Food energy

Food energy is the amount of energy obtained from food that is available through cellular respiration.

Like other forms of energy, food energy is expressed in calories or joules. The calorie is a very small measure of energy so the food calorie (kilocalorie, kcal), 1000 calories, is more often used and is what food packaging usually refers to when showing calorific value. 1 kcal is equal to 4.182 kilojoules (kJ). The kilojoule is the unit officially recommended by the World Health Organization and other international organizations. In some countries only the kilojoule is used on food packaging, while in others the calorie is the most common unit.

Carbohydrates, fibre, fats, proteins, organic acids, polyols, and ethanol all release energy during respiration — this is often called 'food energy'. When the food (providing fuel) reacts with oxygen in the cells of living things energy is released. A small amount of energy is available through anaerobic respiration. Fats and ethanol have the greatest amount of food energy per mass, 9 and 7 kcal/g (38 and 30 kJ/g) respectively. Proteins and most carbohydrates have about 4 kcal/g (17 kJ/g). Carbohydrates that are not easily absorbed, such as fibre or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids have less than 4 kcal/g.

Each food item has a specific metabolizable energy intake (MEI). Normally this value is obtained by multiplying the total amount of energy associated with a food item by 85%, which is the typical amount of energy actually obtained by a human after respiration has been completed.

Nutrition labels

Many governments require food manufacturers to label the energy content of their products, to help consumers control their energy intake. In the European Union, manufacturers of prepackaged food must label the nutritional energy of their products in both kilocalories and kilojoules, when required. In the United States, the equivalent mandatory labels display only "Calories", often as a substitute for the name of the quantity being measured, food energy; an additional kilojoules figure is optional and is rarely used. The energy content of food is usually given on labels for 100g, for a typical serving size (according to the manufacturer), and/or for the entire pack contents.

The amount of food energy associated with a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry. However, the values given on food labels are not determined this way, because it overestimates the amount of fuel that actually enters the blood through digestion because it also burns the indigestible dietary fibre so that not all food eaten is actually absorbed by the body (fecal losses). Instead, standardized chemical tests or an analysis of the recipe using reference tables for common ingredients are used to estimate the product's digestible constituents (protein, carbohydrate, fat, etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities.

Food component	Energy Density
	kJ/g	kcal/g
Fat	37	9
Ethanol (alcohol)	29	7
Proteins	17	4
Carbohydrates	17	4
Organic acids	13	3
Polyols ( sugar alcohols, sweeteners)	10	2.4
Fibre	8	2
Erythritol	0.8	0.2

All the other nutrients in food are non-caloric and are thus not counted.

Recommended daily energy intake values for young adults and men are: 2500 kcal/day (10 MJ/day) and 2000 kcal/day (8 MJ/day) for women. Children and sedentary and older people require less energy, physically active people more. In addition to physical activity, increased mental activity has been linked with moderately increased brain energy consumption.

Energy usage in the human body

The human body uses the energy released by respiration for a wide range of purposes: about twenty percent of the energy is used for brain metabolism, and much of the rest is used for the basal metabolic requirements of other organs and tissues. In cold environments, metabolism may increase simply to produce heat to maintain body temperature. Among the diverse uses for energy, one is the production of mechanical energy by skeletal muscle in order to maintain posture and produce motion.

The conversion efficiency of energy from respiration into mechanical (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 percent 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 food energy, 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 percent, one watt of mechanical power is equivalent to 4.3 kcal per hour. For example, a manufacturer of rowing equipment shows calories released from 'burning' food as four times the actual mechanical work, plus 300 kcal per hour, which amounts to about 20 percent efficiency at 250 watts of mechanical output. It can take up to 20 hours of little physical output (e.g. walking) to "burn off" 4000 kcal (i.e. fuel) more than a body would otherwise have.

The differing energy density of foods (fat, alcohols, carbohydrates and proteins) lies in their varying proportions of oxidizable carbon atoms. Release of energy from food follows transfer of electrons from carbon and hydrogen to carbon dioxide and water.

Swings in body temperature – either hotter or cooler – increase the metabolic rate, thus burning more energy. Prolonged exposure to extremely warm or very cold environments increases the basal metabolic rate (BMR). People who live in these types of settings often have BMRs that are 5–20% higher than those in other climates. Physical activity also significantly increases body temperature, which in turn uses more energy from respiration.