Food energy

Food energy is the amount of energy in food that is available through digestion.

Like other forms of energy, food energy is expressed in calories or joules. Some countries use the food calorie, which is equal to 1 kilocalorie (kcal), or 1,000 gram calories. In the context of nutrition, and especially food labeling, the calories are large calories approximately equal to 4.1868 kilojoules (kJ). The kilojoule is the unit officially recommended by the World Health Organization^[1] and other international organizations. In some countries only the kilojoule is normally used on food packaging, but the calorie is still the most common unit in many countries.

Fiber, fats, proteins, organic acids, polyols, and ethanol contain food energy. All foods are made up of a combination of these six caloric nutrients and non-caloric nutrients.^{[dubious – discuss]} Non-caloric food includes (but not limited to) water, vitamins, minerals, antioxidants, caffeine, spices and natural flavors. Tea and coffee also have no calories without sugar or milk added. Nutritionists usually talk about the number of calories in a gram of a nutrient. Fats and ethanol have the greatest amount of food energy per gram, 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 fiber or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids have fewer 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 contained in a food item by 85%, which is the typical amount of energy actually obtained by a human after the digestive processes have been completed.

Measuring food energy

The following process details how to measure food energy, as specified by the United States Department of Agriculture (USDA) in the early 1900s:

The particular food being measured must be burned in a calorimeter, so that the heat released from the food can be accurately measured. This amount is used to ascertain the G.E.V.^{[clarification needed]} of the specified food. This number is then multiplied by, usually, 85%; which represents the loss happening during human digestion.

Food labels

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

The "calorie" has become a common household term because dietitians recommend in cases of obesity to reduce body weight by increasing exercise (energy expenditure) and reducing energy intake. Many governments require food manufacturers to label the energy content of their products, to help consumers control their energy intake.^[2] 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",^[3] 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 100 g and/or for what the manufacturer claims is a typical serving size.

The amount of food energy in a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry.^[4] However, the values given on food labels are not determined this way, because it overestimates the amount of energy that the human digestive system can extract, by also burning dietary fiber. Moreover, not all food energy eaten is actually resorbed by the body (fecal and urinal losses). Instead, standardized chemical tests or an analysis of the recipe using reference tables for common ingredients^[5] 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.^[6][7]

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, 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.^[8]

Energy usage in the human body

Energy intake to the body that is not used up is mostly stored as fat in the fat tissue.^{[citation needed]}

The conversion efficiency of food energy into physical power depends on the form of energy source (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 food energy is converted into mechanical energy.^[9] This low efficency is the result of about 40% effiency 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 overal 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 burned calories as four times the actual mechanical work, plus 300 kcal per hour,^[10] 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" 500 calories more than your body should 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. ^[11]

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 food energy.^[12]

See also

• Chemical energy
• Basal metabolic rate
• Food chain
• Atwater system
• Nutrition facts label

References

External links

• FAO Food and Nutrition Paper 77: Food energy - methods of analysis and conversion factors
• http://www.ajcn.org/cgi/content/full/79/5/899S?ijkey=f3919ec7617632925bb12e0ffb8deeb08a678686 Is a calorie a calorie?