See that voice in the back of your head is a little cognitive dissonance. People’s behavior changes based on their diet, but outside of starvation metabolism doesn’t speed up with extra calories they just result in weight gain.

Look all kinds of stuff happens when you go to extremes. But, the human diet has a lot of flexibility which is why we don’t have the one true diet despite a lot of research. At the middle of the healthy range +/- a few percent doesn’t do anything. That’s why so much nutrition research is inconclusive we can’t optimize for an ideal peak of perfect nutrition because close enough gives identical results.

Which is what I am talking about. Take a perfectly reasonable heathy diet that wouldn’t result in long term weight gain or loss and then toss in 100 calories of protein and you get weight gain the same way adding 100 of any other macronutrients or even an even split of all three. That’s true because a perfectly reasonable diet is far from the limits on how much protein you can digest per day.