The theory of “Calories In, Calories Out” really doesn’t work, primarily because we are not machines with a single fuel source and only one way to store and burn fuel. Our bodies are hormonal and those hormones have a tremendous effect on how we burn and store fuel, both glycogen and fat.
The calories in, calories out approach assumes that if calories are reduced below the body’s needs, whether through exercise or reduced intake, that your body won’t adapt and reduce its caloric need to match the calories available. The reality is that our metabolisms do adjust and an approach of caloric restriction can set you up for “yo-yo dieting”.
The ideal approach would be to restrict carbohydrates to a level that allows stored fat to be burned for the majority of the day while still eating sufficient calories to maintain a healthy metabolic rate and feed tissues.