When it comes to how the human body acquires the energy that allows us to go about our business each day, the basics are similar to those related to any other type of machine. Energy is obtained through the intake and subsequent breaking down of fuel, and in the case of the human body, the fuel is food instead of gasoline as it would be in a car.

How much energy can be obtained from a particular food depends on the make-up of the food itself. We’ve all certainly heard of calories and know that some foods are higher in calories while others are lower, but what exactly does that mean? A calorie is the unit of measurement used to determine how much energy can be obtained from a particular food item. (To get even more specific and scientific, a calorie is the amount of food energy necessary to raise the temperature of 1 kilogram of water by 1 degree Celsius.) The more calories the food item in question has, the more energy it has the potential to provide for the body.

As we all know, when it comes to calories, not all foods are created equal. Some types of foods are far more calorie rich than others. For instance, fats generally contain the highest amount of calorie energy per gram, while food types such as sugars and certain proteins are usually far less dense. However, it is also important to note that all calories are exactly the same. One calorie acquired from protein or carbohydrate provides the body with exactly the same amount of energy that one calorie derived from fat does. The current recommended daily caloric intake for adults is approximately 2500 calories for men and 2000 calories for women, although what’s best for a particular individual can also depend on factors such as body type, age, and activity level.

How many calories a particular body burns also depends on the type of activity being performed at a given moment. We burn calories constantly, all day and all night, even if we’re sleeping or watching reruns of our favorite television programs, but we’re all aware that we burn far more if we’re working out, running, or performing physical labor. The more strenuous the activity, the more energy your muscles and your body will need to burn in order to get the job done. Changes in core body temperature, due to activity or climate, also raise the body’s metabolic rate and call for the burning of higher numbers of calories. Studies have shown that people who reside in areas that feature extremes in hot or cold typically have a metabolic rate that is 5-20% higher than that of people living in temperate climates.

Calories that are not burned immediately are, of course, typically stored in the body as fatty tissue. One pound of body fat contains approximately 4,000 calories. To gain a pound of body fat, you must consume 4,000 more calories than you burn, and to lose a pound of body fat, you must burn that same amount of calories in excess. When you consider the fact that that’s a pretty decent number of actual calories, it’s easy to see why both weight gain and weight loss are so gradual.