"It's not obvious, actually" Yes it is. Changes in metabolism are a second-order effect, at most. There is only one first-order effect: fat = calories in - cal

"It's not obvious, actually" Yes it is. Changes in metabolism are a second-order effect, at most. There is only one first-order effect: fat = calories in - calories out. Once that equation is OK, one can go look at all the other factors - BMR, how macros are split, genetics, ...