Calorie is a form of heat measurement. It is the amount of energy that is required to raise one milliliter of water by one degree Celsius. Calorie measures the amount of heat or energy that is released when you burn food down.
Calories are burnt through a complicated process that oxidizes food to its basic components. For example, if you burn a sugar, it will turn into carbon dioxide. However in the body, the food burnt is converted to forms of energy.
So therefore when you consume food, the food is stored as body fat or glycogen (stored in liver and muscles) to be burnt for energy. Your body's preference is to burn glycogen first than body fat when you expand energy such as exercising.
The more muscle mass you have, the more calories you will burn. This is because muscle cells are metabolically active where fat cells are not because fat cells do not have much function and are thus not active.
Different food type carries different amount of calories. There are about 4 calories in a gram of protein or carbohydrate and about nine calories in a gram of fat and naturally, the more fat you eat the more calories your body will store.
Some people raise their metabolic rate by eating several small meals a day instead of three big meals. By eating more frequently, your insulin level is also regulated and less insulin will then be converted to body fat.
So how can you burn more calories and further raise your metabolism to naturally burn more calories? Well, build more muscles as explained above, eat frequent small meals and exercise to expand energy and thus burning calories in the process.
