The mechanism that regulates the use of calories in the body is metabolism, the set of chemical reactions that sustain life. It governs the amount of energy you need just to complete your most basic functions. However, this does not include all additional physical exertion, such as exercise, that you might do throughout the day.
Calories are a measure of the energy present in nutrients. In technical terms, a calorie is the amount of energy needed to raise 1 g of water a single degree Celsius. The cells convert the calories from food into a usable form of energy through the process of cellular metabolism. This first requires some kind of precursor molecule, usually the sugar glucose, but fatty acids and amino acids can be break down to produce the same effect.
During energy metabolism, the precursor molecule oxidizes -- essentially meaning it burns -- to synthesize ATP, or adenosine triphosphate, often called the energy currency of life. When one of the three phosphorous units in ATP severs, energy releases from within it. This action is essential to sustain activities such as muscle contractions for the purposes of locomotion and respiration.
The basal metabolic rate, or BMR, is the minimum amount of calories you need to burn to sustain basic bodily functions. The BMR consists of a convergent suite of complex factors such as age, weight, gender, body temperature and genetics, so it is impossible arrive at a figure with complete accuracy. However, it is possible to derive a general figure.
One often-used formula to calculate BMR is the Harris-Benedict Formula. For men, the math formula is 66 plus 6.23 multiplied by weight in lbs., plus 12.7 multiplied by height in inches, minus 6.8 multiplied by age in years. For women it is 655 plus 4.35 multiplied by weight in lbs., plus 4.7 multiplied by height in inches, minus 4.7 multiplied by age in years. This will tell you the minimum calories needed for your body to function.
Calories can come from carbohydrates, proteins or fats. Both carbohydrates and proteins contain four calories per gram. However, fat is more calorie-dense. It contains nine calories per gram, yielding much more energy per unit area. That is why the body stores any excess energy as fat. You cannot store ATP, so when the body no longer has easy calories on hand to perform its functions, it turns to backup sources, such as carbohydrates stored in the muscles, liver and body fat. This will supply the calories you need.