Food energy is the amount of energy obtained from food that is available through cellular respiration.
Food energy is expressed in food calories (labeling: EU kcal, U.S. calories) or kilojoules (kJ). Food calories, or the "calorie" units used often in nutritional contexts, measure amounts of energy 1000 times greater than the units in scientific contexts known also as calories, or gram calories (cal). Food calories are thereby referred to less ambiguously in some formal contexts as kilocalories (kcal) or kilogram calories. One food calorie is equal to 4.184 kilojoules. Within the European Union, both the kilocalorie (kcal) and kilojoule (kJ) appear on nutrition labels. In many countries, only one of the units is displayed.
Carbohydrates, fiber, fats, proteins, organic acids, polyols, and ethanol all release energy during respiration—this is often called 'food energy'.[1] When nutrients react with oxygen in the cells of living things energy is released. A small amount of energy is available through anaerobic respiration. Fats and ethanol have the greatest amount of food energy per mass, 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 less food energy. Polyols (including sugar alcohols) and organic acids have less than 4 kcal/g.
Theoretically there are different ways that food energy could be measured, such as Gibbs free energy of combustion, or the amount of ATP generated by metabolizing the food. But the convention is to use the heat of the oxidation reaction, with the water substance produced being in the liquid phase. In fact, conventional food energy is not even that, but is based on values that take into consideration absorption and production of urea and other substances in the urine. These were worked out in the late 19th century by the American chemist Wilbur Atwater.[2] See Atwater system for more detail.
Each food item has a specific metabolizable energy intake (MEI). Normally this value is obtained by multiplying the total amount of energy associated with a food item by 85%, which is the typical amount of energy actually obtained by a human after respiration has been completed.
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Many governments require food manufacturers to label the energy content of their products, to help consumers control their energy intake.[3] In the European Union, manufacturers of prepackaged food must label the nutritional energy of their products in both kilocalories and kilojoules, when required. In the United States, the equivalent mandatory labels display only "Calories",[4] 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, for a typical serving size (according to the manufacturer), and/or for the entire pack contents. In the USA, it has also been proposed to also display the time needed to burn the food energy contained in the product by running. The burning of the food energy however only makes sense after crossing the 2000kcal/day limit.[5]
The amount of food energy associated with a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry.[6] However, the values given on food labels are not determined in this way. The reason for this is that direct calorimetry also burns the indigestible dietary fiber, and so does not allow for fecal losses (i.e. the fact that not all food eaten is actually absorbed by the body); thus direct calorimetry would give systematic overestimates of the amount of fuel that actually enters the blood through digestion. What are used instead are standardized chemical tests or an analysis of the recipe using reference tables for common ingredients[7] 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 the following standardized table of energy densities.[8][9]
| Food component | Energy density | |
|---|---|---|
| kJ/g | kcal/g | |
| Fat | 37 | 9 |
| Ethanol (alcohol) | 29 | 7 |
| Proteins | 17 | 4 |
| Carbohydrates | 17 | 4 |
| Organic acids | 13 | 3 |
| Polyols (sugar alcohols, sweeteners) | 10 | 2.4 |
| Fiber | 8 | 2 |
All the other nutrients in food are non-caloric and are thus not counted.
The recommended daily energy intake for young adults and men is 2500 kcal (10 MJ) and 2000 kcal (8 MJ) for women. Children, those with a sedentary lifestyle, 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.[10]
The human body uses the energy released by respiration for a wide range of purposes: about twenty percent of the energy is used for brain metabolism, and much of the rest is used for the basal metabolic requirements of other organs and tissues. In cold environments, metabolism may increase simply to produce heat to maintain body temperature. Among the diverse uses for energy, one is the production of mechanical energy by skeletal muscle in order to maintain posture and produce motion.
The conversion efficiency of energy from respiration into mechanical (physical) power depends on the 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: only 18 to 26 percent of the energy available from respiration is converted into mechanical energy.[11] This low efficiency is the result of about 40% efficiency of generating ATP from food energy, losses in converting energy from ATP into mechanical work inside the muscle, and mechanical losses inside the body. The latter two losses are dependent on the type of exercise and the type of muscle fibers being used (fast-twitch or slow-twitch). For an overall efficiency of 20 percent, one watt of mechanical power is equivalent to 4.3 kcal per hour. For example, a manufacturer of rowing equipment shows calories released from 'burning' food as four times the actual mechanical work, plus 300 kcal per hour,[12] which amounts to about 20 percent efficiency at 250 watts of mechanical output. It can take up to 20 hours of little physical output (e.g. walking) to "burn off" 4000 kcal[13] more than a body would otherwise consume.
The differing energy density of foods (fat, alcohols, carbohydrates and proteins) lies mainly in their varying proportions of carbon, hydrogen, and oxygen atoms.
Swings in body temperature – either hotter or cooler – increase the metabolic rate, thus burning more energy. Prolonged exposure to extremely warm or very cold environments increases the basal metabolic rate (BMR). People who live in these types of settings often have BMRs that are 5–20% higher than those in other climates. Physical activity also significantly increases body temperature, which in turn uses more energy from respiration.
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