Basal metabolism: what it is, how it is measured and why it allows us to survive

Living things are not sealed compartments, because we have to get energy from the environment to stay in time.

Some taxa obtain energy by converting inorganic matter to organic matter (as is the case with plants and photosynthesis), while all vertebrates obtain this energy through nutrition, either from vegetables, products. meat and many other more organic sources.

In addition to needing a minimum of energy not to die, we need a more or less daily calorie intake based on our work and our efforts.o. It has nothing to do, for example, with the amount of food consumed by a racehorse and a snake in its lair. Besides having different metabolic pathways, the physical work performed by the two is not even comparable.

In connection with this whole subject, today we come to bring you a term which is really interesting from a biological point of view for researchers, physicians and zoologists. Let’s see how it is basal metabolism: As complex as this concept may seem, we assure you that you will have a detailed idea of ​​it after reading these lines.

    What is Basal Metabolism

    General metabolism or metabolic requirements can be defined as the energy requirements required by an organism for all the biochemical reactions and physicochemical processes that take place in its cells. This set of reactions allows living things to grow, reproduce, keep their physical structures in order, and respond to external stimuli, among others.

    Basal Metabolism or Basal Metabolic Rate (BMR) refers to the rate of energy expenditure per unit of time required for endothermic animals to remain at rest. In other words, it is the amount of heat (expressed in calories) generated in one hour by the subject kept at rest, At a temperature of 18 degrees on an empty stomach after 12-14 hours, standardized conditions.

    We have just introduced a striking term that needs to be explained: endothermia. We lay the groundwork before continuing with more complex terminology.

    Endothermic vs ectothermic

    Endotherms are living things that use internally generated heat to maintain their body temperature., Which tends to stay within a constant range despite inclement weather, unless a pathological picture occurs. This physiological phenomenon is based on a premise: metabolic reactions are not 100% efficient and therefore energy “escapes” from the body in the form of heat.

    On the other hand, ectotherms are animals that are not able to generate heat in their metabolic processes, so their only way of thermoregulation is to go to or away from energy sources.

    It is no coincidence that sunny behaviors are observed mainly in reptiles: when a lizard is leaning on a stone “sunbathing”, what it actually does is obtain the necessary energy in the form of of heat to carry out processes. movement, among others.

    Therefore, when we talk about basal metabolism (BMR), we only refer to the heat produced by endotherms, What are mammals and birds.

    If we wanted to quantify the amount of energy required for a reptile or amphibian to survive, we would use another different term and methodology: Standard Metabolic Rate (MSR). This follows similar criteria to the basal metabolic rate, but the ambient temperature must be taken into account, as it completely modulates the energy availability of the ectothermic animal.

      What other parameters accompany the basal metabolism?

      This value is very useful, as we will see later, however it does not fully explain the energy demand of the individual. Before continuing, you should consider the following:

      Total energy expenditure (GET): basal metabolic rate (BMR) + endogenous thermogenesis (ET) + physical activity (AF)

      The World Health Organization (WHO) gives a very good definition of total energy expenditure, in which the TMB plays a very important role: “it is the level of energy necessary to maintain the balance between consumption and expenditure. energy, when the individual has weight, body composition and physical activity compatible with good health. “

      As you can imagine, this value fluctuates between individuals, As you have to pay attention to age, gender, ethnicity, place of origin and many other things.

      What is basal metabolism used for?

      Basal metabolism represents the minimum cost to the body to perform vital maintenance functions that do not consciously shut down, such as cardiovascular, respiratory, endocrine, central nervous system, kidney, liver, immune, and thermogenesis ( heat formation).

      These “basal” conditions require the quantification of a specific environment.: The individual must have slept 10 to 12 hours, be in a post-absorption state (not having eaten any food for at least 12 hours) and be in thermoneutral conditions and in a state of physical and emotional rest. Of course, a person’s basal metabolic rate cannot be measured if they are on a snow-capped mountain running away from a bear, as their energy demand is much higher and the situation is beyond normal.

      How is basal metabolism measured?

      To effectively measure a basal metabolic rate (without using predictive formulas), it is necessary to resort to direct or indirect calorimetry. In the first case, the amount of heat produced by the body in airtight chambers with insulating walls is measured. Here, the stored energy and that lost by convection are recorded, after a stabilization period of at least 6 hours.

      On the other hand, TMB can also be obtained by indirect calorimetry, a minimally invasive but very efficient method. This is based on the absorption of oxygen and the release of carbon dioxide: as anabolism requires oxygen and CO2 is released, the amount of heat produced is related to the oxygen consumed and the CO2 released. here the respiratory coefficient comes into play, a parameter that we reserve for another opportunity.

      Basic metabolic rate formulas

      If you search the internet, you will see that there are several calculators that attempt to estimate your basal metabolic rate without performing any medical tests. Although the indirect calorimetry method is suitable for obtaining a truly reliable value, these programs are based on mathematical formulas which take into account the following factors:

      • Q: Total heat output at complete rest.
      • M: the mass in kilograms of the individual.
      • H: the individual’s height in centimeters.
      • A: age, in years.

      These parameters give us formulas like the following:

      • MEN TMB = (10 x kg weight) + (6.25 x height in cm) – (5 x age in years) + 5.
      • WOMEN TMB = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) – 161.

      If you want to stick to a number, we can tell you that some studies have calculated average basal metabolic rates in individuals in studies ranging from 1027 kilocalories / day to 2499 kilocalories / day. It is the energy needed to simply existSo, depending on the exercise and activity, you need to add a varying calorie range.

      It should be noted that approximately TMB decreases by 1 to 2% for every decade after the age of 20, mainly due to pure fat loss.

      Basal metabolic rate and height

      It is common (and correct) to think that the net basal metabolic rate will be much higher in an elephant than in a mouse, as its weight moves on different scales and naturally a mammoth animal will produce and lose much more heat than a mouse. animal. child, right?

      In values absolute, this assumption is correct, but the thing becomes interesting if the TMB is divided by the mass of the animal. If we divide it by weight, we observe that the metabolic rate of a mouse per gram of tissue is 10 times that of an elephant.. Although the mechanisms underlying this process are not yet fully understood, it is known that the surface-to-volume ratio of a small animal is higher, which promotes heat loss.

      Thus, small endothermic animals generally have much shorter life cycles because their cell metabolism is very rapid and tissues and organs fail earlier. If, on the other hand, you see a gecko with the same weight as a mouse, you will find that it lives up to 7 times longer. Why ?: Since the reptile does not generate heat, the demand for energy and the work done by your body is much lower.


      Fascinating, isn’t it? It turns out that, as anecdotal as it may sound, the basal metabolic rate in endotherms determines their life expectancy and vital strategies. On a more medical than biological level, moreover, this parameter allows nutritionists and sports professionals to know how many calories to eat per day to maintain, increase or decrease the total mass of a particular organism.

      Bibliographical references:

      • Bonfanti, N., Fernández, JM, Gomez-Delgado, F., and Pérez-Jiménez, F. (2014). Effect of two low-calorie diets and their combination with exercise on basal metabolic rate and body composition. Hospital nutrition, 29 (3), 635-643.
      • López-Fontana, CM, Martínez-González, MA, and Martínez, JA (2003). Obesity, energy metabolism and measurement of physical activity. Spanish Journal of Obesity, 1 (1), 29-36.
      • Vargas, M., Lancheros, L. and del Pilar Barrera, M. (2011). Resting energy expenditure and body composition in adults. Journal of the Faculty of Medicine, 59 (1), S43-S58.

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