Aerobic metabolismOxidative metabolism

Aerobic metabolism is energy production that uses oxygen. Fuel, mostly fat and carbohydrate, is broken down and fed into the Krebs cycle inside mitochondria, and the electrons released drive oxidative phosphorylation, which regenerates ATP in bulk.

It is slower to ramp up than the anaerobic routes and vastly more productive. Burning a molecule of glucose aerobically yields roughly fifteen times the ATP that breaking it down without oxygen does, and fat can only be used this way, which is why the aerobic system is the one that can keep going for hours.

It supplies most of your energy nearly all of the time, including while you sleep and while you read this. The idea that it switches on after twenty minutes of exercise is a misreading: it is always running, and what changes with intensity is how much of the total the anaerobic systems have to add on top.

Its capacity is trainable and the adaptations are specific and physical: more mitochondria, more capillaries around each fibre, more oxidative enzymes, more blood volume, and a heart that moves more blood per beat. Those changes are why the same pace feels easier after three months.

Both easy and hard training build it, through partly different routes. Long easy volume accumulates the signal through duration and is cheap to recover from; intervals near maximum stress the central delivery side. Programmes that use only one tend to plateau.

Fat oxidation rises with aerobic fitness, which is worth stating carefully: being better at using fat during exercise is a performance advantage, not a fat loss mechanism. What you lose over weeks is decided by energy balance, not by which fuel you burned on Tuesday.

In practice, most of your aerobic volume should be comfortable enough to hold a conversation, with our heart rate zone calculator setting the band.