A metabolically healthy person burns primarily fat at rest and during low-intensity activity, and efficiently switches to carbohydrate oxidation during high-intensity effort. Metabolic inflexibility — the inability to make this switch efficiently — is now understood to be an early and sensitive marker of metabolic dysfunction, preceding detectable changes in fasting glucose or insulin.
How to assess it
The gold standard is indirect calorimetry — measuring oxygen consumption and CO₂ production to calculate the respiratory exchange ratio (RER) at different exercise intensities. An RER close to 0.7 indicates fat oxidation; close to 1.0 indicates carbohydrate oxidation. We assess this as part of our VO₂ max testing protocol.
How to improve it
The primary training intervention is Zone 2 aerobic work, which preferentially develops the oxidative capacity of slow-twitch muscle fibres. Nutritional periodisation — strategic variation in carbohydrate availability — can also accelerate adaptation. Fasted training, while not essential, can be a useful tool in the right context.