I've always personally believed in low-carb diets, but I still agree that calories in/out is the main factor for weight gain. That being said, some calories are not calculated right. I remember reading a study on Almonds that said something like 33% of the calories from Almonds are not absorbed, so "100-calorie" packs of almonds are only 66 calories. In this way, not all calories are the same because the way we calculate them isn't right all of the time. Also, calories in/out doesn't account for foods that are unhealthy for other reasons, or could cause you to eat more than you would otherwise, like HFCS.