The Difference Between Chronological and Biological Age & Why It Matters
Updated: Jan 23
The second that we are born, two clocks start ticking: one chronological, the other biological. Chronological age is simple - it's the amount of time that has passed since you were born. This is measured in standardized terms: years, months, days, minutes, seconds. This is more or less how most people and cultures currently think about age. This is a standardized measurement for everyone.
The biological age clock, on the other hand, is much more complex. This one is measured by physiological markers, and is largely affected by genetics, as well as external factors that influence our genetics, like diet, exercise, sleep habits, stress levels, and drug and alcohol use. While not standardized, medical professionals and scientists use specific biomarkers to determine what age your body functions as. For example, a 40-year-old who has lived a high stress lifestyle, smokes heavily, and has spent a lifetime eating highly processed low-nutrition food may have the biological age of someone ten years his senior. Whereas a 70-year-old who has never smoked, swims daily, and maintains a largely Mediterranean diet, may have the biological age of someone ten years her junior. In short, biological age is a measure of each individual’s ageing process - cellular age if you will - and an overall proxy for individual health.
Let's dig deeper: how then, do we measure biological age?
At present, there is no objective measurement for the subjective determination of biological age. However, over the past decade, scientists have begun to offer frameworks in which to quantify this qualitative information. At present, the main means of measurement relies upon DNA methylation. Methylation is the chemical modification of DNA, and therefore the biological mechanism which individual cells use for gene expression. While you inherit your unique genetic makeup - or genes - from your parents, methylation affects how these genes are expressed across your lifespan. While your genetic makeup (or DNA) remains constant throughout your life, different environmental factors affect gene expression: the activity of DNA can change, without any change to the sequence (think, on-off switches). The study of this is known as epigenetics.
While genetic testing can give you clues to your genealogy, epigenetic testing can give you clues to your health and longevity, and the speed at which you are aging. Due to extensive epigenetic study of genes and methylation across the lifetimes of thousands of individuals, science has come to predict ages based purely on methylation of genes. This has resulted in what is called the “epigenetic clock”. The epigenetic clock provides us with an impression of our true age at the cellular level (hence, biological age). It is well documented that lifestyle and environmental factors - diet, exercise, pollution, disease - can accelerate or decelerate aging, and the overall epigenetic clock is a sort of cumulative measurement of cellular ageing due to these combined factors. While there isn’t one globally accepted method for measuring the epigenetic clock, commercial tests have become available based upon the general scientific consensus surrounding DNA methylation/demethylation. These tests are explored in a separate blog post, but have the potential to give individuals compelling data on how your lifestyle measures up against your chronological age.
More recently, scientists have developed a new “clock” which uses a few hundred of the thousands of proteins that circulate in our block to measure our biological age. This new “proteomic clock” follows the changes in levels of key blood plasma protein biomarkers over time. The researchers looked at 2,925 proteins in blood and identified 373 proteins levels which together were accurate predictors of age across the cohort of over 4,200 adults (aged 18-95). When looked at at the individual level, the biomarkers of some blood appeared younger or older than expected when compared to chronological age. Those cases where biomarkers measured younger than the participants’ chronological age, there was evidence of better cognitive and physical abilities, corroborating younger biological age. As with the more widely studied epigenetic clock, the proteomic clock has great potential to provide a snapshot of health and risk of age-related disease. In addition, it provides the possibility, along with epigenetic testing, to triangulate biological age. As the study authors say, “Thus, a deeper understanding of aging is likely to provide insights into mechanisms of disease and to facilitate the development of new antiaging therapeutics”.
It is worth highlighting that researchers were motivated to explore plasma proteins as a potential for measuring biological age following astonishing studies which showed that the transfusion of the blood of young mice to old mice reversed aspects of aging across multiple body tissues. In addition, studies showed that plasma infusions from old mice accelerated brain aging in young mice. These studies provided preliminary evidence that the proteins found in blood plasma contained key aging regulators, and that the identification of age relevant protein signatures was possible.
One important fact about the mechanism of biological ageing is that the epigenetic and proteomic clocks do not tick in a smooth and linear way the way that chronological age does. Biological ageing happens in bursts. Big changes happen in childhood and adolescence, as the body develops and changes rapidly. In addition, researchers have found that major shifts in protein signatures occured around three points in life: age 34, 60, and 78. This suggests that we should expect major biological changes in waves around these points, and that by lowering our biological age through age-decelerating habits, we can postpone the onset of these “waves”.
Lastly, these studies are yet to answer the question of whether the methylation process and protein biomarkers are simply a reflection of the aging process on the body, or if they are themselves driving aging. In any case, the core concept of agehacking is this: while we cannot stop the slow, steady march of your chronological age, your biological age - which is a good proxy for your overall health - is malleable. And this, this is worth exploring.