Fitness predicts health—but does not guarantee it

Cardiorespiratory fitness describes how well the heart, lungs, blood and muscles work together during sustained exercise. A 2024 overview identified 26 systematic reviews with meta-analysis, representing more than 20.9 million observations from 199 unique adult cohort studies.

Why the number needs restraint

The underlying evidence is prospective cohort research. Higher fitness precedes lower disease and mortality rates, and the association is consistent, but participants were not randomized to remain more or less fit for years. Genetics, disease, activity, smoking and socioeconomic factors may influence both fitness and outcomes.

Measured is not the same as estimated

A separate 2024 systematic review and meta-analysis covered 42 studies, 35 cohorts and 3.8 million observations. Directly measured, exercise-estimated and non-exercise-estimated fitness all predicted mortality, but they are not interchangeable. A watch-generated VO₂ max is an estimate, not a laboratory cardiopulmonary exercise test.

Practical implication

Improving aerobic capacity is a defensible training goal, especially from a low baseline. Regular walking, running, cycling, rowing and other sustained activities can contribute, with intensity and progression matched to experience. The population evidence cannot select an individual program, clear symptoms or replace clinical exercise testing when that is indicated.

Use fitness as a direction, not a destiny score

Cardiorespiratory fitness is valuable because it integrates the work of the heart, lungs, circulation and working muscle during sustained effort. A higher measured capacity is consistently associated with better outcomes across populations, but an individual result is not a forecast of exactly how long someone will live. Age, disease, smoking, medication, body composition, genetics and socioeconomic factors still matter.

The practical use is comparison against an appropriate baseline. A repeated test performed under similar conditions can show whether capacity is improving, stable or declining. Comparing a laboratory measurement with a watch estimate or a different exercise protocol can create apparent change that belongs to the method rather than the person.

How to improve the signal

For generally healthy adults, the defensible programme is not a single “longevity workout.” It is regular aerobic training that can be sustained and progressed, supported by resistance training and ordinary daily movement. Easier work builds repeatable volume; harder intervals can raise the ceiling but also add fatigue. The mixture depends on current fitness, goals, health and recovery.

Someone starting from a low baseline does not need an elite protocol. A modest, consistent increase can be meaningful. People with symptoms, known cardiovascular disease or major risk factors should not use a generic internet target as clearance for maximal testing.

What watches can and cannot do

Consumer devices can provide a useful trend when the same device, activity and wearing conditions are repeated. Their estimated VO2 max is still a model built from heart rate, pace or power and personal inputs. It should not be presented as interchangeable with direct gas-exchange measurement. The number is most useful when it changes behaviour or flags a trend worth confirming—not when it becomes a daily verdict on health.

Reader decision

If the goal is general health, choose a sustainable aerobic activity and measure progress with a repeatable field marker: pace at a given effort, distance covered, heart-rate response or a formal test when appropriate. Do not buy a premium wearable solely because it promises a longevity score. Buy only if its measurements and feedback help you train more consistently or communicate useful information to a qualified professional.

Laboratory test, field test or estimate?

Direct cardiopulmonary exercise testing measures respiratory gases during graded work and can provide the most detailed assessment, but it requires equipment and appropriate supervision. Field tests use performance over a set task. Wearables estimate capacity from indirect inputs. Each can be useful when the result is interpreted according to the method.

The mistake is false interchangeability. A watch estimate changing from 44 to 46 does not necessarily equal the same change on a laboratory test. A faster field performance can reflect pacing, skill or conditions as well as physiology. Track like with like.

Strength still belongs in the programme

Aerobic capacity is not the entire healthy-ageing case. Strength, muscle, balance and power affect function and resilience. A programme built only around a high VO2 estimate can neglect the ability to produce force, tolerate load and perform daily tasks. The best general plan combines modalities rather than turning one marker into a religion.

Update trigger

Reassess the plan when capacity declines unexpectedly, symptoms appear or the testing method changes. Sudden deterioration deserves explanation; it should not be dismissed as a bad readiness score or automatically treated as a motivation problem.

Practical checklist

• Choose one repeatable aerobic marker and keep the method stable.
• Build regular easy-to-moderate work before chasing maximal intervals.
• Retain resistance training, balance and ordinary movement.
• Treat wearable estimates as trends, not diagnoses.
• Escalate unexpected decline or symptoms instead of training through them blindly.

What to buy—and what not to buy

Equipment is optional. Comfortable footwear, a bicycle, pool access or a basic heart-rate monitor may lower friction, but none creates fitness without repeated work. A premium watch earns its cost only if the data improves pacing, consistency or communication with a professional. A longevity score alone is not a defensible purchase reason.