Although sometimes we perceive the heartbeat, as a rule we are not aware of what is happening inside us. The normal thing is that we do not perceive, for example, the bustle of the neurons, nor the transfer of salts and water in the kidneys. The physics and chemistry underlying these processes are what make up the basal metabolism, the ones that keep us alive. The energy we spend on them is the price we pay for being alive at all times and represents between 50% and 70% of all that we use.
Other physical and chemical processes allow us to carry out activities such as speaking, walking and working. In the long run, we also need them to live, although not at all times; the corresponding metabolic activity is linked to those physical activities. We call routine metabolism, or habitual, to the sum of both fractions.
Metabolism in children and adults
In the fetal phase metabolism is intense. On the one hand, small organisms have, by themselves, a very high metabolic activity; it is a universal biological phenomenon. And on the other hand, the fetus is a growing organism, inside it new organs and tissues are being formed rapidly and this activity is metabolically expensive. However, if the effect of size is discounted, the total metabolic intensity of the smallest is not so high; it would be below, even, that of adults. It is influenced by the fact that in the womb physical activity is, quite understandably, limited.
After birth, both basal and habitual metabolic levels rise very rapidly, reaching peak values around the first year of life. Creatures of that age have a metabolism that is, discounting the aforementioned effect of size, 50% more intense than that of adults. The great production of new tissue that growth demands is at the base of such a high metabolism. This is an important aspect, because the more intense the metabolic activity, the more food must be supplied to the baby in construction.
From 15 months of life, the metabolism decreases until the age of 20, to then remain practically constant until the fifth or sixth decade. Around this age, a decrease in the metabolic level begins again, which seems to be related to the organic deterioration that occurs associated with aging. Except during the fetal phase, in which metabolism linked to activity is less important, during the rest of life both fractions, the one corresponding to vital maintenance and the one associated with physical activity, follow fairly parallel courses.
The effect of senescence
The decline in basal metabolic activity with age occurs largely because the biological activity of all organs is reduced, especially the most metabolically active ones, such as the brain and liver. And that of the component linked to muscle activity, because after reaching maximum values between the ages of 5 and 10, it drops rapidly until 18, then stabilizes, and drops again more slowly after 45 years, approximately. .
In short, the vital period of maximum basal metabolic intensity is the first year of life, due to the strong growth of highly metabolically active organs. The years of maximum physical activity are the years before puberty. The minimum metabolic intensity is reached at advanced ages, past 70, for both components of metabolism, because both the activity of organs and tissues and that of the muscular system have already been in decline for years. (YO)
The original version of this article was published in the Scientific Culture Notebook of the UPV/EHU.
- Juan Ignacio Perez Iglesias
- President of the Advisory Committee of The Conversation Spain. Professor of Physiology, University of the Basque Country / Euskal Herriko Unibertsitatea