Food energy ischemical energy that animals andhumans derive fromfood to sustain theirmetabolism andmuscular activity.[1]
Most animals derive most of their energy fromaerobic respiration, namely combining thecarbohydrates,fats, andproteins withoxygen fromair or dissolved inwater.[2] Other smaller components of the diet, such asorganic acids,polyols, andethanol (drinking alcohol) may contribute to the energy input. Somediet components that provide little or no food energy, such aswater,minerals,vitamins,cholesterol, andfiber, may still be necessary for health and survival for other reasons. Some organisms have insteadanaerobic respiration, which extracts energy from food by reactions that do not require oxygen.
The energy contents of a given mass of food is usually expressed in themetric (SI) unit of energy, thejoule (J), and its multiple thekilojoule (kJ); or in the traditional unit of heat energy, thecalorie (cal). In nutritional contexts, the latter is often (especially in US) the "large" variant of the unit, also written "Calorie" (with symbol Cal, both with capital "C") or "kilocalorie" (kcal), and equivalent to 4184 J or 4.184 kJ.[3] Thus, for example, fats and ethanol have the greatest amount of food energy per unit mass, 37 and 29 kJ/g (9 and 7 kcal/g), respectively. Proteins and most carbohydrates have about 17 kJ/g (4 kcal/g), though there are differences between different kinds. For example, the values for glucose, sucrose, and starch are 15.57, 16.48 and 17.48 kilojoules per gram (3.72, 3.94 and 4.18 kcal/g) respectively. The differingenergy density of foods (fat, alcohols, carbohydrates and proteins) lies mainly in their varying proportions of carbon, hydrogen, and oxygen atoms. Carbohydrates that are not easily absorbed, such as fibre, orlactose inlactose-intolerant individuals, contribute less food energy.Polyols (includingsugar alcohols) and organic acids contribute 10 kJ/g (2.4 kcal/g) and 13 kJ/g (3.1 kcal/g) respectively.[4]
The energy contents of a complex dish or meal can be approximated by adding the energy contents of its components.
The first determinations of the energy content of food were made by burning a dried sample in abomb calorimeter and measuring the temperature change in the water surrounding the apparatus, a method known as directcalorimetry.[5]
However, the direct calorimetric method generally overestimates the actual energy that the body can obtain from the food, because it also counts the energy contents ofdietary fiber and other indigestible components, and does not allow for partial absorption and/or incomplete metabolism of certain substances. For this reason, today the energy content of food is instead obtained indirectly, by using chemical analysis to determine the amount of each digestible dietary component (such as protein, carbohydrates, and fats), and adding the respective food energy contents, previously obtained by measurement of metabolic heat released by the body.[6][7] In particular, the fibre content is excluded. This method is known as theModified Atwater system, afterWilbur Atwater who pioneered these measurements in the late 19th century.[1][8]
The system was later improved byAnnabel Merrill andBernice Watt of theUSDA, who derived a system whereby specific calorie conversion factors for different foods were proposed.[9]
The typical humandiet consists chiefly of carbohydrates, fats, proteins, water, ethanol, and indigestible components such asbones,seeds, and fibre (mostlycellulose). Carbohydrates, fats, and proteins typically comprise ninety percent of the dry weight of food.[10]Ruminants can extract food energy from the respiration of cellulose because ofbacteria in theirrumens that decompose it into digestible carbohydrates.
Other minor components of the human diet that contribute to its energy content are organic acids such ascitric andtartaric, and polyols such asglycerol,xylitol,inositol, andsorbitol.
Some nutrients have regulatory roles affected bycell signaling, in addition to providing energy for the body.[11] For example,leucine plays an important role in the regulation of protein metabolism and suppresses an individual's appetite.[12] Small amounts ofessential fatty acids, constituents of some fats that cannot be synthesized by the human body, are used (and necessary) for other biochemical processes.
The approximate food energy contents of various human diet components, to be used in package labeling according to the EU regulations[13] and UK regulations,[14] are:
| Food component | Energy density | |
|---|---|---|
| kJ/g | kcal/g | |
| Fat | 37 | 9 |
| Ethanol | 29 | 7 |
| Proteins | 17 | 4 |
| Carbohydrates | 17 | 4 |
| Organic acids | 13 | 3 |
| Polyols (sugar alcohols,sweeteners) (1) | 10 | 2.4 |
| Fiber (2) | 8 | 2 |
(1) Some polyols, likeerythritol, are not digested and should be excluded from the count.
(2) This entry exists in the EU regulations of 2008,[13] but not in the UK regulations, according to which fibre shall not be counted.[14]
More detailed tables for specific foods have been published by many organizations, such as theUnited Nations Food and Agriculture Organization also has published a similar table.[3]
Other components of the human diet are either noncaloric, or are usually consumed in such small amounts that they can be neglected.
The food energy actually obtained by respiration is used by the human body for a wide range of purposes, includingbasal metabolism of various organs and tissues, maintaining the internalbody temperature, and exertingmuscular force to maintain posture and produce motion. About 20% is used for brain metabolism.[3]
The conversion efficiency of energy from respiration into muscular (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 usedaerobically oranaerobically). In general, the efficiency of muscles is rather low: only 18 to 26% of the energy available from respiration is converted into mechanical energy.[15] This low efficiency is the result of about 40% efficiency of generatingATP from the respiration of food, 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%, one watt of mechanical power is equivalent to 18 kJ/h (4.3 kcal/h). For example, a manufacturer of rowing equipment shows calories released from "burning" food as four times the actual mechanical work, plus 1,300 kJ (300 kcal) per hour,[16] which amounts to about 20% efficiency at 250 watts of mechanical output. It can take up to 20 hours of little physical output (e.g., walking) to "burn off" 17,000 kJ (4,000 kcal)[17] more than a body would otherwise consume. For reference, each kilogram of body fat is roughly equivalent to 32,300 kilojoules of food energy (i.e., 3,500 kilocalories per pound or 7,700 kilocalories per kilogram).[18]
Many countries and health organizations have published recommendations for healthy levels of daily intake of food energy. For example, the United States government estimates 8,400 and 10,900 kJ (2,000 and 2,600 kcal) needed for women and men, respectively, between ages 26 and 45, whose total physical activity is equivalent to walking around 2.5 to 5 km (1+1⁄2 to 3 mi) per day in addition to the activities of sedentary living. These estimates are for a "reference woman" who is 1.63 m (5 ft 4 in) tall and weighs 57 kg (126 lb) and a "reference man" who is 1.78 m (5 ft 10 in) tall and weighs 70 kg (154 lb).[19] Because caloric requirements vary by height, activity, age, pregnancy status, and other factors, the USDA created the DRI Calculator for Healthcare Professionals in order to determine individual caloric needs.[20][21]
According to theFood and Agriculture Organization of theUnited Nations, the average minimum energy requirement per person per day is about 7,500 kJ (1,800 kcal).[22] Although the U.S. has changed over time with a growth in population and processed foods or food in general, Americans today have available roughly the same level of calories as the older generation.[1]
Older people and those withsedentary lifestyles require less energy; children and physically active people require more. Recognizing these factors, Australia'sNational Health and Medical Research Council recommends different daily energy intakes for each age and gender group.[23] Notwithstanding, nutrition labels on Australian food products typically recommend the average daily energy intake of 8,800 kJ (2,100 kcal).
The minimum food energy intake is also higher in cold environments. Increased mental activity has been linked with moderately increasedbrain energy consumption.[24]
Many governmentsrequire food manufacturers to label the energy content of their products, to help consumers control their energy intake. To facilitate evaluation by consumers, food energy values (and other nutritional properties) in package labels or tables are often quoted for convenient amounts of the food, rather than per gram or kilogram; such as in "calories per serving" or "kcal per 100 g", or "kJ per package". The units vary depending on country:
| Country | Mandatory unit (symbol) | Second unit (symbol) | Common usage |
|---|---|---|---|
| United States | Calorie (Cal)[25] | kilojoule (kJ), optional[25] | calorie (cal)[26] |
| Canada | Calorie (Cal)[citation needed] | kilojoule (kJ), optional[citation needed] | calorie (cal)[citation needed] |
| Australia andNew Zealand | kilojoule (kJ)[27][28] | kilocalorie (kcal), optional[27][28] | AU: kilocalorie (kcal)[citation needed] |
| United Kingdom | kJ[14] | kcal, mandatory[14] | |
| European Union | kilojoule (kJ)[29] | kilocalorie (kcal), mandatory[29] | |
| Brazil | caloria or quilocaloria (kcal)[30] | caloria |
Metabolism map | ||
|---|---|---|
 Majormetabolic pathways inmetro-style map. Click any text (name of pathway or metabolites) to link to the corresponding article. Single lines: pathways common to most lifeforms. Double lines: pathways not in humans (occurs in e.g. plants, fungi, prokaryotes).  Orange nodes:carbohydrate metabolism. Violet nodes:photosynthesis. Red nodes:cellular respiration. Pink nodes:cell signaling. Blue nodes:amino acid metabolism. Grey nodes:vitamin andcofactor metabolism. Brown nodes:nucleotide andprotein metabolism. Green nodes:lipid metabolism. | ||
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