>Evolutionarily, it wouldn’t make sense for our systems to evolve features that discard ingested calories or burn calories without good reason. Food wasn’t plentiful for the vast majority of our evolution.

I don't understand the argument. I agree with you that the human body seems to have several mechanisms (in terms of behavior changes) for reducing food consumption and/or up-regulating calorie expenditure to avoid gaining excess weight. And presumably there were excellent evolutionary reasons for humans to evolve these mechanisms, even though food was often in short supply! (As someone who has spent time trying to do physical activity while overweight, I can attest to this.)

But then you say that it wouldn't make sense for humans to evolve other physical mechanisms (such as changing digestion efficiency, fat storage efficiency) to accomplish those same well-motivated goals. Why not? I'm genuinely curious.