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{{short description|Chemical energy animals derive from food}}
{{Use British English Oxford spelling|date=December 2019}}
'''Food energy''' is [[chemical energy]] that animals derive from [[food]] to sustain their [[metabolism]] and [[muscle|muscular]] activity.<ref name=marsh2020/> This is usually measured in [[joules]] or [[calories]].  

Most animals derive most of their energy from [[aerobic respiration]], namely combining the [[carbohydrate]]s, [[fat]]s, and [[protein in nutrition|protein]]s with [[oxygen]] from [[air]] or dissolved in [[water]].<ref name=ross2000/>  Other smaller components of the diet, such as [[organic acid]]s, [[polyol]]s, and [[ethanol]] (drinking alcohol) may contribute to the energy input. Some [[diet (nutrition)|diet]] components that provide little or no food energy, such as [[water]], [[dietary mineral|minerals]], [[vitamin]]s, [[cholesterol]], and [[dietary fiber|fiber]], may still be necessary for health and survival for other reasons. Some organisms have instead [[anaerobic 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 the [[International System of Units|metric (SI)]] unit of energy, the [[joule]] (J), and its multiple the [[kilojoule]] (kJ); or in the traditional unit of heat energy, the [[calorie]] (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.<ref name=FAO2003/> Thus, for example, fats and ethanol have the greatest amount of food energy per unit mass, {{convert|37|and|29|kJ/g|kcal/g|0|abbr=on}}, respectively. Proteins and most carbohydrates have about {{convert|17|kJ/g|kcal/g|abbr=on|0}}, though there are differences between different kinds. For example, the values for [[glucose]], sucrose, and starch are {{convert|15.57|,|16.48|and|17.48|kJ/g|kcal/g}} respectively. The differing [[energy 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, or [[lactose]] in [[Lactose intolerance|lactose-intolerant individuals]], contribute less food energy. [[Polyol]]s (including [[sugar alcohol]]s) and organic acids contribute {{convert|10|kJ/g|kcal/g|abbr=on}} and {{convert|13|kJ/g|kcal/g|abbr=on}} respectively.<ref name="uk">{{cite web |title=Schedule 7: Nutrition labelling |url=http://www.legislation.gov.uk/uksi/1996/1499/schedule/7/made |website=Legislation.gov.uk |publisher=The National Archives |access-date=13 December 2019 |date=1 July 1996}}</ref>

The energy contents of a food or meal can be approximated by adding the energy contents of its components, though the entire amount of calories calculated may not be absorbed during digestion.

== History and methods of measurement ==

===Direct calorimetry of combustion===

The first determinations of the energy content of food were made by burning a dried sample in a [[calorimeter|bomb calorimeter]] and measuring the temperature change in the water surrounding the apparatus, a method known as direct [[calorimetry]].<ref name=youd2021/>

===The Atwater system===
{{main|Atwater system}}

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 of [[dietary 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.<ref name=CANH1997/><ref name=SciAm/> In particular, the fibre content is excluded. This method is known as the [[Modified Atwater]] system, after [[Wilbur Atwater]] who pioneered these measurements in the late 19th century.<ref name=marsh2020/><ref name=triv2009/>

The system was later improved by [[Annabel Merrill]] and [[Bernice Watt]] of the [[USDA]], who derived a system whereby specific calorie conversion factors for different foods were proposed.<ref>{{cite book|author1=Annabel Merrill|author2=Bernice Watt|title=Energy Values of Food ... basis and derivation|date=1973|publisher=United States Department of Agriculture|url=https://www.ars.usda.gov/ARSUserFiles/80400525/Data/Classics/ah74.pdf|archiveurl=https://web.archive.org/web/20161122060858/https://www.ars.usda.gov/ARSUserFiles/80400525/Data/Classics/ah74.pdf|archivedate=22 November 2016|url-status=dead}}</ref>

== Dietary sources of energy ==
The typical human [[diet (nutrition)|diet]] consists chiefly of carbohydrates, fats, proteins, water, ethanol, and indigestible components such as [[bone]]s, [[seed]]s, and fibre (mostly [[cellulose]]). Carbohydrates, fats, and proteins typically comprise ninety percent of the dry weight of food.<ref>{{cite web|url= http://www.merck.com/mmhe/sec12/ch152/ch152b.html |title=Carbohydrates, Proteins, Nutrition|work=  The Merck Manual}}</ref> [[Ruminant]]s can extract food energy from the respiration of cellulose because of [[bacteria]] in their [[rumen]]s that decompose it into digestible carbohydrates. 

Other minor components of the human diet that contribute to its energy content are organic acids such as [[citric acid|citric]] and [[tartaric acid|tartaric]], and polyols such as [[glycerol]], [[xylitol]], [[inositol]], and [[sorbitol]]. 

Some nutrients have regulatory roles affected by [[cell signaling]], in addition to providing energy for the body.<ref name=jeff2006/> For example, [[leucine]] plays an important role in the regulation of protein metabolism and suppresses an individual's appetite.<ref name=garl2005/>  Small amounts of [[essential 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<ref name=EUtab1990/> and UK regulations,<ref name=UKSI1996/> are:
{| class="wikitable"
|-
! rowspan=2 | Food component
! colspan=2 | [[Specific energy#Energy density of food|Energy density]]
|-
! kJ/g
! kcal/g
|-
| [[Fat]]
| align=center|37
| align=center|9
|-
| [[Ethanol]]
| align=center|29
| align=center|7
|-
| [[Protein (nutrient)|Protein]]s
| align=center|17
| align=center|4
|-
| [[Carbohydrate]]s
| align=center|17
| align=center|4
|-
| [[Organic acid]]s
| align=center|13
| align=center|3
|-
| [[Polyol]]s ([[sugar alcohol]]s, [[sweeteners]]) (1)
| align=center|10
| align=center|2.4
|-
| [[dietary fiber|Fiber]] (2)
| align=center|8
| align=center|2
|}

(1) Some polyols, like [[erythritol]], are not digested and should be excluded from the count.

(2) This entry exists in the EU regulations of 2008,<ref name=EUtab1990/> but not in the UK regulations, according to which fibre shall not be counted.<ref name=UKSI1996/>

More detailed tables for specific foods have been published by many organizations, such as the [[United Nations Food and Agriculture Organization]] also has published a similar table.<ref name=FAO2003/>

Other components of the human diet are either noncaloric, or are usually consumed in such small amounts that they can be neglected.

== Energy usage in the human body ==
{{Main article|Bioenergetics|Energy balance (biology)}}

The food energy actually obtained by respiration is used by the human body for a wide range of purposes, including [[basal metabolism]] of various organs and tissues, maintaining the internal [[body temperature]], and exerting [[muscle|muscular]] force to maintain posture and produce motion. About 20% is used for brain metabolism.<ref name=FAO2003/>

The conversion efficiency of energy from respiration into muscular (physical) [[power (physics)|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 [[Aerobic exercise|aerobically]] or [[Anaerobic exercise|anaerobically]]). In general, the efficiency of muscles is rather low: only 18 to 26% of the energy available from respiration is converted into mechanical energy.<ref name="seiler"/> This low efficiency is the result of about 40% efficiency of generating [[Adenosine triphosphate|ATP]] 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  {{convert|4.3|kcal/h|kJ/h|order=flip|abbr=on}}. For example, a manufacturer of rowing equipment shows calories released from "burning" food as four times the actual mechanical work, plus {{convert|300|kcal|kJ|abbr=on|order=flip}} per hour,<ref name=concept2/> 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" {{convert|4000|kcal|kJ|abbr=on|order=flip}}<ref name=guyt2006/> 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., {{convert|3,500|kcal/lb|kcal/kg|disp=or}}).<ref name=wish1958/>

==Recommended daily intake==
Many countries and health organizations have published recommendations for healthy levels of daily intake of food energy. For example, the United States government estimates {{convert|2000|and|2600|kcal|kJ|abbr=on|order=flip}} needed for women and men, respectively, between ages 26 and 45, whose total physical activity is equivalent to walking around {{convert|1+1/2|to|3|mi|abbr=on|order=flip|round=0.5}} per day in addition to the activities of sedentary living. These estimates are for a "reference woman" who is {{convert|5|ft|4|in|m|2|abbr=on|order=flip}} tall and weighs {{convert|126|lb|kg|order=flip|abbr=on}} and a "reference man" who is {{convert|5|ft|10|in|m|2|abbr=on|order=flip}} tall and weighs {{convert|154|lb|kg|order=flip|abbr=on}}.<ref name=NIHrdi8/> 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.<ref>{{cite web |title=Dietary Guidelines for Americans 2020 - 2025 |url=https://www.dietaryguidelines.gov/sites/default/files/2020-12/Dietary_Guidelines_for_Americans_2020-2025.pdf |website=dietaryguidelines.gov |publisher=USDA & HHS |access-date=17 May 2022 |archive-date=14 May 2022 |archive-url=https://web.archive.org/web/20220514125315/https://www.dietaryguidelines.gov/sites/default/files/2020-12/Dietary_Guidelines_for_Americans_2020-2025.pdf |url-status=dead }}</ref><ref>{{cite web |title=DRI Calculator for Healthcare Professionals |url=https://www.nal.usda.gov/human-nutrition-and-food-safety/dri-calculator |website=usda.gov |publisher=U.S. Department of Agriculture |access-date=17 May 2022}}</ref>

According to the [[Food and Agriculture Organization]] of the [[United Nations]], the average minimum energy requirement per person per day is about {{convert|1800|kcal|kJ|abbr=on|order=flip}}.<ref name=FAO2014/> 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. [https://www.ncbi.nlm.nih.gov/books/NBK235023/]

Older people and those with [[sedentary lifestyle]]s require less energy; children and physically active people require more. Recognizing these factors, Australia's [[National Health and Medical Research Council]] recommends different daily energy intakes for each age and gender group.<ref>{{cite web|url=http://www.nrv.gov.au/energy.htm|title=Dietary Energy|access-date=27 September 2014}}</ref> Notwithstanding, nutrition labels on Australian food products typically recommend the average daily energy intake of {{convert|2100|kcal|kJ|abbr=on|order=flip}}.

The minimum food energy intake is also higher in cold environments. Increased mental activity has been linked with moderately increased [[Brain#Metabolism|brain energy consumption]].<ref name=Lars1995/>

== Nutrition labels ==

[[Image:Nutrition-label.jpg|thumb|right|220px|The nutritional information label on a pack of [[Basmati rice]] in the United Kingdom]]

Many governments [[mandatory labelling|require 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&nbsp;g", or "kJ per package". The units vary depending on country:

{| class=wikitable
|-
! Country !! Mandatory unit (symbol) !! Second unit (symbol) !! Common usage
|-
| [[United States]] || Calorie (Cal)<ref name=CFR101/> || kilojoule (kJ), optional<ref name=CFR101/> || calorie (cal)<ref name=FDA2019/>
|-
| [[Canada]] || Calorie (Cal) {{cn|date=January 2022}} || kilojoule (kJ), optional {{cn|date=January 2022}} || calorie (cal) {{cn|date=January 2022}}
|-
| [[Australia]] and [[New Zealand]] ||  kilojoule (kJ)<ref name=AUNZstd/><ref name=QLDH2017/> || kilocalorie (kcal), optional<ref name=AUNZstd/><ref name=QLDH2017/> || AU: kilocalorie (kcal) {{cn|date=January 2022}}
|-
| [[United Kingdom]] ||  kJ<ref name=UKSI1996/> || kcal, mandatory<ref name=UKSI1996/> || 
|-
| [[European Union]] ||  kilojoule (kJ)<ref name=EU2011/>  || kilocalorie (kcal), mandatory<ref name=EU2011/>  || 
|-
| [[Brazil]] ||  caloria or quilocaloria (kcal)<ref name=BRIN2020/>  ||   || caloria
|}

== See also ==
* [[Basal metabolic rate]]
* [[Food chain]]
* [[Food composition]]
* [[Heat of combustion#Heating value|Heat of combustion]]
* [[Nutrition facts label]]
* [[Satiety value]]
* [[Table of food nutrients]]
* [[List of countries by food energy intake]]

== References ==
{{Reflist|2|refs = 
<ref name=FDA2019>U. S. Food and Drug Administration (2019): "[https://www.fda.gov/food/nutrition-education-resources-materials/calories-menu Calories on the Menu - Information for ] {{Webarchive|url=https://web.archive.org/web/20220120213703/http://www.fda.gov/food/nutrition-education-resources-materials/calories-menu |date=2022-01-20 }}". Online document at the [https://www.fda.gov FDA Website] {{Webarchive|url=https://web.archive.org/web/20130915112715/https://www.fda.gov/ |date=2013-09-15 }}, dated 5 August 2019. Accessed on 2022-01-20.</ref>

<ref name=QLDH2017>{{Cite web|title=What's the difference between a calorie and a kilojoule|url=https://www.health.qld.gov.au/news-events/news/calorie-kilojoule-difference-convert|date=21 February 2017|website=[[Queensland Health]]|language=en-AU|access-date=29 May 2020}}</ref>

<ref name=AUNZstd>{{Cite web|title=Australia New Zealand Food Standards Code – Standard 1.2.8 – Nutrition information requirements|url=http://www.legislation.gov.au/Details/F2018C00944/Html/Text|last=Health|website=www.legislation.gov.au|language=en|access-date=29 May 2020}}</ref>

<ref name=EU2011>European Union Parliament (2011): "[https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:02011R1169-20180101&from=EN Regulation (EU) No 1169/2011] {{Webarchive|url=https://web.archive.org/web/20220111164645/https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:02011R1169-20180101&from=EN |date=2022-01-11 }}" Document 02011R1169-20180101</ref>

<ref name=CFR101>United States Federal Government (1977), "[https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-101 Code of Federal Regulations - Part 101 - Food labeling] {{Webarchive|url=https://web.archive.org/web/20220121072942/https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-101 |date=2022-01-21 }}", from Federal Register 14308, 15 March 1977.</ref>

<ref name=youd2021>Adrienne Youdim (2021): "[http://www.merckmanuals.com/home/disorders_of_nutrition/overview_of_nutrition/calories.html Calories] {{Webarchive|url=https://web.archive.org/web/20130804173559/http://www.merckmanuals.com/home/disorders_of_nutrition/overview_of_nutrition/calories.html |date=2013-08-04 }}". Article in the ''Merck Manual Home Edition'' online, dated Dec/2011. Accessed on 21 February 2022</ref>

<ref name=SciAm>{{Cite news|url=https://www.scientificamerican.com/article/how-do-food-manufacturers/|title=How Do Food Manufacturers Calculate the Calorie Count of Packaged Foods?|work=Scientific American|access-date=8 September 2017|language=en}}</ref>

<ref name=CANH1997>{{cite web|work=Health Canada, [[PDF]] p. 4|year =1997|url=http://www.hc-sc.gc.ca/fn-an/nutrition/fiche-nutri-data/nutrient_value-valeurs_nutritives-eng.php|format=PDF|title=Nutrient Value of Some Common Foods|access-date=25 January 2015}}</ref>

<ref name=marsh2020>Allison Marsh (2020): "[https://spectrum.ieee.org/how-counting-calories-became-a-science How Counting Calories Became a Science: Calorimeters defined the nutritional value of food and the output of steam generators] {{Webarchive|url=https://web.archive.org/web/20220121000718/https://spectrum.ieee.org/how-counting-calories-became-a-science |date=2022-01-21 }}"  Online article on the [https://spectrum.ieee.org IEEE Spectrum] {{Webarchive|url=https://web.archive.org/web/20220120232140/https://spectrum.ieee.org/ |date=2022-01-20 }} website, dated 29 December 2020. Accessed on 2022-01-20.</ref>

<ref name=triv2009>[http://www.newscientist.com/article/mg20327171.200-the-calorie-delusion-why-food-labels-are-wrong.html?full=true "Why food labels are wrong"] {{Webarchive|url=https://web.archive.org/web/20111113173259/http://www.newscientist.com/article/mg20327171.200-the-calorie-delusion-why-food-labels-are-wrong.html?full=true |date=2011-11-13 }} by Bijal Trivedi, ''[[New Scientist]]'', 18 July 2009, pp. 30–3.</ref>

<ref name=UKSI1996>United Kingdom [http://www.legislation.gov.uk/uksi/1996/1499/contents/made The Food Labelling Regulations 1996] {{Webarchive|url=https://web.archive.org/web/20130921164025/http://www.legislation.gov.uk/uksi/1996/1499/contents/made |date=2013-09-21 }} &ndash; [http://www.legislation.gov.uk/uksi/1996/1499/schedule/7/made Schedule 7: Nutrition labelling] {{Webarchive|url=https://web.archive.org/web/20130317110033/http://www.legislation.gov.uk/uksi/1996/1499/schedule/7/made |date=2013-03-17 }}</ref>

<ref name=EUtab1990>{{Cite web |url=http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1990L0496:20081211:EN:PDF |title=Council directive 90/496/EEC of 24 September 1990 on nutrition labelling for foodstuffs |access-date=18 March 2010 |archive-date=3 October 2011 |archive-url=https://web.archive.org/web/20111003093749/http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1990L0496:20081211:EN:PDF |url-status=live }}</ref>

<ref name=BRIN2020>Ministério da Saúde, Brazil (2020): "[https://www.in.gov.br/en/web/dou/-/instrucao-normativa-in-n-75-de-8-de-outubro-de-2020-282071143 Instrução Normativa Nº 75 - Estabelece os requisitos técnicos para declaração da rotulagem nutricional nos alimentos embalados] {{Webarchive|url=https://web.archive.org/web/20220121023715/https://www.in.gov.br/en/web/dou/-/instrucao-normativa-in-n-75-de-8-de-outubro-de-2020-282071143 |date=2022-01-21 }}", dated 2020-10-08, published on Diário Oficial da União on 2020-10-09, page 113.</ref>

<ref name="seiler">Stephen Seiler, [http://home.hia.no/~stephens/effiperf.htm Efficiency, Economy and Endurance Performance] {{Webarchive|url=https://web.archive.org/web/20071221225027/http://home.hia.no/~stephens/effiperf.htm |date=2007-12-21 }} (1996, 2005).</ref>

<ref name=concept2>[http://www.concept2.com/us/support/manuals/pdf/B_UsersManual.pdf Concept II Rowing Ergometer, user manual] {{Webarchive|url=https://web.archive.org/web/20101226185824/http://www.concept2.com/us/support/manuals/pdf/B_UsersManual.pdf |date=2010-12-26 }} (1993).</ref>

<ref name=wish1958>Wishnofsky, M. Caloric Equivalents of Gained or Lost Weight. The American Journal of Clinical Nutrition, (1958).</ref>

<ref name=guyt2006>Guyton A. C., Hall J. E. Textbook of medical physiology, 11 ed., p. 887, Elsevier Saunders, 2006.</ref>

<ref name=jeff2006>{{cite journal | author = Jeffrey S. F. | year = 2006 | title = Regulating Energy Balance: The Substrate Strikes Back | journal = Science | pages = 861–864 }}</ref>

<ref name=garl2005>Garlick, P. J. The role of leucine in the regulation of protein metabolism. Journal of Nutrition, 2005. 135(6): 1553S–6S.</ref>

<ref name=ross2000>Ross, K. A. (2000c) Energy and fuel, in Littledyke M., Ross K. A. and Lakin E. (eds), Science Knowledge and the Environment. London: David Fulton Publishers.</ref>

<ref name=NIHrdi8>US National Institutes of Health (2015): "[http://health.gov/dietaryguidelines/2015/guidelines/appendix-2/ Dietary guidelines] {{Webarchive|url=https://web.archive.org/web/20160301132440/http://health.gov/dietaryguidelines/2015/guidelines/appendix-2/ |date=2016-03-01 }}"</ref>

<ref name=FAO2003>United Nations Food and Agriculture Organization (2003): "[http://www.fao.org/docrep/006/Y5022E/y5022e04.htm FAO Food and Nutrition Paper 77: Food energy - methods of analysis and conversion factors] {{Webarchive|url=https://web.archive.org/web/20100524003622/http://www.fao.org/DOCREP/006/Y5022E/y5022e04.htm |date=2010-05-24 }}". Accessed on 21 January 2022.</ref>

<ref name=Lars1995>[http://www.ingentaconnect.com/content/els/01602896/1995/00000021/00000003/art90017;jsessionid=1vc0khhvsj1vg.alice?format=print Evaluation of a ''mental effort'' hypothesis for correlations between cortical metabolism and intelligence] {{Webarchive|url=https://web.archive.org/web/20121023174100/http://www.ingentaconnect.com/content/els/01602896/1995/00000021/00000003/art90017;jsessionid=1vc0khhvsj1vg.alice?format=print |date=2012-10-23 }}, Intelligence, Volume 21, Number 3, November 1995 , pp. 267–278(12), 1995.</ref>

<ref name=FAO2014>United Nations Food and Agriculture Organization (2014): "[http://www.fao.org/hunger/en/#jfmulticontent_c130584-2 Hunger] {{Webarchive|url=https://web.archive.org/web/20091220202823/http://www.fao.org/hunger/en/#jfmulticontent_c130584-2 |date=2009-12-20 }}". Accessed on 27 September 2014</ref>
}}

== External links ==
* [http://www.ajcn.org/cgi/content/full/79/5/899S?ijkey=f3919ec7617632925bb12e0ffb8deeb08a678686 Is a calorie a calorie?]
* [https://www.nal.usda.gov/human-nutrition-and-food-safety/dri-calculator DRI Calculator for Healthcare Professionals]

{{Food science}}
{{Cellular respiration}}
{{MetabolismMap}}
{{citric acid cycle}}
{{Citric acid cycle enzymes}}

{{DEFAULTSORT:Food Energy}}
[[Category:Nutrition]]
[[Category:Food analysis]]
[[Category:Nutritional physiology]]