Energy in, energy out. The body normally burns a mix of carbohydrate, as glucose, and fat for fuel. How much of either depends on your physical activity and if, or what you have eaten recently. When you use more energy than you take in from food and drink, the body burns stored fat and carbohydrates, and then even protein, to fuel your everyday activities even if you are not exercising. Thats what happens when people starve of course; the body starts to eat itself. Depending on your family history -- your genetics -- and the way you eat and exercise to create this energy deficit, your body may decide to get conservative and drop your metabolic rate to try to hold onto body weight. Some of us seem to have inherited this tendency more than others, the origins of which may be in the early periods of human evolution where 'feast or famine' was more or less the norm.