Food energy

The physiological caloric value of food indicates the specific energy or energy density in their metabolism ( cellular respiration ) can be made available in the body of an organism. The energy expenditure that the body on the other hand must operate this, it is ignored; So is gross values ​​.

The physiological calorific value is generally less than the physical calorific value from the complete combustion in a flame.

In the nutrition labeling of the EU, the information in kJ ( kilojoules ) per 100 g or at liquid foodstuffs in kJ per 100 ml shall be made; in addition kJ per serving are permitted provided that the product is portioned or the portion size accepted is given. However, they also have the corresponding values ​​in the outdated, but in nutritional science to provide some still common unit calorie, each with kcal ( kilocalories ) instead of kJ.

Determination of physiological calorific values

In practice, the question arises, calorific values ​​can be determined as in products of daily consumption. For the determination of the thermodynamic energy value a bomb calorimeter is used, in which the food is burnt to ashes. For the physiological caloric value of the result of the estimated calorific value of the digested remains will be deducted.

The physical energy value is thus somewhat objectively determinable; However, the estimated calorific value of the digested remains depends on the creatures. The determined value by the calorimeter is the energy that is released with oxygen at reacting the respective substance.

The calorific value of the digested remains is estimated as follows: An average digestion with average diet is assumed as a base, then the portion of the feces, which comes from a particular food, appreciated. Otherwise, you would have all intestinal bacteria contained therein secrete (about 30 %) and also desquamated intestinal cells. Then you could burn the rest of the calorimeter and deduct the value from the physical calorific value of the interest food.

The physiological calorific value is only a rough guide for people. It does the individual digestive system a role. Even for a single person is not subject to general values; the digestive system is temporally as well as food specifically different effective. Moreover, the composition of foods sometimes considerable natural fluctuations. So calorific value figures are only a rough approximation of the in individual cases actually extracted specific energy dar.

An extreme example of the difference of thermal and physiological calorific value would be the consumption of hard coal briquettes, in the bomb calorimeter has a very high calorific value, but which is excreted undigested from the human body. Similarly, the consumption of cellulose, which the human body - in contrast to ruminants - can not catch up.

With respect to the human organism

The calorific value of foods disregarded specific energy components, such as the thermal energy, which depends on the temperature. Thus, the human body can gain from no water for metabolism directly usable energy. This food has therefore. For humans regardless of the temperature is always a calorific value of zero, although hot water has saved more energy than cold water In contrast, just these differences with the same units are expressed in the calorimetry used in the physiological caloric value.

Other creatures, such as bacteria or ruminants can choose from various dietary components that are unusable for humans, gain energy because they have different metabolic processes from those of humans. These materials are referred to in the human digestive system and as a fiber. Thus, cellulose is indigestible to humans, has for him so no calorific value. On the other hand can win from cellulosic energy for their metabolism ruminants using the rumen microbes. The calorific value claims on food are therefore always be seen only in relation to the specific characteristics of human metabolism.

Concept of calorific value in the dietetics

The concept of the energy value of food is not to be understood in the direct sense of the word, because food is not "burned" in the organism. The concept of heat and the associated energy value created before the 20th century and was used to describe the energy sales primarily of steam engines by heating water. For heating serve combustion processes (oxidation) from respective combustion materials such as wood or coal. Contrast, have creatures like man an entirely different type of energy than steam engines: food will not be burned and taken advantage of the thermal expansion for obtaining a mechanical work, but the metabolism in the cells converts them into chemically significantly more complex operations to. Most part, the transformations and energy recoveries run in multiple, time-staggered levels; a waste heat apply only to a small degree. The efficiency of this energy is significantly higher than the thermal energy and the upper bound in the Carnot cycle - especially when one considers the small temperature difference between the body temperature of 37 ° C and the usual ambient temperatures.

The first systematic studies of the physiological caloric value of nutrients were conducted at the end of the 19./Beginn the 20th century.

In a catalytic oxidation (combustion ) of the water content does not interfere, the calorific value of the account, it merely reduces the amount of oxidizable material. Therefore, for example, the nutritional value of an apple, with its high water content is less than the fries by french fries.

Criticism of validity and use

Simply because of these inaccuracies is debatable to what extent the physiological calorific value at all has significance, for about diets. The criticism in a nutshell: Even the physical calorific value for a particular food falls in individual cases very different from, depending on growing conditions, processing, etc. The following consumption of the digestive retired shares are an estimate only and will vary greatly from person to person. The rest is not burned in the body, but (often under energy release ) mined in many different ways, and vice versa ( with energy use ) then reassembled, partly excreted in the urine. Significant parts of the food are not used for energy, but used as building blocks in the body.

Everything was put together, so the criticism is a valid for everyone, even a semi- plausible physiological calorific value not scientifically derived. A fortiori left to the common, often from source to source outlying numbers not statements of fat metabolism. In addition, is also the energy consumption of a people very differently about certain physical activities, from case to case. In total, more quackery and profiteering was any "calorie calculation " as genuine science.

Energy sales

The amount of energy needed by the human body per day at complete rest to maintain its function is called basal metabolic rate. As a guideline, one can be 100 kJ per day and kg body mass, ie daily 7000 kJ ( about 1.9 kWh) for a 70 kg person - in women less than men. Energy expenditure depends heavily on the particular person whose physical size, condition, and physical activity and the ambient temperature. Physical exertion through exercise or physical labor, this value can nearly double. Extreme values ​​are achieved in elite athletes (eg, cyclists during the Tour de France) or when working with extreme need for thermoregulation (eg blast furnaces ).

The largest share in the turnover in the human body have liver and skeletal muscle, each with about 26 %, followed by the brain by 18 %, the heart with 9% and the kidneys with 7%. The remaining 14 % is for the rest of the organism.

Energy needs of the people

The energy requirement depends on basic and advanced sales, which vary according to age, gender and other factors. So is, according to the FAO, the average energy needs of a woman between 20 and 30 years of age - at a weight of 55 kg and a moderate physical activity - at 10,090 kJ ( 2410 kcal) per day. For a man 20 to 25 in age - with a weight of 68 kg and a moderate physical activity - is the energy requirement at 13,000 kJ ( 3,105 kcal).

Calorific value table

Note: The variation of the internal values ​​within a category are partly still much larger. This is a rough overview, based on common foods. Should also be noted that the calorific value can be partially vary greatly due to manufacturing, processing and maturity of natural products. In addition, the specific calorific value from person to person, as the digestion is not exactly win the same amount of energy from a specific food for each person.

Condensing information in the nutrition labeling of the EU

In the nutrition labeling of the EU not by a bomb calorimeter (see above) measured energy value of a food is given. Rather, the calorific values ​​of the components of a food (fat, carbs, protein, etc.) are added according to their share in the food. The calorific values ​​of the respective components ( see table below) are defined in Article 5, Section 1 of the Guidelines on Nutrition Labelling.

Note that the two values ​​are rounded separately and therefore calculated ratios of 4.0 ( dietary fiber) to 4.333 ( organic acids) kJ / kcal result - a range that significantly exceeds that of the various definitions of a calorie. Depending on the composition often give the information on the products thus two quite different energy values ​​, although the 3 - to 4 - digit numbers suggest a high accuracy.

Energy consumption as a welfare indicator

Due to the energy consumption of food per capita can be determined how good the food situation in a region. It can therefore be an indicator of prosperity for the classification of states.

Negative condensing

From some foods, especially various vegetable varieties, is sometimes claimed to have a negative calorific value, because the body would consume more energy for digestion when ingested, as it receives from them. In fact, however remains the effort that must operate the body for food intake and their utilization, by definition, completely ignored when specifying physiological calorific values; So is gross data. Extent one is no food with a specified negative condensing find, even if the energetic benefit to the body in a particular case should be negative.

As an example of a negative calorific value and cold water is called because the body has to expend energy to heat the water to body temperature drunk cold. To heat a liter of tap water of 12 ° C to 37 ° C, for example, 105 kJ are required. Depending on the ambient temperature and activity of the body this amount of heat but does not necessarily generate additional, but instead can reduce the heat loss to the environment by reducing the blood flow to the outer layers of the skin ( thermoregulation ).