Calories: Definition, basal metabolic rate and daily requirement
We read about it almost every day, but only a few people know in detail what is meant by the term “calorie”. When we eat food, we do so not only to replenish lost substances, but above all to provide our bodies with the energy they need. However, it does not matter which nutrients are actually ingested. In terms of the energy required, the origin is less important than the actual amount consumed.
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Units give energy a tangible value
The term calorie corresponds to a unit of supplied energy and is thus made tangible. It should be mentioned that the unit of measurement calorie is actually obsolete and has been replaced by the SI unit* joule. Nevertheless, in everyday life we encounter the unit kilocalorie (1 kcal = 1,000 cal), which still plays a role especially in the food industry. One calorie corresponds to approx. 4.2 joules. By definition, a calorie is the amount of energy needed to heat one gram of water by one degree Celsius.
The calorific value determines the energy
The individual calorific value of a food tells us how much energy the biochemical cycle of the human organism can draw from it. A distinction is made between physical and physiological calorific value. The former describes the energy content of a nutrient that is actually contained. In science, we speak of released energy as soon as a nutrient has been completely burnt to CO2 and water. The physiological calorific value, on the other hand, represents the amount of energy of a nutrient that can actually be used by the body and is indicated on food packaging. The total energy contained in a food is made up of the energies of the macronutrients it contains. One gram of carbohydrates and proteins each provide the body with 4.1 kcal, while one gram of fat, as the largest energy supplier, provides 9.3 kcal.
The basal metabolic rate reflects the minimum daily energy requirement
In order to ensure that the biochemical processes taking place in the body and the maintenance of our vital systems such as respiration, digestion and cardiovascular system can continue throughout the day, there is a minimum amount of energy. In this context, we speak of the so-called basal metabolic rate. This is the amount of energy required daily by our systems and circulations to maintain all life-sustaining, building and breaking down processes in a state of rest. In theory, the intake of this amount of energy does not lead to excess fat deposits, as the energy flows one-to-one into maintaining the daily necessary body processes and is thus utilised. The parameters gender, age, height and weight are used to determine the basal metabolic rate. However, this is only an estimate of the individual daily requirement, as complex person-related measurements in the context of calorimetry* are not the norm.
Adjusting the basal metabolic rate
It is important to remember, however, that the basal metabolic rate is the daily amount of energy required by the body at rest. Since every person performs individual activities every day and is confronted with certain stressful situations and challenges, it is imperative to adjust the individual basal metabolic rate. Consequently, this value corresponds to the actual daily requirement. The amount of energy added thus depends on the individual daily routine. Special attention should be paid to sporting activities and major physical exertion.
On average, each of us burns more than 300 calories during a 30-minute running session. However, generalisations regarding calorific values per sports unit or exercise situation must be determined according to the circumstances and individual prerequisites of each person. The data circulating on the internet are average values and therefore guidelines. For example, it may be that a daily guideline of 10,000 steps is given. However, it does not make sense to infer a general number of calories burned from this. Here too, attention must be paid to how the individual steps are taken in order to estimate the amount of calories burned.
The following should be kept in mind:
1. The energy contained in food is still stated in kilocalories — in the meantime, this has been replaced by the SI unit joule in science.
2. The calorific value stated on packaging corresponds to the physiological calorific value.
3. The daily basal metabolic rate corresponds to the energy consumed by the body at rest.
4. When determining the basal metabolic rate, formulas and general parameters are used, which means that the accuracy is lost, unlike when determining the basal metabolic rate by indirect calorimetry.
5. To determine the daily requirement, amounts of energy consumed through physical activity must be added to the basal metabolic rate.
* SI units: These are physical quantities that belong to a pool of globally recognised and used measurement quantities.
* By means of so-called indirect calorimetry, a measuring method, the energy metabolism can be determined in detail.
Text sources:
(1) Alberts et al, Textbook of Molecular Cell Biology, 4th edition, 2012, p. 51.
(2) Löffler / Weiss, Physiological Chemistry, 2nd edition, 1979, p. 4
(3) Grimm et al, Pocket Atlas of Nutrition, p. 30 ff.
(4) Brandenburger and Bajorat, Casebook Biochemistry, 2006, p. 112 f.
(5) Fürst et al, Nutritional Medicine, 3rd edition, 2004, p. 28 ff.
(6) Max Rubner Institute, How much energy does food provide?
(7) Wechsler, Adipositas — Ursachen und Therapie, 2nd edition, 2003, p. 107
Picture sources:
(9) Designed by rawpixel.com / Freepik
