Encyclopedia of Food & Culture | Calorie

CALORIE. The calorie is a unit for measuring heat energy, and it is usually used as the unit for food energy and of energy expenditure. Media and lay attention to food, exercise, and health, as well as the greater prevalence of obesity during the past few decades, has resulted in a cultural preoccupation with caloric intake and expenditure in industrialized nations. Heat is that which produces a change in temperature. Heat was formerly regarded as a substance called "caloric," but it came to be viewed as the random motion of molecules.

The calorie has traditionally been defined as the amount of heat required to raise the temperature of 1 gram of water by 1.8°F (1.0°C), usually defined as from 58.1°F to 59.9°F (14.5°C to 15.5°C), under normal atmospheric conditions. Because electrical measurements can be standardized more accurately than heat measurements, a calorie is officially defined as equivalent to 4.186 joule. A joule is defined, in "force × distance" units, as 1 Newton meter, which is equal to (1 kg m/s²) × (1m) or 1 kg m²/s². Energy values are expressed as joules when the Système International d'Unités, which is recommended for all scientific purposes, is required.

Food energy values and energy expenditures are commonly expressed as the number of kilocalories (kcal). One kcal is equal to 1000 calories or 4.186 kJ or 0.004186 MJ. Although the terms "calorie" and "large calorie" have frequently been used in place of kilocalorie in the nutrition literature and for food labeling purposes, these alternative terms are confusing, and their use is discouraged.

Measurement of Energy Values of Foods

The energy in foods is present as chemical energy; it can be measured by the heat evolved when the food is oxidized or combusted. Although energy transformations normally involve friction and heat conduction, which cause the changes of one form of energy to another to be incomplete, various forms of energy normally can be converted completely to heat. The caloric value of a food may be determined by burning weighed samples of the food in an oxygen atmosphere in an apparatus called a calorimeter, which is designed to allow measurement of the heat released by combustion of the fuel or food. The total amount of heat produced or consumed when a chemical system changes from an initial state to a final state is independent of the way this change is brought about (the law of Hess or the law of constant heat sums). Thus the complete oxidation of a compound, such as glucose, to CO₂ and H₂O produces the same amount of heat whether the process is carried out in a calorimeter or by metabolism within the body.

Heats of combustion are not accurate reflections of the amount of energy available to the body, however, because the body does not completely absorb and metabolize ingested nutrients. The energy lost in the excreta (feces and urine) must be subtracted from the total energy value of the food to obtain the amount of energy available to the body from consumption of the food. The caloric values of foods reported in food composition tables are "physiological fuel values," also referred to as "available energy" or "metabolizable energy" values. They are not total energy values.

Physiological Fuel Values of Foods

The physiological fuel value of a food or a food component may be determined by measuring the heat of combustion of the food in a calorimeter and then multiplying the heat of combustion by correction factors for incomplete digestion and incomplete oxidation of the food in the body. In about 1900, Wilbur Olin Atwater and his associates at the Connecticut (Storrs) Agriculture Experiment Station used this approach to determine the physiological fuel values of a number of food components (i.e., the protein, fat, and carbohydrate isolated from various foods). They determined factors appropriate for individual foods or groups of foods, and they proposed the general physiological fuel equivalents of 4.0, 8.9, and 4.0 kcal per gram of dietary protein, fat, and carbohydrate respectively for application to the mixed American diet. These factors are commonly rounded to 4, 9, and 4 kcal per gram (17, 36, and 17 kJ per gram) respectively for protein, fat, and carbohydrate. The conversion factors determined by Atwater and his associates remain in use in the twenty-first century, and energy values of foods are calculated using these factors. The energy values (physiological fuel values) reported in food composition tables are commonly estimated by determination of the proximate composition of each food (i.e., the water, protein, fat, carbohydrate, and ash contents) followed by multiplication of the amount of each energy-yielding component by the appropriate conversion factor.

See also Caloric Intake; Dietary Assessment; Nutrition.

Martha H. Stipanuk