producing processes, requiring external export of heat and entropy to the environment. System stability, including health and healing, is hypothesized to relate to the mass-normalized rate of entropy production and export, and the capacity to augment both when required. Using a novel methodological framework that allows the continuous measurement of heat fluxes and body temperature, we can estimate internal entropy production and external entropy export in humans during exercise under heat stress. We report the impairment of entropy export leading to entropy accumulation in association with increasing age, reduced fitness, and the presence of diabetes. In all conditions, impairment was also found to be negatively correlated with the VO(2) max fitness measure. Our analyses make use of direct calorimetry to quantify rates of heat loss and indirect calorimetry to measure metabolic heat production and core temperature in real time. Our results highlight the potential relevance of the entropy balance to the definition of health and open the possibility of designing novel therapeutic approaches based on nonequilibrium thermodynamics to improve patient care.