In 1930, Giaja published a paper entitled “Muscle fatigue and thermogenic potential”, in which he found that the maximal thermogenic potential decreases with intense muscle fatigue. This was his first foray into the research of adaptation, which he considered to be essential; in fact, he wrote that physiology could be defined as the science of functional adaptation of living beings. In the same paper, he showed that laboratory rats living at a variable room temperature could survive in temperatures close to 0°C without weight loss or hypothermia, while animals kept at the temperature of their thermal neutrality (32°C) die or experience hypothermia at the same low external temperature. Thus, Giaja concluded that the prolonged maintenance of animals at thermal neutrality significantly decreased their thermogenic potential by lowering the peak metabolism by 30%. In other work, he demonstrated that the growth of animals kept at thermal neutrality is slowed in comparison to that of animals exposed to lower temperatures. In the series of papers that followed, Giaja conducted a comparative study on different homeothermic organisms and demonstrated that the limit of resistance to cold depends on the previous thermal adaptation of the subject. Giaja termed this decrease in resistance to cold during combating low temperatures “thermogenic fatigue”.