I'm more than 100% sure that the kilograms I eat don't get directly converted to feces and urine.
I'm sure some of it goes to regenerating tissue that is constantly being disposed as well, and maybe some of it is also being exhaled.
But I have a feeling that $E=mc^2$ might have something to do with the disposal of ingested kilograms.
We are constantly radiating electromagnetic waves in the nonvisible spectrum, we generate heat, we generate chemical bonds, all of these processes can convert mass to energy.
So I was wondering if $E=mc^2$ is relevant to biology?