Members often ask for their biological age before anything else. It is a fair question and an easy number to print. It is also one of the easiest to misread.

Three kinds of clock

There is no single biological age. There are several methods, and they measure different things.

• Blood based clocks combine ten or so routine markers, such as albumin, creatinine, glucose, CRP and white cell count, into a predicted age. They are cheap and repeatable.
• Epigenetic clocks read methylation patterns on DNA. They track age closely across populations but can shift by two or three years between repeat samples from the same person.
• Functional measures, such as VO2 max, grip strength and walking speed, describe what the body can do rather than what it is made of.

The same person can read four years younger on one and two years older on another. Neither is wrong. They are answering different questions.

Where the number comes from

Most blood based clocks were built by training a model to predict death in a large cohort, then converting that risk back into years. A biological age of 46 for a 52 year old means your markers look like those of the average 46 year old in that cohort. It does not mean any organ is six years younger.

A biological age is a risk estimate written in years.

What moves it, and what only appears to

Because the number rests on a handful of markers, a change in one of them moves the whole result. A mild infection raises CRP and white cells and can add two years overnight. A week of heavy training can do the same through creatinine. We never compare two results unless both were taken in the same conditions.

How we report it

We give a biological age only alongside the markers that built it, so you can see what is driving it. Member 0198 read 49 at 55. Most of the gap came from a low CRP and good kidney function. His fasting glucose was pulling the other way, and that is where we started.