What is a calorie? It’s a unit of energy. The human body is constantly using energy for activities like metabolism, heart beat, brain function, organ function, motion production and body heat creation in the cold. So your body requires a perpetual source of calories (energy) just to stay alive, even while you sleep.
Where does this energy come from? Calories in food and adipose tissue. Essentially, calories indicate how much energy your body will get from eating a particular food, known as caloric intake. The amount of calories in food is derived from its composition, generally the amount of carbohydrate, protein or fat it contains. Each gram of carbohydrate and protein contain 4, while fat contains 9.
On average, a caloric intake of between 2,000 to 3,000 is needed to keep a body running. If you are basically sedentary, go low. If active, go higher. Weight loss occurs if consumption falls below your needs. Or when your body needs more, as is the case with exercise.
Each pound of body fat contains approximately 3,500 calories. So to lose a pound your caloric intake must be in a shortfall of your body’s needs by 3,500 calories. Sounds ominous, but not really. Let’s put the concept to work by using a simple real world situation. Suppose each day you refrain from consuming a single high calorie item (e.g. trade 3 cans of soda for water, large slice of chocolate cake, couple of donuts) and take a walk lasting 30 minutes. You’ll come up short a pound worth of energy each week so long as the rest of your daily caloric intake is kept relatively constant.