Nutritionists measure the energy content of food today in both "calories" (large calories) or "kilojoules" (kJ), a usage widely known in the general population and (confusingly) often referred to as a "kilocalorie" as though small calories were being used. Sporadic attempts have been made, particularly in the United States, to write the kilogram calorie capitalized as Calorie and abbreviated as Cal, to alleviate the confusion with the chemical definition of the term "calorie" as a small calorie. This alternate notation has no legal standing, and is not endorsed by any standards body.
The amount of food energy in a particular food is measured by completely burning the food in a bomb calorimeter, a method known as direct calorimetry [1]. For example, dietary fat has about 9 kcal/g, while proteins and carbohydrates have about 4 kcal/g. Recommended daily energy intake values for average adults are 2500 kcal/d (10 MJ/d) for men and 2000 kcal/d (8 MJ/d) for women. We consume calories in the form proteins, fats, and carbohydrates, three basic nutrients that are necessary for life. There are three other nutrients, vitamins, minerals, and water, are necessary for bodily function but do not contribute any calories to our daily diet.
The "calorie" has become a common household term, because dietitians recommend in cases of obesity to reduce body weight by increasing exercise (energy expenditure) and reducing energy intake.