Well, look. Painting is just redistributing colored fluids onto two-dimensional surfaces. Simple, right? We know all we need to know about painting now.

Except of course we don’t. Not nearly.

So the next question is: what determines how much you eat, and what determines how much energy you burn? And the answer, to both, is it’s really complicated. So the “weight gain does not violate thermodynamics” observation is close to useless.

What controls food consumption? Why don’t you balloon ridiculously or starve away to nothing — it’s mostly handled automatically by “feeling hungry”, but what causes that? You eat 80–100 kilograms of fat-equivalent energy per year. How the HECK do you balance that year after year without messing up by a few kilograms one way or the other every year? If you’re an athlete, you can easily double or even triple that. How do athletes not waste away to nothing?! How does hunger work, and how does satiety work? What causes net metabolism of fat stores without a concomitant increase in hunger that will lead them to be replaced? If you can’t answer questions like this — because these are things that impact energy balance — then you can’t even get started.

And noting that “there exists an energy balance” and then throwing simplistic ideas at it is tempting — so tempting that the entire field has done mostly that for most of its existence — and wholly inadequate. Indeed it can lead to completely backwards advice, like “avoid calorie-dense foods”, because that only would necessarily be good advice if the limiting factor was how much you could stuff in your mouth or stomach, and it’s not.