Brown adipose tissue (BAT), also known as brown fat, is a specialized type of body fat that is found in small amounts in humans and other mammals. BAT is different from the more common white adipose tissue (WAT), which is primarily used for energy storage.
BAT is activated when the body needs to generate heat to maintain its core temperature. This process, known as thermogenesis, is particularly important in cold weather or when the body is otherwise exposed to cold temperatures. When BAT is activated, it burns stored energy in the form of fatty acids to produce heat.
In contrast, WAT is primarily responsible for storing energy and is not as metabolically active as BAT. White adipose tissue does not play a significant role in thermogenesis.
Therefore, on a cold day, BAT would be expected to be the most active tissue in the body, as it is specialized in generating heat to maintain body temperature.