Food energy is the amount of energy in food that is available through digestion. The values for food energy are expressed in kilojoules (kJ) or food Calories (kcal).
One food Calorie (1 kcal or 1,000 calories) is the amount of digestively available food energy (heat) that will raise the temperature of one kilogram of water one degree Celsius. Some advocate the convention of the capitalizing the C in these so that one food Calorie is equal to 1000 lowercase calories, but that convention is not generally followed. The large Calorie is sometimes abbreviated kcal, to indicate clearly that is 1000 times as large as the small calorie formerly common in chemistry and physics usage. Consequently, the prefix kilo- is not used with large Calories. Food calories are also more specifically called kilocalories on the basis of the small calorie usage. This term, which makes it clear that large calories are intended, is widely used by professional nutritionists when speaking in terms of calories rather than joules, but the term kilocalorie for the large calorie is less often used by laypersons.
The unit from the International System of Units, the joule and the more practical kilojoule (kJ), is becoming more common and is the unit officially recommended by the World Health Organization and other international organizations.[1] One (food) Calorie is approximately equal to 4.1868 kJ. In some countries (Australia, for example) only the kilojoule is normally used on food packaging.
Some types of food contain more food energy per gram than others: fats and ethanol have particularly high values for food energy density: 9 and 7 kcal/g (38 and 30 kJ/g), respectively. Sugars and proteins have about 4 kcal/g (17 kJ/g).
Each food item has a specific metabolizable energy intake (MEI). Normally this value is obtained by multiplying the total amount of energy contained in a food item by 85%, which is the typical amount of energy actually obtained by a human after the digestive processes have been completed.
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The following process details how to measure food energy, as specified by the United States Department of Agriculture (USDA) in the early 1900s:
The particular food being measured must be burned in a calorimeter, so that the heat released from the food can be accurately measured. This amount is used to ascertain the G.E.V. of the specified food. This number is then multiplied by, usually, 85%; which represents the loss happening during human digestion.
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.[2] 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"[3], often as a substitute for the name of the quantity being measured, food energy; an additional kilojoules figure is optional and is rarely used. The energy content of food is usually given on labels for 100 g and/or for what the manufacturer claims is a typical serving 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.[4] 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. Moreover, not all food energy eaten is actually resorbed by the body (fecal and urinal losses). Instead, standardized chemical tests or an analysis of the recipe using reference tables for common ingredients[5] are used to estimate the product's digestible constituents (protein, carbohydrate, fat, etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities:[6]
| food component | energy density | |
|---|---|---|
| kcal/g | kJ/g | |
| fat | 9 | 38 |
| ethanol (alcohol) | 6.3 | 26 |
| proteins | 3.2 | 13 |
| carbohydrates | 4 | 17 |
| organic acids | 2.1 | 9 |
| polyols (sugar alcohols, sweeteners) | 2.4 | 10 |
Other substances found in food (water, non-digestible fibre, minerals, vitamins) do not contribute to this calculated energy density.
Recommended daily energy intake values for young adults are: 2500 kcal/day (10 MJ/day) for men and 2000 kcal/day (8 MJ/day) for women. Children, sedentary and older people require less energy, physically active people more. In addition to physical activity, increased mental activity has been linked with moderately increased Brain energy consumption[7].
Energy intake to the body that is not used up is mostly stored as fat in the fat tissue. Consider the following theoretical calculation.
This assumes that all the weight gained and lost is in the form of fat. In reality, this is a mixture of protein, carbohydrates etc. (in muscle and organ etc).
The conversion efficiency of food energy into physical power depends on the form of energy source (type of food) and on the type of physical energy usage (e.g. which muscles are used, whether the muscle is used aerobically or anaerobically). In general, the efficiency of muscles is rather low, and roughly speaking, only about 15% of the food energy is actually converted into mechanical energy. For example, when calculating food energy burnt per unit time gym equipment manufacturers multiply the value of physical power by a factor of eight (assuming 12.5% efficiency). Thus if an exercise bike registers a 150-watt physical power output it might display 17 kcal/min as the rate of food energy burnt per unit time (since 150 W × 8 = 1200 W ≈ 17 kcal/min).