Is that true, though? Your body needs energy for various tasks and those have different mechanisms of spending the energy. Muscles, for example, move, which creates heat. But that heat is not simply breathed out.
The heat is literally produced by oxidizing (burning) carbon that you then breathe out as carbondioxide.
Producing heat isn’t where the mass goes though - mass is conserved. You only lose mass to energy in a nuclear reaction.
Something has to go in there, if not losing energy to radiant heat transfer, then how e=m(c^2)?
I’m not sure what you mean by in there but yes, the heat would be transferred to the environment.
E=m(c^2) describes how much energy is contained in matter. It’s useful for nuclear reactions, but your body isn’t a nuclear reactor and you aren’t consuming substantial quantities of radioactive isotopes, like uranium ore, that will decay on their own so it isn’t relevant here.