physical energy

Food energy

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. 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.

Only carbohydrates (including fiber), fats, proteins, organic acids, polyols, and ethanol contain calories. All foods are made up of a combination of these five nutrients. Everything else in food is non-caloric, including (but not limited to) water, vitamins, minerals, antioxidants, caffeine, spices and natural flavors, and enzymes. Tea and coffee also have no calories without sugar or cream added. Nutritionists usually talk about the number of calories in a gram of a nutrient. Fats and ethanol have the most calories per gram, 9 and 7 kcal/g (38 and 30 kJ/g), respectively. Proteins and most carbohydrates have about 4 kcal/g (17 kJ/g). Carbohydrates that are not easily absorbed, such as fiber or lactose in lactose-intolerant individuals, contribute fewer calories. Polyols (including sugar alcohols) and organic acids have fewer than 4 kcal/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.

Measuring food energy

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. Foodstore is a chemical energy

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", 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. 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 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:

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


All the other nutrients in food are non-caloric and are thus not counted.

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.

Energy usage in the human body

Energy intake to the body that is not used up is mostly stored as fat in the fat tissue. Consider the following theoretical calculation.

  • Fat contains about 3,500 kilocalories per pound ( MJ/kg).
    • If you eat 3,500 kcal more than your body needs, you will put on of fat (assuming 30% digestion efficiency, or less depending on food conversion inefficiency not included in the "body needs" category).
    • If you burn 3,500 kcal more than you eat, you lose about of fat, assuming that only fat is burnt (this is close to 100% since even the waste heat counts toward the 4,000 kcal). However, energy sources can come from catabolism of protein (muscles), and fat may be preferentially saved. The use of different body materials as available must be considered.

Mathematically, it is much easier to lose weight than to gain weight. Energy is even utilized to digest new food and convert body fat into energy. The equations above assume 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 tissue, organs, 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).

Swings in body temperature - either hotter or cooler - increase the metabolic rate burning more energy, Prolonged exposure to extremely warm or very cold environments increases the BMR. People who live in these type of settings often have BMR's that are 5-20% higher than those in other climates. Physical activity also significantly increases body temperature which in turn burns more calories.

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