5 functional biomarkers that track biological age (most are free)

Your doctor checks your blood pressure, cholesterol, maybe fasting glucose. These are useful. They are also lagging indicators. By the time they’re abnormal, the underlying process has been running for years.

There are earlier signals. Functional signals. That your body gives off well before any blood test turns red. Most of them you can measure yourself, at home, for free or close to it. They shift years before a diagnosis. They’re the markers where intervention has the most leverage. And most people write them off as “just getting older.”

They’re not. They’re measurable. They’re modifiable.

Five matter most.

1. Grip strength

The first marker is grip strength, and the data on this might surprise you.

A study published in The Lancet in 2015 followed 139,691 adults across 17 countries for a median of four years. The finding: grip strength predicted all-cause mortality and cardiovascular mortality better than systolic blood pressure. Each 5 kg reduction in grip strength was associated with a 16% increase in all-cause mortality and a 17% increase in cardiovascular mortality.¹

Grip strength is a stand-in for whole-body neuromuscular health, hormonal status, nutritional adequacy, and mitochondrial function. When grip is declining, something systemic is shifting. When it holds or improves, your physiology is supporting it.

How to measure: a hand dynamometer (under £30 / $30) gives an accurate reading. Squeeze hard for 3 seconds, three times per hand, take the highest. Track monthly. Trend matters more than absolute number.

Rough thresholds. Below 30 kg for men or 20 kg for women is typically associated with elevated mortality risk in cohort data. Most healthy adults under 50 should comfortably exceed 40 kg (men) or 25 kg (women).

2. Sleep architecture

Not just sleep duration. Sleep architecture. How much of your night is spent in deep slow-wave sleep, how often you wake, how quickly you fall asleep.

Slow-wave sleep is the deepest, most physically restorative phase. It regulates growth hormone, clears inflammatory markers, and supports memory consolidation. From the late 30s, slow-wave sleep declines naturally with age, but the rate of decline varies enormously between people, and most of that variability is behavioural.

How to track: consumer wearables (Oura, Whoop, Apple Watch, Garmin) all estimate sleep stages reasonably well. They’re not laboratory-accurate, but the trends in your own slow-wave sleep are informative. If your deep sleep has been dropping over months, it’s worth investigating.

Levers that shift it: consistent sleep timing, cool dark bedroom, reduced evening stimulation, no caffeine after 2 pm, no alcohol close to bed.

3. Heart rate variability (HRV)

Heart rate variability is the natural variation in time between consecutive heartbeats. Higher HRV indicates the autonomic nervous system is flexible. Able to ramp up under demand and downshift under recovery. Lower HRV signals chronic stress, poor sleep, overtraining, or systemic illness.

The absolute number depends on age, fitness, and genetics, so comparing yourself to population norms is less useful than comparing you to you over time.

How to track: most wearables now report overnight HRV. Watch the weekly trend rather than spot readings. A sustained 2-week dip usually signals something. Accumulated stress, brewing illness, training load that’s become unrecoverable.

4. Recovery capacity

How long does it take you to feel like yourself after a hard day, a hard workout, or a poor night’s sleep?

This is hard to measure objectively, but it’s one of the most honest internal signals you have. The same training stimulus that once required a single rest day now requires two. The same poor sleep that used to clear in 24 hours now needs 48. The function is sliding.

Levers: the same five basics. Sleep, training, protein, stress, relationships. Plus deliberate down-regulation through paced breathing, time in nature, and structured rest. Recovery capacity is the slowest biomarker to shift and the most rewarding when it does.

5. VO2 max (cardiorespiratory fitness)

VO2 max is the maximum rate at which your body can use oxygen during effort. It’s the single strongest modifiable predictor of all-cause and cardiovascular mortality across the literature.

Per one-MET increase in cardiorespiratory fitness, the risk of dying from any cause drops by approximately 14%, and from cardiovascular disease by 16%. The effect is linear across the range most people will ever reach. (See Hard exercise vs long exercise for the underlying data.)

How to measure: lab testing is the gold standard, but most modern wearables (Garmin, Apple Watch, Fitbit) estimate VO2 max from running pace and heart rate. The estimate is reasonable for tracking trends over months, even if not laboratory-grade.

Levers: structured aerobic training, particularly with intervals. Going from sedentary to moderately fit produces the largest single shift in this number anyone will ever achieve.

How to start

Don’t track all five at once. Pick one.

The lowest-barrier entry points are grip strength (a £30 dynamometer, 30 seconds, monthly) and consistent sleep timing (a notebook). Both reward small consistent investment with month-on-month signal.

Add others when those two are running smoothly. The “measure-everything” phase is appealing but is one of the surest ways to abandon the practice altogether.

The bottom line

Standard medicine is excellent at diagnosing disease and worse at tracking the slow erosion that precedes it. Functional biomarkers fill that gap. They shift earlier than blood tests. They respond faster to behaviour change. And most of them you can measure yourself.

Pick one. Track it for three months. Then add the next.