Biological age: what the number is, and what it is not

Biological age is the number people most want to see and the one most easily misread. It appears in the Clare app next to your health scores, and this post explains how it is estimated, what it can and cannot tell you, and why the direction it moves is worth more than the number itself.

Two ages

Chronological age is the number of years since you were born. It is precise and nothing changes it. Biological age is an estimate of how far along the ageing process your body appears to be, built from things that can be measured. Two people born in the same month can have very different biological ages, and the same person can move theirs.

How it is estimated

There is no single agreed method, and any product that presents its figure as a fact rather than an estimate is overselling. The main approaches are:

Blood marker clocks

These use routine laboratory values that shift predictably with age and with the risk of disease. The best known is PhenoAge, published in 2018, which combines nine markers with chronological age: albumin, creatinine, glucose, C-reactive protein, lymphocyte percentage, mean cell volume, red cell distribution width, alkaline phosphatase and white cell count. All nine are in a comprehensive panel, which is why blood-based clocks are the practical choice for a membership built around one. Clare's estimate is of this kind and is labelled as calculated.

Epigenetic clocks

These read chemical marks on DNA, called methylation, at hundreds of sites that change with age. The first, published by Steve Horvath in 2013, is accurate to within a few years of chronological age across tissues. Later clocks such as GrimAge are trained to predict time to death rather than age, and are better at picking up risk. They need a separate sample and a specialised laboratory, and cost several hundred dirhams each.

Function-based measures

Grip strength, VO2 max, walking speed and how quickly you can stand from the floor all track ageing well and are free to measure. They are not blood tests, but a wearable that estimates VO2 max adds a useful second opinion beside a blood clock.

What the number means

A biological age below your chronological age suggests your markers look like those of a younger group in the population the clock was built from. Above it suggests the reverse. That is all. It is a summary of markers you can already see individually, and it will move when they move.

What it is not:

• It is not a diagnosis, and no clinician will treat a biological age.
• It is not a prediction of how long you will live.
• It is not precise. A blood clock has an error of several years for an individual, so a reading of 38 for a 41-year-old is best read as "in the range you would hope for" rather than "three years younger".
• It is not comparable between products. Different clocks use different markers and different reference populations, and a number from one app cannot be set against another.

Why the UAE context matters

Most clocks were built from American or European datasets. Reference populations in the Gulf differ in body composition, insulin sensitivity and vitamin D, among other things, and a clock can misplace someone whose markers differ from its training set for reasons that have nothing to do with ageing. This is another reason to read the trend rather than the level. Your own baseline removes most of that problem, because whatever the clock's bias, it is the same bias at every reading.

What moves it

Because a blood clock is built from ordinary markers, it moves when they do. The largest contributions usually come from:

• Glucose and insulin sensitivity, through fasting glucose and, indirectly, through inflammation.
• Inflammation, through C-reactive protein and the white cell measures.
• Kidney and liver function, through creatinine, albumin and alkaline phosphatase.
• Red cell health, through mean cell volume and distribution width, which respond to iron, B12 and folate.

Sleep, resistance training, walking, body weight and alcohol move those markers over months. A short course of an infection or a hard training week can move them for a few days, which is a reason not to draw blood the day after either.

How to use it

1. Note the first estimate and then largely ignore it. It is one point.
2. Read the markers underneath it. They tell you what is driving it and what to change.
3. Retest at the same time of day, fasted, after a normal week, so the comparison is fair.
4. Judge the direction over a year, not the change over a month.

The most useful thing about biological age is that it gives people a reason to care about albumin and red cell distribution width, which nobody would otherwise look at. Treat it as a summary that points you to the markers, and it earns its place on the screen.