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Calories in the body are a measure of how much energy a given food provides when it is “burned.” The extent to which this occurs comprises the “metabolic rate.” Let us take, for example, a 160 pound woman who expends 2,000 calories per day. If weight-stable, she is consuming 2,000 calories of food per day, and is creating 2,000 calories of heat per day (that is not a coincidence). The calories expended provide energy for all aspects physical life, breathing, daily activity, walking, eating, etc., etc. Extra calories are used to fuel physical activities, as opposed to non-physical activities such as reading… even if you are reading really, really fast. More intense activities like running or climbing stairs require more energy, and thus burn more calories, relative to lower intensity activities like walking or sitting on the couch. These calories can be derived directly from the food in your most recent meal, or from the body’s storage depots. So, basically, you consume calories in the form of food, and expend calories in the form of mechanical work and heat. That is calorie balance.