Treatment & Procedure

Zone 2 & VO2max training

Q: Is a high VO2max really that important?

As a marker it is exceptionally well supported. In a cohort with 122,007 exercise ECGs, mortality with low fitness was around five times higher than with elite fitness, and below-average fitness weighed as heavily as smoking or diabetes. These are observational data: they predict mortality but do not prove a cause.

Q: Does endurance training lower mortality?

That is less well supported than the marker data suggest. The cohort studies measure fitness, not training. The large Norwegian intervention study Generation 100 with 1,567 older adults over five years missed its primary endpoint of all-cause mortality. Training remains sensible, but proof of lower mortality from intervention data is lacking.

Q: Does Zone 2 do more than other intensities?

There is no evidence for this. In a meta-analysis of 17 studies with 437 people, polarized training was on a par with other distributions for all performance endpoints. Only VO2peak rose slightly, and exclusively in highly trained athletes and programs lasting under twelve weeks.

Q: What does 80/20 mean in training?

It refers to distributing training volume across mostly easy and a few very hard sessions, also called polarized training. The split is popular but barely studied: the available comparisons include 437 people in total, run for a few weeks and measure performance, not health.

Q: How much of a MET difference is noticeable?

In a meta-analysis of 33 cohort studies with 102,980 participants, the relative risk of death per additional MET of maximal capacity was 0.87, and the risk of coronary events 0.85. People with low fitness had a relative risk of 1.70 compared with people with high fitness. The analysis did not test a training method.

Q: Is hard interval training suitable for everyone?

This page gives no training recommendation. The evidence reviewed contains no harm data, but also no data on untrained people or people with pre-existing conditions at high intensity. Anyone over forty, with pre-existing conditions or after a long break should have a sports medicine evaluation before hard intervals.

Biohacking

Maximal oxygen uptake is one of the best-supported mortality markers of all. That a particular training distribution such as Zone 2 or 80/20 produces this advantage, by contrast, has barely been studied.

What Zone 2 and VO2max refer to

VO2max is the maximum amount of oxygen the body can use under exertion; it is often given in METs, a multiple of resting metabolic rate. Zone 2 means easy endurance training below the second threshold; polarized training means distributing the volume across mostly easy and a few very hard sessions.

This page rates the state of knowledge, not the sport. A high evidence score means “well studied”, not “the advertised concept is correct”.

Marker and measure are not the same

“Fitness predicts mortality” and “training lowers mortality” are two different statements. The first comes from large cohorts and is extraordinarily robust. The second requires intervention studies with hard endpoints, and the Norwegian Generation 100 study with 1,567 older adults over five years missed its primary endpoint of all-cause mortality.

It gets narrower still on the question of the right intensity distribution. There are comparative studies here, but only with performance endpoints, small groups and short durations.

What is well supported

VO2max is one of the best-supported mortality markers of all. Mandsager 2018 analyzed 122,007 patients with exercise ECGs over a median of 8.4 years, 13,637 deaths. Elite fitness compared with low fitness was associated with an adjusted hazard ratio of 0.20 (0.16–0.24; p<0.001); calculated the other way round, the risk with low fitness was five times higher (5.04; 4.10–6.20).
The effect is of the order of classic risk factors. In the same analysis, below-average compared with above-average fitness carried a hazard ratio of 1.41 (1.34–1.49; p<0.001) — on a par with smoking (1.41) and above diabetes (1.40) and coronary heart disease (1.29). Few preventive measurements have comparable predictive power.
The association has been independently confirmed and is dose-dependent. Kodama 2009 pooled 33 cohort studies, 102,980 participants with 6,910 deaths for mortality and 84,323 for cardiovascular endpoints. For each 1 MET higher maximal capacity, the relative risk fell to 0.87 (0.84–0.90) for all-cause mortality and to 0.85 (0.82–0.88) for coronary heart disease and cardiovascular disease. Low compared with high fitness: 1.70 (1.51–1.92; p<0.001).
The professional society has recognized the marker. In 2016, the American Heart Association classified cardiorespiratory fitness as a clinical vital sign in a dedicated statement, that is, as a measurement that belongs in routine assessment. The paper assesses the measurement, not a particular training protocol.

What the studies show

VO2max as a predictor: a very large effect

Mandsager 2018 retrospectively studied 122,007 patients with exercise ECGs over a median of 8.4 years, 13,637 deaths. Elite fitness compared with low fitness was associated with an adjusted hazard ratio of 0.20 (0.16–0.24); conversely, the risk with low fitness was five times higher. Below-average fitness weighed as heavily as smoking or diabetes and more heavily than coronary heart disease.

A measurable difference per MET

Kodama 2009 pooled 33 cohort studies, 102,980 participants and 6,910 deaths for mortality. For each 1 MET higher maximal capacity, the relative risk of all-cause mortality fell to 0.87 (0.84–0.90) and of coronary events to 0.85 (0.82–0.88). Low compared with high fitness: 1.70 (1.51–1.92). This does not show causality, nor does it show a training method.

Negative finding on training distribution

Silva Oliveira 2024 compared polarized training with other intensity distributions over at least four weeks in 17 studies with a total of 437 people. VO2peak rose, just significantly, by 0.24 standard deviations (0.01–0.48; p=0.040), and only for durations under twelve weeks and in highly trained athletes. Time-trial performance (p=0.92), time to exhaustion (p=0.24) and power at the second threshold (p=0.75) remained unchanged.

Position paper instead of study data

Ross 2016 is a statement by the American Heart Association that classifies cardiorespiratory fitness as a clinical vital sign. It contains no study data of its own and is to be read as a professional assessment, not as evidence of efficacy for a training protocol.

Where the data stop

That increasing fitness lowers mortality. The strong figures come from observational data, partly from clinical populations. The large intervention study Generation 100 missed its primary endpoint of all-cause mortality.
That the 80/20 distribution is superior for recreational athletes. The only comparison reviewed found a small VO2peak advantage only in highly trained athletes and short programs; all performance endpoints remained unchanged.
The special status of Zone 2. That easy endurance training in particular is the decisive stimulus has not been shown in the evidence reviewed; the direct comparison tends to favor higher intensities.

Status, approval and legal

Training is not a product requiring approval, so no official benefit assessment exists. The only institutional assessment in the evidence reviewed is the 2016 AHA statement, which recommends measuring cardiorespiratory fitness as a clinical vital sign. It assesses the marker, not a particular training concept.

Safety

The evidence reviewed contains no harm data on endurance training. The caveat concerns exercise intensity: anyone over forty, with pre-existing conditions or after a long break should have a sports medicine examination before hard intervals. In any case, the benefit develops over years, not weeks.

BK-Score Supported, with caveats

Human evidence		7
Mechanism		7
Safety data		8
Hype gap		4
Track record of use		9

Two levels that are often conflated. VO2max as a marker is excellently supported (Kodama et al., JAMA 2009, 33 cohorts, around 103,000 people) – but observational. The intervention study Generation 100 (Stensvold et al., BMJ 2020, 1,567 older adults, five years) missed its primary endpoint of all-cause mortality. And for Zone 2 specifically, the direct comparison tends to favor higher intensities. Training is right; the special status of Zone 2 is not supported.

The score rates the state of knowledge, not the substance. “Safety data 9” means well studied – not harmless. “Track record of use 9” means used long and widely – that is not proof of efficacy.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about Zone 2 & VO2max training

Is a high VO2max really that important?

As a marker it is exceptionally well supported. In a cohort with 122,007 exercise ECGs, mortality with low fitness was around five times higher than with elite fitness, and below-average fitness weighed as heavily as smoking or diabetes. These are observational data: they predict mortality but do not prove a cause.

Does endurance training lower mortality?

That is less well supported than the marker data suggest. The cohort studies measure fitness, not training. The large Norwegian intervention study Generation 100 with 1,567 older adults over five years missed its primary endpoint of all-cause mortality. Training remains sensible, but proof of lower mortality from intervention data is lacking.

Does Zone 2 do more than other intensities?

There is no evidence for this. In a meta-analysis of 17 studies with 437 people, polarized training was on a par with other distributions for all performance endpoints. Only VO2peak rose slightly, and exclusively in highly trained athletes and programs lasting under twelve weeks.

What does 80/20 mean in training?