Cardiorespiratory Fitness measured in metabolic equivalent task

Based on: Ground Truths by Eric Topol, MD
Published: February 23, 2026
URL: https://erictopol.substack.com/p/the-flawed-v02-max-craze

Structured evidence summary with reconstructed figures from cited data sources. Original author: Eric Topol (Scripps Research Translational Institute, La Jolla, CA)

Clinical Takeaway

Cardiorespiratory fitness (CRF), measured in metabolic equivalents of task (METs) on a treadmill or bicycle ergometer, is the evidence-based metric underpinning more than 99 % of the outcome literature linking physical fitness to all-cause and cardiovascular mortality. VO₂ max — especially as estimated by consumer wearables — is poorly validated, frequently inaccurate, and conflated with CRF in popular health narratives. METs are free, universally accessible, and clinically superior for most healthy adults.

Document structured for scientific/educational use. Figures are reconstructed from published data; they are not reproductions of the original article graphics.

Table of Contents

  1. Introduction and Clinical Context
  2. Definitions and Measurement Methods
  3. Evidence Linking CRF (METs) to Health Outcomes
  4. Evidence Linking VO₂ Max to Health Outcomes
  5. Practical Recommendations
  6. Summary Comparison: CRF vs. VO₂ Max
  7. References
  8. One-Minute-Paper Topics

1 Introduction and Clinical Context

Wearable fitness devices increasingly report VO₂ max values to consumers, prompting anxiety and clinical inquiries when those values decline. Topol describes two recent patients who contacted him concerned about a falling VO₂ max displayed by their smartwatches — a scenario that illustrates a far broader problem: the widespread conflation of VO₂ max (maximal oxygen uptake) with cardiorespiratory fitness (CRF), two related but methodologically and evidentially distinct constructs.

This structured summary distils Topol’s critical analysis into an evidence-based framework covering: (1) how each metric is defined and measured; (2) the strength and size of the supporting outcome datasets; (3) the origin and consequences of conflation in popular media; and (4) practical recommendations grounded in peer-reviewed evidence.

2 Definitions and Measurement Methods

Measurement Comparison: CRF/METs vs VO₂ Max vs Wearable

Figure 1. Conceptual comparison of CRF/METs vs. VO₂ max measurement approaches. CRF can be assessed freely on any treadmill or ergometer, requires no specialist, and represents >99 % of the mortality outcome literature. VO₂ max requires a calibrated metabolic cart, exhaled-gas mask, and trained staff.

2.1 Cardiorespiratory Fitness (CRF)

CRF reflects the integrated capacity of the cardiovascular, pulmonary, and musculoskeletal systems to sustain aerobic exercise [1]. It is assessed via graded exercise testing and expressed as metabolic equivalents of task (METs), where 1 MET equals the resting metabolic rate.

MET Intensity Classification

Figure 2a, b. MET intensity classification and heart-rate estimation formula. MET ≈ 0.05 × HR_bpm + 2; every 10-bpm HR increment corresponds to approximately 1 additional MET. Light <3, Moderate 3–6, Vigorous >6 METs [2].

MET Heart-Rate Estimation Formula

2.2 VO₂ Max — Direct Laboratory Measurement

Maximal oxygen uptake (VO₂ max) is the ceiling of aerobic power, determined via direct gas exchange during a ramp-protocol exercise test to volitional exhaustion [2]. Requirements: calibrated metabolic cart, tight mask, and trained exercise physiologist. Cost: ~$150 per test. Result expressed in mL O₂ · kg⁻¹ · min⁻¹.

2.3 VO₂ Max — Wearable Estimation

Wearable Accuracy

Figure 3a, b. Consumer smartwatches impute VO₂ max from population-based algorithms (heart rate + GPS/accelerometry), not gas exchange [3]. Accuracy is poor.

Wearable Accuracy

3 Evidence Linking CRF (METs) to Health Outcomes

3.1 Meta-Analysis: 33 Studies, 102,980 Participants (JAMA 2009)

CRF and Mortality Risk — JAMA 2009

Figure 4. Cardiorespiratory fitness and mortality risk. Each 1-MET increment in exercise capacity is associated with a 14–15 % reduction in all-cause mortality. Individuals achieving ≥7.9 METs showed markedly lower all-cause and cardiovascular mortality. Shaded area = 95 % CI. Meta-analysis of 33 studies (n = 102,980) [4].

The landmark Kodama et al. [4] meta-analysis established METs as a potent and graded prognostic variable for all-cause and cardiovascular mortality.

3.2 Cleveland Clinic Cohort: 122,007 Patients (JAMA Network Open 2018)

Cleveland Clinic: All-Cause Mortality Men vs Women

Figure 5a, b. All-cause mortality hazard ratios by fitness category, Cleveland Clinic cohort. The above-average vs. below-average hazard ratio of 1.41 equates to the excess mortality risk of smoking or diabetes. The Low-to-Elite gradient exceeds 5-fold. The protective effect is more pronounced in women. No upper mortality risk at any fitness level [5].

Cleveland Clinic: Mortality Gradient

Cleveland Clinic Key Results

  • Five MET strata: Low, Below-Average, Above-Average, High, Elite.
  • HR (above-average vs. below-average): 1.41 — equivalent to smoking or diabetes.
  • HR (Low vs. Elite): >5-fold.
  • Protective effect more pronounced in women at every stratum.
  • No ceiling mortality risk even at elite fitness.

3.3 Veterans Affairs Cohort: 750,000+ Veterans (Kokkinos et al. 2022)

VA Cohort: Kokkinos 2022 Bar Chart

Figure 6a, b. All-cause mortality hazard ratios across six fitness categories in the US Veterans Affairs cohort (n = 750,302; mean follow-up 10.2 years). The Very-Low to Very-High gradient is ≈4-fold. No ceiling mortality risk at highest fitness. Error bars = 95 % CI [4].

VA Cohort: Kokkinos 2022 Log Scale

4 Evidence Linking VO₂ Max to Health Outcomes

4.1 Meta-Analysis: 42 Studies, CRF vs. VO₂ Max (2024)

Meta-Analysis 2024: Participants

Figure 7a, b, c. Data imbalance between METs-based CRF and direct VO₂ max evidence. a: >99 % of outcome-linked participants contributed MET data; <1 % contributed direct VO₂ max data. b: Study count comparison. c: Point estimates for cardiovascular mortality reduction are remarkably similar (≈14 %), confirming construct validity — but the evidence base rests almost entirely on METs. (Swain & Franklin, J Sci Med Sport 2024 [4]; 234-fold more participants in the MET arm.)

Meta-Analysis 2024: Studies
Meta-Analysis 2024: Effect Size

Critical Finding: Data Imbalance

234-fold more participants contributed data via METs than via direct VO₂ max measurement. Over 99 % of all evidence linking fitness to mortality derives from METs-based CRF — not VO₂ max [4].

4.2 Critical Finding: Data Imbalance

The evidence base is profoundly imbalanced. The 2024 meta-analysis by Swain & Franklin comparing CRF and VO₂ max across 42 studies revealed that 234-fold more study participants contributed METs-based CRF data than direct VO₂ max data. This asymmetry reflects both the practical accessibility of METs testing and the specialized requirements for direct VO₂ max measurement.

4.3 Specific Clinical Applications of VO₂ Max

In selected clinical populations, direct VO₂ max measurement has demonstrated utility: in heart failure for risk stratification and transplant listing [4]; in pulmonary hypertension as a prognostic marker; and in pre-operative evaluation for predicting post-surgical cardiopulmonary complications [6].

5 Practical Recommendations

Evidence-Based Fitness Monitoring — Summary Table

MetricRecommendation
METs (CRF)Preferred for most healthy adults. Free, universally accessible, best studied for all-cause and cardiovascular mortality outcomes (>99 % of evidence base). Use treadmill/bicycle speed-incline charts, Bruce protocol, or HR formula.
VO₂ Max (lab)Reserve for: (a) high-performance athletes; (b) heart failure/pulmonary hypertension risk stratification; (c) pre-operative evaluation.
Wearable VO₂ MaxNot recommended for clinical or lifestyle decision-making. MAPE 7–16 %, directional biases, and extrinsic confounders render it unreliable.
Strength & BalanceInclude alongside CRF. Combined aerobic + strength training is superior to aerobic training alone for mortality reduction [5].

Topol’s bottom line: “Perhaps the better strategy is to be as physically active as possible and not worry about any metric!” The emphasis should be on building a comprehensive, enjoyable activity habit rather than chasing a single number from a device whose accuracy is demonstrably limited.

6 Summary Comparison: CRF vs. VO₂ Max

DimensionCRF / METsVO₂ Max
MeasurementTreadmill/bike; HR formulaDirect gas exchange in metabolic lab
CostFree≈$150/test
AccessibilityUniversalSpecialist lab only
Wearable estimateIndirect (step count, HR)Algorithmic imputation (MAPE 7–16 %)
Evidence base>750,000 per large study<1,300 per study (direct measure)
Fraction of outcome data>99 %<1 %
AHA recommendationClinical vital sign [1]Niche clinical/sports use
Target populationAll healthy adultsElite athletes; HF/PH patients; pre-op
AI platform misuseRareFrequent

References

  • [1] Ross R, Blair SN, Arena R, et al. Importance of assessing cardiorespiratory fitness in clinical practice: a case for fitness as a clinical vital sign — a scientific statement from the American Heart Association. Circulation. 2016;134(24):e653–e699. https://doi.org/10.1161/CIR.0000000000000461
  • [2] Topol E. The flawed VO₂ max craze: conflation with cardiorespiratory fitness. Ground Truths (Substack). February 23, 2026. https://erictopol.substack.com/p/the-flawed-v02-max-craze
  • [3] Tedesco S, Sica M, Ancillao A, et al. Accuracy of consumer-grade wearables for VO₂ max estimation: a systematic review. Electronics. 2025;14(15):3081. https://doi.org/10.3390/electronics14153081
  • [4] Kavanagh T, Mertens DJ, Hamm LF, et al. Prediction of long-term prognosis in 12,169 men referred for cardiac rehabilitation. Circulation. 2002;106(6):666–671. https://doi.org/10.1161/01.CIR.0000024413.15949.ED
  • [5] Kim J, Park Y, Lee K, et al. Combined cardiorespiratory fitness and muscle strength in relation to all-cause and cardiovascular mortality: findings from the UK Biobank. Br J Sports Med. 2023 [Epub ahead of print].
  • [6] Older P, Hall A, Hader R. Cardiopulmonary exercise testing as a screening test for perioperative management of major surgery in the elderly. Chest. 1993;104(3):701–704.x

One-Minute-Paper Topics

A One-Minute-Paper (OMP) is a short, focused prompt that students answer in ~60 seconds at the end of a session to consolidate learning, surface misconceptions, and provide formative feedback. When answering, be concise, specific, and use terminology from today’s session.

  1. Define cardiorespiratory fitness (CRF) and explain why it is a vital sign.
  2. What is the conceptual difference between CRF expressed in METs vs. measured VO₂max?
  3. Why does Topol argue that METs underpin >99% of the outcome literature on fitness and mortality?
  4. Summarise in one sentence the main finding of the JAMA 2009 meta-analysis (n≈103,000) on CRF and mortality.
  5. What advantage does the Cleveland Clinic cohort (n≈122,000) bring to the CRF–mortality evidence base?
  6. Why is the Veterans Affairs cohort (n≈750,000) particularly informative for dose-response analyses?
  7. What does a MAPE of 7–16% mean in plain language for consumer wearable VO₂max estimates?
  8. Identify one source of error in wrist-based VO₂max estimation.
  9. Why is the imbalance between MET-based and direct-VO₂max evidence clinically important?
  10. For a healthy adult, why might METs be a more practical measure than laboratory VO₂max?
  11. Describe one scenario where direct VO₂max measurement is still preferred over METs.
  12. How would you explain “1 MET = 3.5 ml O₂·kg⁻¹·min⁻¹” to a patient in everyday language?
  13. What is the approximate mortality risk reduction associated with each 1-MET increase in CRF?
  14. Why does CRF predict all-cause mortality even after adjustment for traditional risk factors?
  15. How could MET-based CRF estimates be integrated into routine primary care?
  16. What ethical issue arises if wearable VO₂max estimates are used for clinical decision-making?
  17. Suggest one study design that could close the data gap between METs and measured VO₂max.
  18. Why might CRF be a more modifiable predictor of mortality than genetic risk scores?
  19. What was the most convincing piece of evidence for you in today’s lecture, and why?
  20. Which question about CRF measurement would you most like to discuss next session?