Irrespective of the author's take on calories-in calories-out. The argument against it, or at least the simple version of it, that I've heard is the following.

Irrespective of the author's take on calories-in calories-out. The argument against it, or at least the simple version of it, that I've heard is the following. 1. Bomb calorimetry calorie measurements are not a good measurement of the net calories that your body gets out of the food. 2. Just because protein has 4 kCal/g and carbs have 4 kCal/g does not mean that they impact with your body in the same way (same idea when looking between different carbs, e.g. sugar vs complex starches). 3. Different people process and extract different amounts of energies and at different rates than others (e.g. lactose intolerance people, or anecdotally having the same food as another person and only one of you getting a food coma)