Simply put, food calories are the measure of energy in food. Every human being needs calories in order to perform very basic functions like breathing and digestion. In addition, people need calories to perform other “optional” tasks, like walking and lifting objects. Food calories provide this energy for bodies to function. When the intake of calories exceed those burned by the body, weight gain occurs. Just the opposite happens when not enough food calories are consumed; weight loss occurs.
A calorie can have two different definitions, whether referring to physics or to food. In scientific communities, this is usually differentiated by capitalizing the word when discussing food calories. Precisely speaking, food calories equal the amount of energy it takes to increase the temperature of a gram of water by 1° Celsius.
There are eight substances in food that contain food calories: fat, ethanol, proteins, carbohydrates, organic acids, polyols, fiber and erythritol. These eight components release energy during cellular respiration, and are therefore counted as food calories. All other components of food do not contain calories, and are not counted when assessing a food's caloric content.
Food calories are mandated by the US Food and Drug Administration (FDA) to be labeled on all food packaging. Other governments throughout the world have different regulations. For example, in the European Union, food packaging must contain the number of kilocalories, which are the same as regular calories in the US, and kilojoules.
A calorie can be measured using a device called a bomb calorimeter, which burns dried food and measures the amount of energy it takes to burn it. This method, however, is imprecise when dealing with food calories, because it doesn't take into account human waste. A certain amount of food is passed through the digestive system and excreted as waste; therefore, it should not be counted. Scientists have devised standards for measuring the calories of certain base components, and most manufacturers will measure the calories of the base components of the food product and add them together when determining the number of calories in the food.
Ideally, a dieting person will consume fewer calories than are spent in energy throughout the day. For an average person, this is anywhere from 1000-1400 calories. Excess calories consumed turn into fat, which is stored energy the body can use if it becomes necessary. When the body has to dip into its fat reserves for energy, weight loss occurs. In contrast, if a person consumes more calories than he or she burns in a particular day, he or she gains weight.