My take: oxidation, a slow burning of food. Everything we eat is combustible. We exhale carbon dioxide, a product of the slow combustion.
The body needs to operate in a very narrow temperature range for these processes to take place. If the heat is taken from the body too fast, death results. Think of the victims of the Titanic.
Where did your original heat come from?
It was a gift from your mother. She originally got the heat from her mother. And her mother originally got the heat from her mother. An unbroken chain, on and on, back into the mists of time.
I'm not a mystical or overly spiritual person, but I find it interesting that this chain-of-heat energy is passed to our children, who will carry it forward into time, long after we are cold and dead...