I wish I could like this more than once.
It has to be a matter of thermodynamics because if that weren't true we'd be talking about a major world-shaking change in our understanding of the universe and how it works. (OK, possibly thermodynamics doesn't apply here, maybe this is something else like, I don't know, maybe the general expansion of the universe over time expressing itself at personal scale, but that would be really extraordinary and require some world-shaking proof, and would not be a safe starting assumption.)
However, it's not simply a matter of thermodynamics, because the thermodynamic model still needs to include what happens once you eat, with consideration that the system itself is changed by the environment, and that food being inside your body is not the same thing as having been digested and incorporated into your body proper. Your gut microbiome still gets to have its say, for example, and of course much of what you eat never becomes part of your body at all, simply exiting instead. And obviously the thermodynamic system defined at the boundary of your body takes as inputs the things you choose to feed it, which choices are heavily influenced by environment, social and otherwise.