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| 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. | 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. | ||
| == Nutrition and food labels == | |||
| ] | |||
| The "calorie" has become a common household term, because ]s recommend in cases of ] to reduce body weight by increasing exercise (energy expenditure) and reducing energy intake. Many governments require food manufacturers to label the energy content of their products, to help consumers control their energy intake. In ], manufacturers of prepackaged food must label the nutritional energy of their products in both kilocalories ("kcal") and kilojoules ("kJ"). In the ], the equivalent mandatory labels display only "calories" (meaning kilocalories); an additional kilojoules figure is optional. The energy content of food is usually given on labels for 100 g and for a typical service size. | |||
| The amount of ] in a particular food could be measured by completely burning the dried food in a ], a method known as direct ] . However, the values given on food labels are not determined this way, because it overestimates the amount of energy that the human digestive system can extract, by also burning ]. Instead, standardized chemical tests and an analysis of the recipe are used to estimate the product's digestable constitutents (], ], ], etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities: | |||
| {| class=wikitable | |||
| |- | |||
| ! rowspan=2 | food component | |||
| ! colspan=2 | energy density | |||
| |- | |||
| ! kcal/g | |||
| ! kJ/g | |||
| |- | |||
| | ] | |||
| | align=center|9 | |||
| | align=center|37 | |||
| |- | |||
| | ] (alcohol) | |||
| | align=center|7 | |||
| | align=center|29 | |||
| |- | |||
| | ]s | |||
| | align=center|4 | |||
| | align=center|17 | |||
| |- | |||
| | ]s | |||
| | align=center|4 | |||
| | align=center|17 | |||
| |- | |||
| | ]s | |||
| | align=center|3 | |||
| | align=center|13 | |||
| |- | |||
| | ]s (sugar-free sweeteners) | |||
| | align=center|2.4 | |||
| | align=center|10 | |||
| |} | |||
| Other substances found in food (water, non-digestable fibre, minerals, vitamins) do not contribute to this calculated energy density. | |||
| Recommended daily energy intake values for young adults are: 2500 kcal/] (10 MJ/d, 120 W) for men and 2000 kcal/d (8 MJ/d, 100 W) for women. Children, sedentary and older people require less energy, physically active people more. | |||
| == Energy content == | == Energy content == | ||
Revision as of 08:08, 4 March 2006
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.
Nutrition and food labels
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. Many governments require food manufacturers to label the energy content of their products, to help consumers control their energy intake. In Europe, manufacturers of prepackaged food must label the nutritional energy of their products in both kilocalories ("kcal") and kilojoules ("kJ"). In the United States, the equivalent mandatory labels display only "calories" (meaning kilocalories); an additional kilojoules figure is optional. The energy content of food is usually given on labels for 100 g and for a typical service size.
The amount of food energy in a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry . However, the values given on food labels are not determined this way, because it overestimates the amount of energy that the human digestive system can extract, by also burning dietary fiber. Instead, standardized chemical tests and an analysis of the recipe are used to estimate the product's digestable constitutents (protein, carbohydrate, fat, etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities:
| food component | energy density | |
|---|---|---|
| kcal/g | kJ/g | |
| fat | 9 | 37 |
| ethanol (alcohol) | 7 | 29 |
| proteins | 4 | 17 |
| carbohydrates | 4 | 17 |
| organic acids | 3 | 13 |
| polyols (sugar-free sweeteners) | 2.4 | 10 |
Other substances found in food (water, non-digestable fibre, minerals, vitamins) do not contribute to this calculated energy density.
Recommended daily energy intake values for young adults are: 2500 kcal/d (10 MJ/d, 120 W) for men and 2000 kcal/d (8 MJ/d, 100 W) for women. Children, sedentary and older people require less energy, physically active people more.
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
- Chemical energy
- Food chain
- Screaming jelly babies experiment showing food energy.
References
Health Canada (1997). Nutrient Value of Some Common Foods PDF p. 4 Retrieved Jan. 22, 2006.
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