Basal metabolism: what it is, how it is measured, and why it allows us to survive
Let's see what basal metabolism is, and what Biological functions it is linked to.
Living beings are not watertight compartments, since we have to obtain energy from the environment in order to survive over time.
Some taxa obtain energy through the conversion of inorganic matter into organic matter (as is the case of plants and photosynthesis), while all vertebrates obtain this energy through nutrition, be it from vegetables, meat products and many other organic sources.
In addition to needing a minimum amount of energy in order not to die, we need more or less daily caloric intake based on our work and effort.o. For example, the amount of food consumed by a racehorse and a lurking snake in its lair have nothing to do with each other. In addition to having different metabolic pathways, the physical work performed by both is not even comparable.
In relation to all this, 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 metabolismAs complex as this concept may sound, we assure you that you will have a detailed idea of it after reading these lines.
What is basal metabolism?
The general metabolism or metabolic requirements can be defined as the energy demands required by an organism for the set of biochemical reactions and physicochemical processes occurring in its cells.. This set of reactions allows living beings to grow, reproduce, maintain 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 required by 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 maintained at restat a fasting temperature of 18 degrees Celsius after 12-14 hours, standardized conditions.
We have just introduced a term that attracts attention and should be explained: endothermia. Let us lay the groundwork before moving on to more complex terminology.
Endothermy vs. ectothermy
Endotherms are living beings that use internally generated heat to maintain their body temperature.The heat output, which tends to remain in a constant range in spite of environmental conditions, unless a pathological condition occurs. This physiological phenomenon is based on one premise: metabolic reactions are not 100% efficient and, therefore, energy "escapes" from the organism 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 to thermoregulate is to turn to energy sources or to move away from them.
It is no coincidence that sunbathing behaviors are observed mainly in reptiles: when a lizard is resting on a stone "sunbathing", what it is really doing is obtaining the necessary energy in the form of heat to be able to carry out metabolic processes such as digestion or movement, among other things.
Thus, When we speak of basal metabolism (BMR), we refer only to the heat produced by endotherms, which are mammals and birds.which are mammals and birds.
If we wanted to quantify the amount of energy needed for the survival of a reptile or an amphibian, we would use a different term and methodology: the standard metabolic rate (SMR). This follows similar criteria to the basal metabolic rate, but environmental temperature must be taken into account, as it completely modulates the energy availability of the ectothermic animal.
What other parameters accompany basal metabolism?
This value is very useful, as we will see later, but it does not fully explain the energy demand of the ectotherm animal. does not fully explain the energy demand of the individual.. Before continuing, you should bear in mind the following:
Total energy expenditure (TEE): Basal metabolic rate (BMR) + endogenous thermogenesis (ET) + physical activity (PA).
The World Health Organization (WHO) gives a very good definition of total energy expenditure, in which the BMR plays a very important role: "it is the level of energy needed to maintain the balance between energy intake and energy expenditure, when the individual has weight, body composition and physical activity compatible with a good state of health".
As you can imagine this value fluctuates between individualsAs you can imagine, this value fluctuates between individuals, as we must pay attention to age, gender, ethnicity, place of origin and many other things.
What is the basal metabolism for?
Basal metabolism represents the minimum cost of the organism so that it can carry out vital maintenance functions that are not consciously stopped, such as cardiovascular, respiratory, endocrine, central nervous system, renal, hepatic, immune and thermogenesis (heat formation) phenomena.
These "basal" conditions require a specific environment in order to be quantifiedThe individual must have slept 10-12 hours, be in a state of post-absorption (not having ingested any food for at least 12 hours) and be under thermoneutral conditions 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 snowy mountain running from a bear, as their energy demand is much higher and the situation is far from normal.
How do you measure basal metabolic rate?
To measure a basal metabolic rate effectively (without using predictive formulas), it is necessary to use direct or indirect calorimetry. In the first case the amount of heat produced by the organism is measured in hermetically sealed chambers with insulated walls.. Here, the energy stored and the energy lost by convection is recorded after a stabilization period of at least 6 hours.
On the other hand, MBR can also be obtained by indirect calorimetry, a minimally invasive but very effective method. This is based on oxygen uptake and carbon dioxide release: because anabolism requires oxygen and CO2 is released, the amount of heat produced is related to oxygen consumed and CO2 released. Here Here comes into play the respiratory coefficient, a parameter that we will reserve for another opportunity..
Basal metabolic rate formulas
If you search the internet, you will see that there are several calculators that try to estimate your basal metabolic rate without performing any medical test. Although the indirect calorimetry method is the right one 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 rest.
- 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 such as the following:
- MEN BMR= (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5.
- FEMALES TMB= (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161.
If you want to keep some figures, we can tell you that some studies have calculated average basal metabolic rates in individuals from studies ranging from 1027 kilocalories/day to 2499 kilocalories/day. This is the energy needed to simply existso, based on exercise and activity, a variable caloric range must be added.
It is striking to know that, approximately, BMR decreases by 1-2% per decade after the age of 20, mainly due to the loss of pure 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, as their weight moves on different scales and, naturally, a mastodon animal will produce and lose much more heat than a small one, right?
In absolute values this postulation is correct, but things get interesting if you divide the BMR by the mass of the animal. If you divide by weight, you find 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, as their cellular metabolism is very fast and tissues and organs fail sooner. If, on the other hand, you see a gecko with the same weight as a mouse, you will discover that the latter lives up to 7 times longer. Why? Because the reptile does not generate heat, the energy demand and the work done by its organism is much lower.
Fascinating, isn't it? It turns out that, anecdotal as it may seem, the basal metabolic rate in endotherms determines their life expectancy and life strategies.. On a medical rather than biological level, this parameter is also useful for nutritionists and sports professionals to know how many calories should be ingested per day to maintain, increase or decrease the total mass of a specific organism.
- Bonfanti, N., Fernández, J. M., Gomez-Delgado, F., & Pérez-Jiménez, F. (2014). Effect of two hypocaloric diets and their combination with physical exercise on basal metabolic rate and body composition. Nutrición Hospitalaria, 29(3), 635-643.
- López-Fontana, C. M., Martínez-González, M. A., & Martínez, J. A. (2003). Obesity, energy metabolism and physical activity measurement. Spanish Journal of Obesity, 1(1), 29-36.
- Vargas, M., Lancheros, L., & del Pilar Barrera, M. (2011). Resting energy expenditure and body composition in adults. Journal of the Faculty of Medicine, 59(1), S43-S58.
(Updated at Mar 28 / 2023)