@skoot1123 you are partially correct a calorie is a unit of energy therefore if you do not give your body enough energy, calories, then it uses stored energy, fat and muscle, to keep you going. Everyone has a maintenance calorie level, this is the calorie level at which calories in = calories burned, this level is varying and can only be estimated, in order for you to lose weight your calorie intake must be less than your calories burned and vice versa if you want to gain weight. However creating to much of a calorie deficit is not recommended, also working out does increase your calories burned which in turn raises your maintenance calorie level. A general rule of thumb I use is no more than a 400 calorie deficit in either direction of your maintenance level.