My dad could tolerate long periods of time outside during the winter while wearing shorts and a t-shirt. He told me he started wearing warmer-weather clothing during the winter, and eventually his body simply adapted.
Is this possible, and if so, what is the mechanism behind it?
Your dad has the right of it. Unfortunately, the literature on human cold adaptation is not very lay-person friendly. US military publications seem to provide the most accessible information on the subject:
Habituation, the most common pattern observed in both acclimatization and acclimation studies, is characterized by a blunted shivering and vasoconstrictor response to cold exposure. Habituation appears to require only brief, intermittent cold exposures to he induced, and can develop when only small body regions are exposed unprotected to cold. It allows extremity skin temperatures to be maintained higher during cold exposure. The higher skin temperatures coupled with the absence of shivering are advantageous in that manual dexterity and comfort are enhanced. In one acclimation study in which subjects were exposed to moderate cold conditions for a prolonged period, a metabolic form of cold acclimation appeared to develop. This adaptation was characterized by an enhanced shivering thermogenesis during cold exposure. When individuals acclimatize or acclimate to cold conditions severe enough to repeatedly cause a significantly body temperature fall, an insulative pattern of adaptation develops, characterized by enhanced mechanisms for body heat conservation. The mechanisms determining the pattern of adaptation to chronic cold exposure appear related to type of cold exposure conditions, the amount of body heat lost and the degree to which shivering thermogenesis compensates for heat loss and defends body temperature.
Humans get significantly better at handling heat or cold by repeated exposure to the extreme, and working in those extreme conditions.
Yes, you can. This effect is to do with the brown fat activation(BAT) in the body.
To gain a deeper understanding of how brown fat affects metabolism, a team led by Dr. Shingo Kajimura at the University of California, San Francisco, carefully analyzed blood levels of glucose, fats, and amino acids before and after activation of brown fat in 33 healthy young men (average age about 23 years old). The work was funded in part by NIH’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Results were published online on August 21, 2019, in Nature. Based on measurements taken at normal room temperature, the team classified 17 of the men as having high brown fat activity and 16 as low activity. They then exposed the men to a temperature cold enough to activate their brown fat (but not low enough to cause them to shiver) for two hours. Unexpectedly, the team found that the men with high brown fat activity had reduced levels of compounds called branched chain amino acids (BCAAs) in their bloodstreams. This change wasn’t seen in the men with low brown fat activity. Experiments in obese mice confirmed that cold exposure reduced BCAAs in rodents with brown fat, too.
(https://www.nih.gov/news-events/nih-research-matters/how-brown-fat-improves-metabolism)
One of the leading experts in this field is a dutch guy called Wim Hof.
