Physiological calorific value
The physiological calorific value of food indicates the specific energy or the energy density that can be made available in the body of an organism when it is metabolized ( cell respiration ) . The energetic effort that the body has to make for this is not taken into account; so they are gross values . The physiological calorific value is generally lower than the physical calorific value with complete combustion .
For foodstuffs in the EU's goods traffic, the Food Information Regulation (LMIV) has been binding since December 13, 2014. According to this, the energy content of food is to be given in the SI unit kilojoule (kJ) per 100 g. The outdated unit kilocalorie (kcal) may only be mentioned additionally , but then in brackets after the SI unit kilojoule (kJ), as the following example shows: calorific value 210 kJ / 100 g ( 50 kcal / 100 g ). The previously valid EU directive on nutritional labeling (1990) has thus been replaced by the Food Information Regulation (LMIV).
Determination of physiological calorific values
In practice, the question arises as to how calorific values can be determined for everyday products. To determine the thermodynamic energy value one will bomb calorimeter used in which the food is burned to ashes. For the physiological calorific value , the estimated calorific value of the digested remains is subtracted from the result. The value determined by the calorimeter is the energy that is released when the respective substance is converted with oxygen .
The calorific value of the digested remains is estimated as follows: an average digestion with an average diet is taken as the basis, then the part of the excrement that comes from a certain food is estimated. Otherwise you would have to secrete all the intestinal bacteria contained in it (approx. 30%) as well as intestinal cells that are also flaked off . Then you could burn the rest in the calorimeter and subtract the value from the physical calorific value of the food of interest.
The physiological calorific value is only a rough guide for people. The individual digestive system plays a role. No general values apply to a single person either; the digestive system is differently effective in terms of time and food. In addition, the composition of food is sometimes subject to considerable natural fluctuations. The calorific value figures are therefore only a rough approximation of the specific energy actually extracted in the individual case.
An extreme example of the difference between thermal and physiological calorific value would be the consumption of a hard coal briquette , which has a very high calorific value in the bomb calorimeter, but which is excreted from the human body undigested. The situation is similar when consuming cellulose , which the human body - unlike ruminants - cannot break down.
Relation to the human organism
The indication of the calorific value of food does not take into account certain energy components, such as thermal energy , which depends on the temperature. The human body cannot obtain any energy that can be used directly for metabolism from water. This food therefore always has a calorific value of zero for humans, regardless of the temperature, even though warm water has more energy stored than cold water. In contrast, in calorimetry it is precisely these differences that are expressed using the same units that are used for the physiological calorific value.
Other living beings, such as bacteria or ruminants , can obtain energy from various food components that are unusable for humans because their metabolic processes differ from those of humans. These substances are also called fiber in the human digestive system . Cellulose is indigestible for humans and therefore has no calorific value for them. Can, however ruminants using the rumen - microbial cellulose gain energy for their metabolism. The calorific value information on food should therefore only be seen in relation to the peculiarities of human metabolism.
Concept of calorific value in nutrition
The term calorific value for food is not to be understood in the direct sense of the word, because food is not "burned" in the organism. The term of the amount of heat and the associated calorific value originated before the 20th century and was used to describe the energy conversion primarily of steam engines by heating water. Combustion processes (oxidation) of corresponding fuel materials such as wood or coal are used for heating. On the other hand, living beings as well as humans have a completely different way of generating energy than steam engines: Food is not burned and thermal expansion is used to achieve mechanical work, but the metabolism in the cells converts it into chemically much more complex processes. For the most part, the conversions and energy generation take place in several, staggered stages; only a small amount of waste heat is generated. The efficiency of this energy generation is also significantly higher than with thermal energy generation and its upper limit in the Carnot process - especially when one considers the small temperature difference between the body temperature of 37 ° C and the usual ambient temperatures.
The first systematic studies on the physiological calorific value of nutrients were carried out at the end of the 19th / beginning of the 20th century.
In the case of catalytic oxidation (combustion), the water content, which the calorific value takes into account, does not interfere , it only reduces the amount of oxidizable mass. Therefore, for example, the nutritional value of an apple with its high water content is lower than that of French fries.
Criticism of expressiveness and use
Because of the inaccuracies mentioned, it is controversial to what extent the physiological calorific value is meaningful at all, for example for diets . The criticism in a nutshell: Even the physical calorific value for a certain food is very different in individual cases, depending on the growing conditions, processing, etc. The proportions excreted through digestion after consumption are only estimated and vary greatly from person to person. The rest is not burned in the body, but broken down in a wide variety of ways (often with the release of energy) and, conversely, reassembled again (with the use of energy), sometimes also excreted with the urine. Significant parts of the food are not used energetically at all, but used as building blocks in the body.
Taken together, so the criticism, a physiological calorific value that is valid for everyone, even halfway plausible, cannot at all be scientifically derived. Even more so, the current figures, which often differ strongly from source to source, do not allow any conclusions to be drawn about fat metabolism. In addition, the energy consumption of a person, for example for certain physical activities, varies greatly from case to case. In its most radical form, this criticism rejects any "calculating calories" as quackery or profiteering instead of serious science.
The amount of energy that the human body needs per day in complete rest to maintain its function is called the basal metabolic rate. A guideline value can be set at 100 kJ per day and kg of body mass, i.e. 7000 kJ (around 1.9 kWh) per day for a person weighing 70 kg - slightly less for women than for men.
The total energy expenditure (basic and performance expenditure ) depends heavily on the person in question, their height, condition as well as physical activity and the ambient temperature. This value can almost double during physical exertion through sport or physical work. Extreme values are achieved with top athletes (e.g. cyclists during the Tour de France) or when working with extreme requirements for thermoregulation (e.g. on blast furnaces).
The liver and skeletal muscles account for the largest share of the basal metabolic rate in the human body with about 26% each, followed by the brain with 18%, the heart with 9% and the kidneys with 7%. The remaining 14% are accounted for by the rest of the organism.
Energy needs of humans
The energy requirement is based on the energy expenditure, which varies depending on age, gender and other factors. According to the FAO, the average energy requirement of a woman between the ages of 20 and 30 - with a weight of 55 kg and moderate physical activity - is 10,090 kJ (2,410 kilocalories) per day. For a man between the ages of 20 and 25 - with a weight of 68 kg and moderate physical activity - the daily energy requirement is 13,000 kJ (3,105 kilocalories).
Calorific value table
|(in kJ / 100 g)
|(in kcal / 100 g)
|Pasta , rice (uncooked)
|Potatoes , corn , beans , lentils (dry)
|Cocoa (slightly de-oiled)
|Milk (depending on the fat content)
|Cola / lemonade
|Wine ( white / red / mulled )
|Fruit / berries
|Fruit gums ( gummy bears )
The fluctuations in the calorific values within a category are sometimes even greater. This is a rough overview based on common foods. The calorific value can fluctuate greatly due to the manufacture, processing (water content) and degree of ripeness of the natural products. In addition, the calorific value varies from person to person, since digestion does not extract exactly the same amount of energy from a certain food in every person.
Calorific value information in the nutritional value labeling of the EU
When the Nutrition Labeling of the EU is not marked with a bomb calorimeter (s. O.) Measured calorific value of a foodstuff. Rather, the calorific values of the components of a food (fats, carbohydrates, proteins, etc.) are added according to their proportion. The calorific values of the respective components (see table below) can be found in Regulation (EU) No. 1169/2011 - Annex XIV
|Calorific value in kJ / g
|Calorific value in kcal / g
|Polyhydric alcohols ( polyols )
|Salatrims (low-calorie fat, "short and long chain acyl triglyceride molecules")
|Erythritol (sugar substitute)
Please note that the two values are rounded separately and that the result is ratios of 4.0 (dietary fiber) to 4.333 (organic acids) kJ / kcal - a range that clearly exceeds the various definitions of the outdated unit of calories . Depending on the composition, the information on the products can therefore indicate two quite different energy values, although the 3 to 4-digit numbers suggest a high degree of accuracy.
Energy consumption as an indicator of prosperity
Negative calorific value
Some foods, especially various types of vegetables , are sometimes claimed to have a negative calorific value because the body uses more energy for digestion than it takes from them. In fact, the effort to be made for food intake and its utilization is, by definition, completely disregarded when specifying physiological calorific values, since it is gross information . In this respect, you will not find any foods with a specified negative calorific value, even if the energetic benefit should be negative in individual cases.
(Drunk) cold water is also mentioned as an example of a negative calorific value , because the body has to use energy to warm it up to body temperature . To heat one liter of tap water from 12 ° C to 37 ° C, 105 kJ / 25.08 kcal are required. Depending on the ambient temperature and activity, this amount of heat does not necessarily have to be generated additionally or, instead, the heat release to the environment can be reduced by reducing the blood flow to the outer skin layers ( thermoregulation ).
- (PDF; 67 kB) .
- Wladimir Glikin: Kalorimetrische Methodik: A guide for determining the heat of combustion of organic bodies, including nutrients and metabolic products and for measuring animal heat production (German: Gebrüder Bornträger, Berlin 1911).
- For example: Udo Pollmer , Andrea Fock, Ulrike Gonder, Karin Haug: Cheers meal! Sick from healthy eating . 8th edition. Kiepenheuer and Witsch, Cologne 2006, ISBN 3-462-03012-4 . In Spektrum der Wissenschaft reviewed as follows  : “Without a doubt (the authors) are trying to tear down the rigid chemical-mechanistic structure of established nutrition theory. Rightly so, because it is built on sand. "
- Robert F. Schmidt, Florian Lang, Manfred Heckmann: Physiologie des Menschen. with pathophysiology. 31st edition. SpringerMedizin Verlag, Heidelberg 2010, ISBN 978-3-642-01650-9 , p. 838.
- Energy requirements and dietary energy recommendations. Human energy requirements . Report of a Joint FAO / WHO / UNU Expert Consultation. Rome, October 17-24, 2001.