[ Thus, for example, fats and ethanol have the greatest amount of food energy per unit mass, , respectively. Proteins and most carbohydrates have about , though there are differences between different kinds. For example, the values for glucose, sucrose, and starch are 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-intolerant individuals, contribute less food energy. ]Polyol
In organic chemistry, a polyol is an organic compound containing multiple hydroxyl groups (). The term "polyol" can have slightly different meanings depending on whether it is used in food science or polymer chemistry. Polyols containing two, thr ...
s (including sugar alcohol
Sugar alcohols (also called polyhydric alcohols, polyalcohols, alditols or glycitols) are organic compounds, typically derived from sugars, containing one hydroxyl group (–OH) attached to each carbon atom. They are white, water-soluble solids ...
s) and organic acids contribute and respectively.
The energy contents of a complex dish or meal can be approximated by adding the energy contents of its components.
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 bomb calorimeter and measuring the temperature change in the water surrounding the apparatus, a method known as direct calorimetry
In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in ''state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reac ...
.[
]
The 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.[ In particular, the fibre content is excluded. This method is known as the ]Modified Atwater
The Atwater system, named after Wilbur Olin Atwater, or derivatives of this system are used for the calculation of the available energy of foods. The system was developed largely from the experimental studies of Atwater and his colleagues in the ...
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
The United States Department of Agriculture (USDA) is the federal executive department responsible for developing and executing federal laws related to farming, forestry, rural economic development, and food. It aims to meet the needs of com ...
, who derived a system whereby specific calorie conversion factors for different foods were proposed.
Dietary sources of energy
The typical human diet
Diet may refer to:
Food
* Diet (nutrition), the sum of the food consumed by an organism or group
* Dieting, the deliberate selection of food to control body weight or nutrient intake
** Diet food, foods that aid in creating a diet for weight loss ...
consists chiefly of carbohydrates, fats, proteins, water, ethanol, and indigestible components such as bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
s, seed
A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
s, and fibre (mostly cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
). Carbohydrates, fats, and proteins typically comprise ninety percent of the dry weight of food. Ruminant
Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
s can extract food energy from the respiration of cellulose because of bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
in their rumen
The rumen, also known as a paunch, is the largest stomach compartment in ruminants and the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. The rumen's microbial favoring environment al ...
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 and tartaric, and polyols such as glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
, xylitol
Xylitol is a chemical compound with the formula , or HO(CH2)(CHOH)3(CH2)OH; specifically, one particular stereoisomer with that structural formula. It is a colorless or white crystalline solid that is freely soluble in water. It can be classifi ...
, inositol, and sorbitol.
Some nutrients have regulatory roles affected by cell signaling
In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
, in addition to providing energy for the body.[ For example, ]leucine
Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α- amino group (which is in the protonated −NH3+ form under biological conditions), an α- ...
plays an important role in the regulation of protein metabolism and suppresses an individual's appetite.[ Small amounts of ]essential fatty acids
Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them.
Only two fatty acids are known to be essential for humans: alpha-linolenic ac ...
, 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:
(1) Some polyols, like ]erythritol
Erythritol is an organic compound, a four-carbon sugar alcohol (or polyol) with no optical activity, used as a food additive and sugar substitute. It is naturally occurring. It can be made from corn using enzymes and fermentation. Its formula is ...
, are not digested and should be excluded from the count.
(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
The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...
also has published a similar table.[
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
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
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
, and exerting muscular
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...
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
Power most often refers to:
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power
* Power (social and political), the ability to influence people or events
** Abusive power
Power may a ...
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% of the energy available from respiration is converted into mechanical energy. This low efficiency is the result of about 40% efficiency of generating 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 . For example, a manufacturer of rowing equipment shows calories released from "burning" food as four times the actual mechanical work, plus 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" ][ 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., ).][
]
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 needed for women and men, respectively, between ages 26 and 45, whose total physical activity is equivalent to walking around per day in addition to the activities of sedentary living. These estimates are for a "reference woman" who is tall and weighs and a "reference man" who is tall and weighs .[ 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
The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...
of the United Nations
The United Nations (UN) is an intergovernmental organization whose stated purposes are to maintain international peace and security, develop friendly relations among nations, achieve international cooperation, and be a centre for harmoniz ...
, the average minimum energy requirement per person per day is about .[
Older people and those with ]sedentary lifestyle
Sedentary lifestyle is a Lifestyle (social sciences), lifestyle type, in which one is physically inactive and does little or no physical movement and or exercise. A person living a sedentary lifestyle is often sitting or lying down while enga ...
s require less energy; children and physically active people require more. Recognizing these factors, Australia's National Health and Medical Research Council
The National Health and Medical Research Council (NHMRC) is the main statutory authority of the Australian Government responsible for medical research. It was the eighth largest research funding body in the world in 2016, and NHMRC-funded rese ...
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 .
The minimum food energy intake is also higher in cold environments. Increased mental activity has been linked with moderately increased brain energy consumption.[
]
Nutrition labels
Many governments 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 g", or "kJ per package". The units vary depending on country:
See also
* Atwater system
* Basal metabolic rate
Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest. It is reported in energy units per unit time ranging from watt (joule/second) to ml O2/min or joule per hour per kg body mass J/(h·kg). Pro ...
* Calorie
* Chemical energy
* Food chain
A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), de ...
* Food composition
* Heat of combustion
* Nutrition facts label
* Table of food nutrients The tables below include tabular lists for selected basic foods, compiled from United States Dept. of Agriculture (USDA) sources. Included for each food is its weight in grams, its calories, and (also in grams,) the amount of protein, carbohydrates, ...
* List of countries by food energy intake
Food consumption is the amount of food available for human consumption as estimated by Our World in Data. However, the actual food consumption may be lower than the quantity shown as food availability depends on the magnitude of wastage and los ...
References
External links
Is a calorie a calorie?
DRI Calculator for Healthcare Professionals
{{DEFAULTSORT:Food Energy
Nutrition