Comment by naijaboiler

this. Yes the physics of weight is ridiculously straight forward. Its first law of thermo (calories in = calories out). The biology is incredibly complex and that's before we get to the pyschological, to the point of making CICO too overly simplistic to describe actual practical reality. The body is not some perfect energy converting mechanical machine.

People also forget "metabolism doesn't change with age" narrative is a based on a paper that used population-wide statistical model. That approach measures "average treatment effect" and is directionally useful in general but incredibly unhelpful in addressing individual differences and nuances. Something is true on average does not mean it is true "for every" or even "for any"