The real argument against sugar (or dietary advice that treats sugar just like any other calorie) is not that "calories in, calories out" is severely flawed but

The real argument against sugar (or dietary advice that treats sugar just like any other calorie) is not that "calories in, calories out" is severely flawed but rather that the body is not a static heat engine (duh) and in fact changes its own metabolic optimization in response to the kind of food we eat. High sugar diets can lead to chronically elevated insulin levels, which (basically) causes the body to lock too much energy away in fat cells, leading to increased sensations of hunger for the same caloric intake.