Our body needs heat to work, it is the energy to power up the organs to work normally. If the body does not have enough heat, the organs would not work properly. If the body stays at low heat level for a prolonged period of time, the body would have more or less problems, which appear to be illnesses.
Where does the heat come from? We commonly know it comes from food that is processed by the body digest system, so we need regularly take food into our body. Where the heat goes? It normally goes to powering body organs, and daily activities, such as mind activities, physical body activities, which are necessary. However, there are cases for body to unknowingly and unnecessary lose heat, such as eating cold food, drinking cold water, wearing thin cloth in winter, staying AC environment, wind blowing, heavy sweating from intensive physical exercises or labor, staying up late, and not getting up early in the morning. The list could be long for some individuals. Even over-eating food would consume extra heat for body to process food.