Food energy

Food energy is chemical energy that animals (including humans) derive from their food to sustain their metabolism, including their muscular activity. ^[1]

Most animals derive most of their energy from aerobic respiration, namely combining the carbohydrates, fats, and proteins with oxygen from air or dissolved in water.^[2] Other smaller components of the diet, such as organic acids, polyols, and ethanol (drinking alcohol) may contribute to the energy input. Some diet components that provide little or no food energy, such as water, minerals, vitamins, cholesterol, and fiber, may still be necessary to 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 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 always 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 mass, 37 and 29 kJ/g (9 and 7 kcal/g), respectively. Proteins and most carbohydrates have about 17 kJ/g (4 kcal/g).

The energy contents of a complex dish or meal can be approximated by adding the energy contents of its components.

Measuring the energy content of food

Direct calorimetry of combustion

The first determinations of the energy content of food was determined by burning a dried sample a bomb calorimeter and measuring a temperature change in the water surrounding the apparatus, a method known as direct calorimetry.^[4]

For organic substances, the value obtained by this method depends mainly in proportions of carbon, hydrogen, and oxygen atoms. Namely, combustion of one gram of a substance with elemental formula C_cH_hO_oN_n, with the resulting water in liquid form, will produce about

$${\displaystyle 420\,\mathrm {kJ} \cdot {\frac {c+0.3h-0.5o}{12c+h+16o+14n}},}$$

to a good approximation (±3%).^[5] Theoretically, one could compute this value from the Gibbs free energy of the substance and its combustion products.

Dietary sources of energy

The typical human diet consists chiefly of carbohydrates, fats, proteins, water, ethanol, and indigestible components such as bones, seeds, and fiber (mostly cellulose). Carbohydrates, fats, and proteins typically comprise ninety percent of the dry weight of food.^[9] Ruminants can extract food energy from the respiration of cellulose because of bacteria in their rumens 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, xylitol, inositol, and sorbitol.

Some nutrients have regulatory roles affected by cell signaling, in addition to providing energy for the body.^[10] For example, leucine plays an important role in the regulation of protein metabolism and suppresses an individual's appetite.^[11] 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 ^[12]and UK regulations, ^[13] are:

(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,^[12] but not in the UK regulations, according to which fibre shall not be counted.^[13]

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.^[3]

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, and exerting muscular 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 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.^[14] 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 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,^[15] 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)^[16] 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 (7,700 kcal/kg)).^[17]

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 recommends 10,900 and 8,400 kJ (2,600 and 2,000 kcal) for men and women, respectively, between ages 31 and 35. These recommendations assume the light physical activity associated with typical day-to-day life, including walking about 2.5 to 5 km (1+1⁄2 to 3 mi) per day at 5 to 6 km/h (3 to 4 mph). ^[18]

According to the Food and Agriculture Organization of the United Nations, the average minimum energy requirement per person per day is about 7,500 kJ (1,800 kcal).^[19]

Older people and those with sedentary lifestyles 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.^[20] 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 increased brain energy consumption.^[21]

Nutrition labels

The nutritional information label on a pack of Basmati rice in the United Kingdom

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
• Calorie
• Chemical energy
• Food chain
• Food composition
• Heat of combustion
• Nutrition facts label
• Table of food nutrients
• List of countries by food energy intake

References

External links

• Is a calorie a calorie?