Food energy: Difference between revisions

'''Food energy''' is [[chemical energy]] that animals and [[human]]s derive from [[food]] to sustain their [[metabolism]] and [[muscle|muscular]] activity.

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]].  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. 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.

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]].

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. 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.

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.

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. [[Ruminant]]s can extract food energy from the respiration of cellulose because of [[bacteria]] in their [[rumen]]s that decompose it into digestible carbohydrates.  

Some nutrients have regulatory roles affected by [[cell signaling]], in addition to providing energy for the body. For example, [[leucine]] plays an important role in the regulation of protein metabolism and suppresses an individual's appetite. 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 and UK regulations, are:

(2) This entry exists in the EU regulations of 2008, but not in the UK regulations, according to which fibre shall not be counted.

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.

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.

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. 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, 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}} 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}}).

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}}. 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.

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}}. 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. 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]].

| [[United States]] || Calorie (Cal) || kilojoule (kJ), optional || calorie (cal)

| [[Canada]] || Calorie (Cal) || kilojoule (kJ), optional || calorie (cal)  

| [[Australia]] and [[New Zealand]] ||  kilojoule (kJ) || kilocalorie (kcal), optional || AU: kilocalorie (kcal)  

| [[United Kingdom]] ||  kJ || kcal, mandatory ||  

| [[European Union]] ||  kilojoule (kJ) || kilocalorie (kcal), mandatory ||  

| [[Brazil]] ||  caloria or quilocaloria (kcal) ||  || caloria