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

Living beings are not watertight compartments, as we have to obtain energy from the environment to remain over time.

Some taxa obtain energy by converting inorganic to organic matter (such as plants and photosynthesis), while that all vertebrates get this energy through nutrition, whether based on vegetables, meat products and many other organic sources more.

In addition to needing a minimum amount of energy to avoid dying, we need more or less daily caloric intake based on our work and effort.or. It has nothing to do with, for example, the amount of food consumed by a racehorse and a lurking snake in their lair. In addition to having different metabolic pathways, the physical work done by both is not even comparable.

In relation to all this topic, today we come to bring you a truly interesting term from a biological point of view for researchers, doctors and zoologists alike. Let's see what the basal metabolism: As complex as this concept may sound, we assure you that you will be left with a detailed idea of ​​it after reading these lines.

• Related article: "Major Cell Types of the Human Body"

What is the basal metabolism?

General metabolism or metabolic requirements can be defined as the energy demands that an organism requires for the set of biochemical reactions and physicochemical processes that occur in its cells. This set of reactions allows living beings to grow, reproduce, keep their physical structures in order and respond to external stimuli, among other things.

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

We have just introduced a term that is striking and needs to be explained: endothermy. Let's lay the groundwork before moving on to more complex terminology.

Endothermy vs ectothermy

Endotherms are living things that use internally generated heat to maintain their body temperature., which tends to remain in 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 capable of generating heat in their metabolic processes, so their only way to thermoregulate is to go to energy sources or get away from them.

It is no coincidence that sunbathing behaviors are observed above all in reptiles: when a lizard is leaning on a stone "basking in the sun", it what it really does is obtain the necessary energy in the form of heat to be able to carry out metabolic processes such as digestion or movement, among others things.

So that, When we talk about basal metabolism (BMR), we refer only to the heat produced by endotherms, which are mammals and birds.

If we wanted to quantify the amount of energy required for the survival of a reptile or amphibian, we would use a different term and methodology: the standard metabolic rate (TMS). This follows similar criteria to the basal metabolic rate, but the environmental temperature must be taken into account, since it completely modulates the energy availability of the ectothermic animal.

• You may be interested in: "Rough endoplasmic reticulum: definition, characteristics and functions"

What other parameters accompany basal metabolism?

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

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

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

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

What is basal metabolism for?

The basal metabolism represents the minimum cost of the organism so that it can carry out the vital maintenance functions that are not stopped consciously, such as cardiovascular, respiratory, endocrine, central nervous system, kidney, liver, immune, and thermogenesis phenomena (formation of hot).

These "baseline" conditions require a specific environment to be quantified.: the individual must have slept 10-12 hours, be in a post-absorption state (not having ingested any food in at least 12 hours) and be under conditions of thermoneutrality and in a state of physical and emotional rest. Of course, you cannot measure a person's basal metabolic rate if they are on a mountain full of snow fleeing from a bear, as its energy demand is much higher and the situation escapes from the normal.

How is basal metabolism measured?

To measure a basal metabolic rate effectively (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 is measured in hermetic chambers with insulating walls. Here the stored energy and the loss 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 effective method. This is based on the uptake of oxygen and the release of carbon dioxide: because anabolism requires oxygen and CO2 is released, the amount of heat produced is related to oxygen consumed and CO2 released. Here the respiratory coefficient comes into play, a parameter that we reserve for another opportunity.

Basal metabolic rate formulas

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

• P: the total heat production at complete idle.
• M: the mass in kilograms of the individual.
• H: the height in centimeters of the individual.
• A: age, in years.

These parameters give us formulas like the following:

• MEN TMB = (10 x weight in Kg) + (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 stay with some figure, we can tell you that certain studies have calculated metabolic rates Baseline mean in individuals from studies ranging between 1027 Kilocalories / day and 2499 Kilocalories / day. This is the energy needed to simply existSo, based on exercise and activity, you have to add a variable caloric range.

It is striking to know that, approximately, BMR decreases by 1-2% for each decade after the age of 20, mainly due to the loss of purely fat mass.

Basal metabolic rate and size

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

In absolute values ​​this postulation is correct, but things get interesting if the BMR is divided by the mass of the animal. If divided by weight, it is observed 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 / volume ratio of a small animal is higher, which favors heat loss.

Thus, small endothermic animals tend to have much shorter life cycles, since their cellular metabolism is very fast 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?: as the reptile does not generate heat, the energy demand and the work done by its body is much lower.

Resume

Fascinating, right? It turns out that, anecdotal as it may seem, the basal metabolic rate in endotherms conditions their life expectancy and vital strategies. On a more medical than biological level, in addition, this parameter serves so that nutritionists and sports professionals know how many calories they need to eat per day to maintain, increase or decrease the total mass of an organism concrete.

Bibliographic references:

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