Abstract
The most basic requirement for sustaining life is energy. Over the centuries it has been recognized that the living body is warm and begins to cool at the time of death. Referring to great scientists – Harvey, Boerhaave, Black, Priestley, Scheele, Lavoisier, Liebig, Pettenkofer, Rubner, Voit, Lusk, DuBois – the change in paradigm connected with the concepts of ‘life’, ‘substrate intake’ and ‘body heat’ and their underlying natural phenomena is outlined. The late 19th and early 20th century were extremely eventful in the evolution of the understanding of energy and its application to medicine. Definitions and concepts of metabolic body size, diet-induced thermogenesis, and physical activity are described. Further, metabolism adapted to starvation, postaggression metabolism and energy balance corresponding to their historical development and to the recent knowledge are elucidated.