Metabolic fitness: the health metric hiding in your heart rate

Cardiorespiratory fitness (CRF) is the strongest predictor of all-cause mortality. This is not a fringe claim — it is one of the most robust findings in epidemiology. The American Heart Association has called for CRF to be treated as a clinical vital sign.

The gold standard for CRF is VO2max, measured via a graded exercise test to exhaustion with gas exchange analysis. In practice, this test is rarely performed — it is too expensive, too demanding, and inaccessible to the populations who need it most. Wearables have begun estimating VO2max from pace-to-heart-rate ratios, and TrueZone derives it too, from the same three-parameter model that powers everything else.

But VO2max, whether measured in a lab or estimated from a wearable, tells only part of the story. It measures total aerobic capacity — how much oxygen the body can consume at peak effort. It does not tell you how efficiently that oxygen is used, how the body partitions fuel between fat and carbohydrate, or how much metabolic strain daily life imposes. For that, you need a metabolic scale.

Beyond VO2max: metabolic fitness and metabolic strain

TrueZone introduces two metrics that capture what VO2max alone cannot.

The Metabolic Fitness Index (MFI) is a composite score (0–5) that integrates threshold speed, cardiovascular range, and body size into a single number. MFI correlates strongly with glycemic response markers (R² = 0.99 in validation studies), making it a wearable-native proxy for metabolic flexibility — the body's ability to switch between fuel sources in response to demand.

An MFI of 4–5 indicates excellent metabolic fitness: high oxidative capacity, efficient fat metabolism, strong cardiovascular reserve. An MFI below 2 indicates significant metabolic limitation: poor aerobic capacity, limited fat oxidation, elevated metabolic risk.

The Resting Zone Index (RZI) answers a simple but revealing question: how much of your metabolic capacity is already being used just to exist? RZI shows where resting heart rate falls on the individual's metabolic scale (0–3). A person whose resting heart rate sits deep in the fat zone (low RZI) has substantial metabolic reserve. A person whose resting heart rate approaches their first metabolic threshold (high RZI) is under metabolic strain even at rest.

RZI is fully passive — it requires no exercise, only a resting heart rate and the individual's threshold structure. It declines with deconditioning, ageing, obesity, and cardiovascular disease. It improves with regular aerobic exercise.

Glycolytic power: the missing dimension

TrueZone uniquely separates metabolic demand into aerobic and glycolytic components. At any given intensity, the model estimates how much work is fuelled by oxidative metabolism and how much by glycolysis — replacing direct blood lactate testing with a heart-rate-derived estimate.

This separation reveals something important for health: glycolytic power is blunted in ageing, obesity, and diabetes. The glycolytic heart rate response — how steeply heart rate rises through the high-intensity domain — becomes compressed. The body loses its ability to access high-intensity energy systems.

Tracking glycolytic power over time provides a window into metabolic decline that current wearables cannot see. A declining glycolytic response, even when VO2max remains stable, may signal early metabolic dysfunction.

Where endurance fits

The endurance parameter (E) reflects the proportion of the intensity range covered by aerobic metabolism — predominantly fat oxidation and slow-twitch fiber recruitment. High E means dense mitochondrial networks, efficient fat oxidation, and late onset of glycolysis.

E is a valuable secondary marker of CRF, particularly in older adults where maximum speed (Vmax) declines. As Vmax falls with age, E becomes increasingly important as an indicator of metabolic reserve — those with higher E experience lower metabolic strain at rest. In younger individuals, however, E alone does not fully capture CRF, because a young person with below-average endurance but high Vmax may still have excellent total aerobic capacity.

This is why TrueZone derives both VO2max (reflecting absolute aerobic power) and E (reflecting aerobic efficiency). Together with MFI and RZI, they form a multi-dimensional view of metabolic health that a single number cannot provide.

Mapping the low-intensity domain

Most existing zone systems focus on the moderate-to-high intensity range that matters for athletic training. But for health applications, the most important domain is at the bottom of the intensity scale — where deconditioned, obese, and elderly individuals spend virtually all of their time.

TrueZone defines three metabolic zones that map this low-intensity domain:

• Z0 (Fat zone) — below the first metabolic threshold. Activity at this level imposes minimal metabolic demand beyond resting. For healthy, fit individuals, this zone is narrow. For severely deconditioned individuals, it may encompass most of their daily activity.
• Z1 (Glucose zone) — the first zone of meaningful glucose flux. Activity here involves mixed oxidative metabolism. As fitness declines, resting HR shifts upward into Z1.
• Z2 (Lactate zone) — approaching the endurance threshold. When even easy walking places someone in Z2, it signals significant metabolic strain — common in deconditioning, obesity, and ageing.

The width and position of these zones are fully individualized. For a fit runner, Z0 might correspond to a slow walk. For a deconditioned 70-year-old, Z0 might extend up to a brisk walk — meaning that prescribing "Zone 2 walking" using generic guidelines could put them above their actual aerobic threshold.

Validation

The metabolic calculations — MFI, RZI, energy expenditure, energy intake, and glycolytic power — have been validated in two clinical studies: the METFIT PhD study at the University of Iceland and the FITSILVER Eurostars project with CSEM, Switzerland. Together, these studies comprise 70 participants and three scientific papers (submitted).

Key results:

• MFI vs glycemic response: R² = 0.99
• Energy expenditure MAE: 5.8% (against indirect calorimetry)
• Energy intake MAE: 4.8% (pilot-scale, against controlled meals)

The vision

Every smartwatch, fitness tracker, and chest strap on the market already collects the raw data needed to assess metabolic fitness. The sensors are there. The data is there. What has been missing is the metabolic scale to interpret it.

TrueZone provides that scale. MFI for metabolic fitness. RZI for metabolic strain. Glycolytic power for metabolic capacity. VO2max for aerobic ceiling. All from the same three parameters, derived from ordinary heart rate data — no lab, no blood test, no maximal effort required.

The signal has been sitting on your wrist all along.