What are the differences between chronological age and biological age?

Time is a physical magnitude with which human beings measure the duration or separation between events. This dimension helps us to order events in sequences, establishing a past, a future and a third set of events that are not directly related to these terms.

When we say that "time passes for everyone", we mean, although we do not know it, an essential condition to define life. Organic matter degrades over time, and this event is essential to conceiving one's own existence. Without going any further, few more exact definitions exist of life, at the biological level, than "the interval between birth and death."

Thus, senescence and death are an integral part of living organisms. We are born, we grow, we reproduce and we die, in a so far infinite cycle of organic matter and energy. Even so, it is curious to know that, although time passes for everyone, it does not do so in the same way according to certain conditions of the organism. Here we introduce you the differences between chronological age and biological age: immerse yourself with us in this world of science and metaphysics.

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How are biological and chronological ages different?

The chronological age is the one that is marked by the date of birth of the individual. This is a quite meaningless figure at the biological level, but it serves in the social environment to legislate and categorize population groups based on their knowledge, work aptitude and many other things.

The number of years that defines us is nothing more than a social construct, since a sick person with 30 years can have the health and vital prognosis of an individual of 90.

Chronological age is really a chronometric figure, that is, only quantified based on the rhythm of a clock. Despite the fact that it is a relative and purely mechanical measure, it can be married in a certain way with the biological passage of time.

If you put a clock on one side of the "equation" and a specific physiological process on the other (such as the first ovulation in the female of a given species), a kind of useful bijection is formed.

The distance between birth and the first ovulation has been defined chronologically (or rather chronometric), since a total of X seconds, minutes, days, months and years have passed until reaching that exact point, truth? Yes and no. Hang on, it's time to digress and abstract from typical terms.

While the chronological age is the one that marks the clock and the date of birth of the individual, the biological age is the one that represents their internal functional state, marked by the aging of cells, tissues and organs. The difference in the example previously given is that the physiological processes do not correspond with a physical process that is invariably successive (as is the passage of time at a level chronometric).

Thus, the vital phases of a living being are not determined based on their chronometric distance (year 1, year 2, year 3, etc.), but by the qualities of the physical system in which they are manifested. In other words, what determines the ovulation of a female of a given species is not the past time, but the concentrations of sex hormones in the blood, for example. We know that all this may sound complex, but it is enough for us to make the following message clear: the biological and mechanical clocks do not follow the same time patterns.

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How is the biological age of an organism determined?

So what use is the physical timescale to quantify the age of a living being? Undoubtedly, the calendar can give us various clues regarding the health of an organism, since generally someone who has spent 20 laps around the Sun on Earth will have statistically more years ahead of him than someone who has lived 80. We emphasize the term “statistically”, that is to say, on average, because the cases of early mortality due to pathologies and accidents are there and they are an undeniable reality.

For more complexity and interest, it is essential to emphasize that chronological and biological age differ based on different parameters. Among them, we have the following:

• Epigenetics: modifications of gene expression that do not respond to a change in the organism's DNA. The genome is "fixed" throughout life, but its expression is not.
• Lifestyle: to confirm this parameter, we only need to look at the lung tissue of a chronic smoker and a person who does not use tobacco with the same age.
• Diseases: various diseases can cause cell destruction, which sometimes cannot be recovered. This causes a clear premature aging.
• Environment: it is not the same to grow up near an industrial plant that continuously emits smoke than to live in the countryside. The body notices environmental variations.

There are various biomarkers that show discrepancies between biological and chronological age. One of the most widely used biomarkers in this field is the telomere shortening rate, as you will see below.

Telomere shortening rate and biological age

Telomeres are the ends of chromosomes. They are non-coding regions of DNA (not used for protein synthesis) and highly repetitive (repeating nucleotide sequences) whose function is to provide structural stability to the chromosomes of cells eukaryotes.

Telomeres are one of the most interesting bases of aging. Although we do not want to go into complex genetic terms, it is enough for us to know that, with the duplication of genetic information, it is impossible to transcribe absolutely all the DNA. A) Yes, as a cell line divides and renews, telomeres get shorter. When these reach a critical length, aging processes are triggered, as the integrity of the cell itself is destabilized.

It is even more interesting to know that there is an enzyme called telomerase present in the cells of the germ line, which is found in fetal tissues and is capable of causing elongation telomeric. Telomerase is repressed in mature somatic cells after birth, with telomere shortening after each cell division in adult tissues.

Deficiency in telomerase levels in key developmental stages results in early onset of pathologies, such as aplastic anemia, immune problems or pulmonary fibrosis, or what is the same, accelerated aging and an increase in biological age. Thus, it follows that the telomere shortening rhythm (and the previous action of telomerase) are about perfect bioindicators to predict the biological age of the individual, regardless of what the clock ticks or calendar.

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Each part of our body has an age

As surprising as it may seem (and saving terminological distances), your brain may be "younger" than your left leg, for example. Imagine that we compare the “age” of the liver of an alcoholic person with the one that presents in, say, their eyes.

Chronic alcoholism can lead to liver cirrhosis. When exposed to toxic agents over time, the hepatocytes (main cells of the liver) are destroyed, and this space is replaced by scar tissue. This new tissue does not present purifying functionality so, little by little, the liver loses its capacities. It could be said from an abstract point of view that the liver has aged at a dizzying rate.

That same person can maintain adequate eye health and not have suffered any refractive error or pathology in their eyes throughout their life. While your ocular apparatus remains young, your liver is that of an elderly person. As you will understand, there comes a point where both events go hand in hand, since the chronic failure of a vital organ usually leads to a general systemic collapse.

Resume

Fascinating, right? Time is still a social construct and, as such, it does not define the totality of what surrounds us. The physiological stages of our body are part of a widely interconnected internal system, so they do not have to be governed by the hands of a clock in all cases.

Genotype, heredity, family history, lifestyle, environmental conditions, and many more factors shape the concept of biological age. Thus, although time passes for everyone, we can assure you that it does not do it in the same way in each individual.

Bibliographic references:

• Liver cirrhosis, Clínica Universidad de Navarra (CUN). Picked up on February 23 at https://www.cun.es/enfermedades-tratamientos/enfermedades/cirrosis-hepatica
• Biological age, Genotype.com. Picked up on February 23 at https://genotipia.com/edad-biologica/
• Rodríguez López, A. (2019). Bibliographic review: common methods for estimating biological age.
• Toro, J. M. (1997). Old age and aging from the perspective of the experimental synthesis of behavior. Latin American Journal of Psychology, 29 (3), 459-473.
• Vargas, E., & Espinoza, R. (2013). Time and biological age. Arbor, 189 (760), 022.