Energy in, energy out. In general our body burns a combination of carbohydrate, as glucose, and fat for fuel. How much of both depends on your body movement and if, or what you have recently. When you make use of more energy than you receive from food and drink, the body burns stored fat and carbohydrates, and then even protein, to fuel your daily actions even if you are not exercising.

Really it happens when people have nothing to eat of course; the body starts to eat itself. Depending on your family history your heredity and the mode you eat and exercise to create this energy shortfall, your body may make a decision to get conservative and go down your metabolic rate to try to hold onto body weight. Some of us look as if we have inherited this trend more than others, the origins of which may be in the early periods of human growth where 'feast or famine' was more or less the rule.