I totally agree, Arno, that energy balance is the master formula which determines a persons weight.
What you’re missing is that metabolic response to food can alter that formula.
High GI meals put us immediately into an anabolic state due to the insulin response and insufficient capacity of tissues to soak up the glut of glycogen. By anabolic I mean, broadly, storing energy as body mass, rather than the muscle building term associated with bodybuilding etc.
Simplifying weight loss to ‘daily calories in<daily calories out’ assumes that we’re perfectly able to switch to fat-burning later after a meal, to square that equation and use up the earlier stored calories. However, the ‘calories out’ part of that equation actually changes. BMR drops, potential working capacity of muscle tissues drops, even the brains energy use slows.
This is even more evident in those of us with poor metabolic flexibility (i.e. insulin resistance and degraded fat metabolism) — a condition brought about, in part, by over-consumption of high glycemic impact foods.