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Food energy

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JEREMY'S REPORT/ Food energy is the amount of energy in food that is available through digestion. The values for food energy are expressed in kilocalories (kcal) and kilojoules (kJ).

One calorie is the amount of energy (heat) to raise the temperature of one gram of water one degree Celsius. The magnitude of human energy requirements makes it awkward to use such a small unit, so the convention of the capitalized Calorie is equal to 1000 lowercase calories, and is abbreviated kcal to indicate that is 1000 times as large as the calorie.

The International System of Units unit kilojoule is becoming more common. In some countries (Australia, for example) only the kilojoule is normally used. Some types of food contain more food energy than others: fats and sugars have particularly high values for food energy. One Calorie is approximately equal to 4.1868 kilojoules.

Measuring food energy

In the early twentieth century, the United States Department of Agriculture (USDA) developed a procedure for measuring food energy that remains in use today.

The food being measured is completely burned in a calorimeter so that the heat released through combustion can be accurately measured. This amount is used to determine the gross energy value of the particular food. This number is then multiplied by a coefficient which is based on how the human body actually digests the food.

For example, pure sugar releases about 3.95 kilocalories per gram (16.5 kJ/g) of gross energy but the digestibility coefficient of sugar is about 98% in humans, so the food energy of sugar is 0.98 × 3.95 = 3.87 kilocalories per gram (16.2 kJ/g) of sugar.

Energy content

  • Protein contains about 4 nutritional calories per gram (17 kJ/g)
  • Carbohydrates contains about 4 nutritional calories per gram (17 kJ/g)
  • Fat contains about 9 nutritional calories per gram (38 kJ/g)
  • Alcohol contains about 7 nutritional calories per gram (29 kJ/g)

See also

References

Health Canada (1997). Nutrient Value of Some Common Foods PDF p. 4 Retrieved Jan. 22, 2006.

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