When your body releases energy from food, chemically, it is the same process as if you lit that food on fire. The sugars, fats, and proteins are broken down into simpler molecules like water and carbon dioxide. Instead of releasing that energy as hear and light, our cells store it in chemicals to be used later. The amount of energy stored in food is measured in calories. Your body is always burning calories, and everyone has a baseline of how many calories they need daily just to stay alive. If you do nothing and eat more calories than your base line, you will put on weight. If you consume fewer calories you will lose weight. Alternatively, you can increase your activity while keeping your consumption constant which will lead you to burn more calories than you consume and cause you to lose weight