Last week, I saw a doctor post their BioAge online, treating it like a lab result. Now, gyms, fitness trackers, and mail-in kits all give you some version of your ‘age.’ I wanted to find out how these numbers are really calculated.

What?

Let’s start with the most reliable method. A USC-led study published in npj Aging on July 20 tested the five most popular epigenetic clocks on 3,227 adults from the Health and Retirement Study. These clocks, Horvath, Hannum, PhenoAge, GrimAge, and DunedinPACE, did not share a single gene in common. The authors said there were “more unique than common biological processes.”

These clocks can also give inconsistent results. A 2022 study in Nature Aging found that technical noise alone could make six of these clocks show up to a nine-year difference when testing the same sample twice. Newer versions have reduced this gap to about 1.5 years, so there is a fix. Still, most reports don’t say which version was used for your result.

Consumer options are even less accurate. EGYM gym machines use four fitness tests: strength, cardio, metabolism, and flexibility. WHOOP figures out your age from months of sleep, training, and recovery data. Garmin mostly uses your VO2max. This means the same person could get three different ages from three devices.

So what?

None of these tools truly measure aging. They look at your fitness and habits, then turn those results into years, since that’s a number people understand.

I think that’s okay. If a simple number gets someone to go to the gym on a Tuesday, that’s more useful than a perfect measurement no one pays attention to. Treat it like a game. If your watch pushes you to work out a fourth time this week, it’s working.

The problem is when doctors present a vendor’s score as if it’s a real lab result. Then the suggestion becomes a claim. Most people can’t tell the difference. That’s why credentials matter, and using them to support an unproven number isn’t a good idea.

Now what?

  • See your ‘age’ score as a tool to track your habits, not as something for your official health record.
  • Focus on the numbers behind the score: your VO2max (in ml/kg/min), strength, waist size in centimeters, and blood pressure. Each of these can predict health outcomes on its own.
  • Only compare your results to your own past results, using the same device and conditions. Numbers from different brands can’t be compared.
  • If you get a score, ask your provider two questions: Has this score been tested against illness or death in people outside their database? And which version of the clock are they using?
  • If a gym blood pressure cuff shows 140/90 or higher, check your blood pressure at home over several days. The 2024 European Society of Cardiology guidelines recommend out-of-office readings before making a diagnosis.

I made a similar point in July about the supplements in your cupboard. You can read more in The Doctor’s Edit on Your Longevity Drawer (https://www.andreheeg.com/archive/the-doctors-edit-on-your-longevity-drawer).

Let the number motivate you to go to the gym. That’s its only purpose.

Stay healthy.

Andre

Editor's note

Editorial clarification and evidence update

The Upward ARC is an educational newsletter for a general audience, not a medical publication. It may use ordinary-language metaphors and simplifications to make complex evidence readable. Those phrases are not clinical terminology. The editorial standard remains unchanged: do not fabricate, cite falsely, hide important uncertainty, or simplify a point so far that it changes the practical meaning.

Updated September 3, 2026. The historical edition above remains unchanged.

What the five clocks still predict: The 2026 study found that five DNA-methylation clocks emphasized different genes and pathways. It did not find that the clocks were useless. The same analysis reported that the clocks still predicted aspects of health, aging and mortality, with different strengths for different outcomes. [1]

Where the nine-year figure came from: The nine-year difference was the upper end of repeat-test disagreement for the original clocks in one technical study. Retrained principal-component versions brought most repeat measurements within 1.5 years. Several authors disclosed commercial interests related to biological-age testing, and Elysium Health supplied datasets. [2]

Consumer scores are not the same test: EGYM BioAge combines strength, cardiovascular, metabolic and flexibility measures. WHOOP Age uses six months of sleep, strain and fitness data, and WHOOP describes it as a wellness feature rather than a medical device. These scores measure different constructs. The evidence reviewed here does not establish that one is more accurate against a shared clinical reference. [3]

Risk markers are context, not diagnoses: Cardiorespiratory fitness, strength, waist measures and blood pressure can help describe health risk and change over time. They are not interchangeable, and no single reading or score determines an individual's biological age, diagnosis or prognosis. [4]

How to follow up the 140/90 reading: A gym blood-pressure reading of 140/90 mmHg is a reason to repeat the measurement under standard conditions, not a diagnosis by itself. The 2024 ESC guideline defines office hypertension at 140/90 or higher and a home average at 135/85 or higher. Confirmation should use repeated office readings or validated home or ambulatory monitoring. [4]

References

  1. Arpawong et al. DNA methylation clocks exhibit heterogeneity in associations with mortality and aging-related outcomes. 2026.
  2. Higgins-Chen et al. A computational solution for bolstering reliability of epigenetic clocks. Nature Aging, 2022.
  3. WHOOP. Healthspan: WHOOP Age and Pace of Aging Guide.
  4. European Society of Cardiology. 2024 ESC Guidelines for the management of elevated blood pressure and hypertension.

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