Could simply mean that fat people are more likely to be sick and sick people are more likely to heat their house better. Most believe that humans evolved in tropical regions where its hot and the seasons are vastly different from what most of us now experience. If I remember correctly, there is the hot and humid typhoon season, the hot and humid rainy season, the hot and humid less rainy season, and the cold season in which it can drop to a bone chilling 83 degrees F. Or something like that. One thing I can tell you, no one has a house heater in the tropics but many do have air conditioners.
If you want to get technical, it's probably most natural for humankind to be in a warm environment where we feel comfortable. If we were designed to be in a cold environ, then we would be comfortable in a cold environ. You don't see the polar bear complaining he is a bit chilly on the ice and thata's because he is designed for the ice. He is comfortable in his native environs but probably gets hot easy if the temps rise too high beyond his normal environs. On our side, we are comfortable in our native warm environs but not comfortable out of it. But we, being humans, have more options like heaters and clothing, so we can live in a much wider variety of environs than most other animals.
