Sure, the body loses some energy processing calories (TEF, thermic effect of food). But it's very efficient. The larger flaw behind "CICO" reasoning is that it

Sure, the body loses some energy processing calories (TEF, thermic effect of food). But it's very efficient. The larger flaw behind "CICO" reasoning is that it often ignores volume, hunger, and mood effects. 1200 calories of spinach will make you gain about as much as 1200 calories of sugar. But the former is basically impossible to eat: spinach comes to 23 calories per 100g, so you would have to eat ~5.017kg, or ~11lbs, to eat 1200 calories (sugar is 0.3kg or 0.6lb). When people say things like "500 calories of spinach = 500 calories of chocolate", well technically they're correct, but it's unfair because nobody is eating 500 calories of spinach. And yes, spinach is an extreme example. You can gain weight on healthy "low calorie" foods like chicken and apples. But you'll be stuffed. Most people, when trying to lose weight on processed foods, will constantly battle hunger and low energy. But when they eat unprocessed foods, they have much less hunger and feel better. Even when people say "I'm eating healthy but not losing weight", usually most of the foods they're eating are still high calorie (e.g. cheese and nuts).