"Biological age" has become a popular way to talk about longevity, but the term covers several genuinely different measurement approaches — some backed by substantial research, some still emerging. This guide explains what biological age testing can and can't tell you, and focuses on the one longevity-linked metric with the strongest research behind it: cardiorespiratory fitness, measured as VO2 max.
Chronological age vs. biological age
Chronological age is simply how long you've been alive. Biological age is an attempt to estimate how "old" your body's systems function, independent of the calendar. It's a useful concept for tracking direction over time, but it's important to understand it as an estimate, not a diagnostic measurement with a single validated standard.
How biological age is typically estimated
- Epigenetic clocks — analyze patterns of DNA methylation. Primarily research tools, not standardized or FDA-cleared diagnostic tests.
- Biomarker-based composite scores — combine multiple blood biomarkers (see ONU's blood test results guide) into a single estimate.
- Functional and fitness measures — cardiorespiratory fitness (VO2 max), grip strength, and gait speed.
No single one of these is universally recognized as "the" biological age test.
VO2 max: the longevity metric with the strongest evidence behind it
Cardiorespiratory fitness — the maximum rate at which your body can use oxygen during intense exercise — has some of the most robust supporting research. Large population studies, including a widely cited 2018 analysis published in JAMA Network Open, have found cardiorespiratory fitness strongly and inversely associated with long-term, all-cause mortality risk.
VO2 max check: what wearables actually measure
The gold-standard method is a laboratory cardiopulmonary exercise test. Wearables (Garmin, Apple Watch, Whoop) estimate VO2 max using algorithms based on heart rate response during exercise, pace, and personal data — a reasonable approximation for tracking your own trend, but not as precise as a lab test.
How to increase VO2 max
- Aerobic base training (zone 2) — sustained, moderate-intensity effort. See ONU's guide to zone 2 training and heart rate zones.
- High-intensity interval training (HIIT) — shorter bursts of near-maximal effort.
Consistency over months, not a single dramatic workout, is what actually moves VO2 max.
Does stress actually age you biologically?
There's genuine, still-developing research connecting chronic stress to cellular aging markers, but it isn't a simple, precisely quantifiable relationship. See ONU's guide to acute vs. chronic stress.
What to do with a biological age result
Treat it as a snapshot and a motivational tool for tracking direction, not a verdict on your health.
How ONU approaches longevity tracking
ONU incorporates the underlying inputs that have the strongest evidence behind them — fitness trends, HRV, sleep quality, and lab markers where available — into your ongoing ONU Score.
Frequently asked questions
Every claim, built on the same foundation
Plain-language, evidence-checked explanations of the biology behind your health data — organized by the kind of question that usually brings people here in the first place.
1Blood Test Results Explained
Cholesterol, non-HDL, A1C, and metabolic panels — decoded in plain English, including what a borderline number is actually worth flagging to your doctor.
2Sleep Score Explained
Deep sleep, REM, and why your score moves the way it does — plus research-backed ways to actually improve both, not just chase a higher number.
3Why Am I Always Tired?
Tired but can't sleep, dragging through mornings, waking up unrefreshed — the most common, evidence-backed explanations, in plain language.
4HRV Explained
What heart rate variability measures, what counts as a good resting heart rate, and how HRV connects to zone 2 training and real recovery.
5Cortisol and Stress
What cortisol actually does in your body, what the viral terms get right and wrong, and which foods and supplements genuinely affect stress.
6Acute vs. Chronic Stress
The actual difference between short-term stress and chronic burnout, and whether stress genuinely speeds up aging — without the scare tactics.
7Biological Age Testing
VO2 max, biological age calculators, and the longevity metrics everyone's suddenly discussing — what's evidence-backed, and what's still unproven.
8Full Body Scans & Preventive Checkups
What full-body MRI and CT scans actually involve, what physicians say about screening healthy adults, and where routine tracking fits in between.
9Magnesium & Ashwagandha: A Practical Guide
Which magnesium form actually helps sleep, when to take it, and how much ashwagandha is typically used in studies — no supplement-aisle guesswork.
How this library is built
Every article here goes through the same process: a claim isn't published until it's checked against a primary, authoritative source — the CDC, NIH, WHO, American Diabetes Association, American Heart Association, or peer-reviewed research, depending on the topic — and every article stays inside ONU's wellness-lane guardrail: educational and non-diagnostic, with no disease, treatment, or cure claims anywhere. Where the science is genuinely unsettled or still developing, these guides say so directly, rather than overstating certainty for the sake of a cleaner narrative.
Nothing in this library exists purely as content marketing disconnected from the product. Each article maps directly to something ONU's app actually measures and interprets: the HRV guide explains the exact metric behind your recovery score, the Blood Test Results guide explains the same interpretation logic your uploaded labs go through, and the Cortisol guide explains the physiology behind your daily stress signal. Reading a guide here should make your own ONU Score meaningfully easier to understand, not just deliver abstract information disconnected from your actual data.
If you want a broad orientation rather than jumping straight to one specific question, a reasonable starting sequence is: Blood Test Results Explained (the foundation for understanding any lab work), Sleep Score Explained and HRV Explained (the two most common wearable metrics people misunderstand), then Cortisol and Stress (the physiology behind the terminology dominating health conversations online right now). From there, the remaining guides go deeper into specific, related questions.
See how ONU turns this knowledge into your daily ONU Score in the full product overview.
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