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Food energy
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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 kilogram calorie, which is equal to 1 kilocalorie (kcal), or 1,000 calories as normally defined. In the context of nutrition, and especially food labeling, the terms "calorie" and "kilocalorie" are interchangeable. In either case the unit is approximately equal to 4.1868 kilojoules (kJ).
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Encyclopedia
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 kilogram calorie, which is equal to 1 kilocalorie (kcal), or 1,000 calories as normally defined. In the context of nutrition, and especially food labeling, the terms "calorie" and "kilocalorie" are interchangeable. In either case the unit is approximately equal to 4.1868 kilojoules (kJ). The kilojoule is the unit officially recommended by the World Health Organization and other international organizations. In some countries (Australia, for example) only the kilojoule is normally used on food packaging, but the calorie is still the most common unit in many countries.
Only carbohydrates (including fiber), fats, proteins, organic acids, polyols, and ethanol contain food energy. All foods are made up of a combination of these five nutrients. Everything else in food is non-caloric, including (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. of the specified food. This number is then multiplied by, usually, 89%; which represents the loss happening during human digestion.
Food labels
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. In the European Union, manufacturers of prepackaged food must label the nutritional energy of their products in both kilocalories and kilojoules. 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 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. 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 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:
All the other nutrients in food are non-caloric and are thus not counted.
Recommended daily energy intake values for young adults are: 2500 kcal/day (10 MJ/day) for men 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.
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. Consider the following theoretical calculation.
- Fat contains about 3,500 kilocalories per pound ( kJ/g).
- If you eat 3,500 kcal more than your body needs, you will put on of fat (assuming , or less depending on food conversion inefficiency not included in the "body needs" category).
- If you burn 3,500 kcal more than you eat, you lose about of fat, assuming that only fat is burnt (this is close to 100% since even the waste heat counts toward the 3,500 kcal). However, energy sources can come from catabolism of protein (muscles), and fat may be preferentially saved. The use of different body materials as available must be considered.
Because the body must expend energy to create fat, the amount of energy a person has to expend to lose weight is much smaller than the amount they have to consume in order to gain weight. However, the body may increase its basal metabolic rate to preferentially "burn off" new calories or decrease it to conserve energy to balance out increased activity. Thus, actual weight changes vary between individuals. Also, the computations above assume that all the weight gained and lost is in the form of fat. In reality, this is a mixture of protein, carbohydrates, etc. (in muscle tissue, organs, etc.).
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, and roughly speaking, only about 15% of the food energy is actually converted into mechanical energy. For example, when calculating food energy burnt per unit time gym equipment manufacturers multiply the value of physical power by a factor of eight (assuming 12.5% efficiency). Thus if an exercise bike registers a 150-watt physical power output it might display 17 kcal/min as the rate of food energy burnt per unit time (since 150 W × 8 = 1200 W ˜ 17 kcal/min).
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.
See also
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