PHYS-02 · Principle
Is Zone 2 the fat-burning zone?
Partly. Fat oxidation peaks at a moderate intensity, at a point that differs between people, and harder work runs more on carbohydrate. But burning more fat during a ride is not losing more body fat.
How the body works, agreed across the reviews below; a study entry shows what one experiment measured.
Practical Application
Is Zone 2 the fat-burning zone? Partly. Fat oxidation peaks at a moderate intensity, at a point that differs between people, and harder work runs more on carbohydrate. But burning more fat during a ride is not losing more body fat.
Mechanism
Rising intensity raises glycogen breakdown, recruits fast fibres and lifts sympathetic drive; training enhances fat oxidation and lowers sympathetic responses at the same work (Brooks & Mercier 1994). Fatty-acid transport into cell and mitochondria caps fat use at high intensity (Purdom 2018); in 5 trained subjects adipose lipolysis was maximal by 25% VO2max (REF-450, B).
Fat oxidation peaked at 59-64% VO2max in trained people and 47-52% in a large general sample, higher running than cycling (Achten & Jeukendrup 2004); across 434 cycling studies sex, habitual diet and duration moved the mix most, and at least 41% stayed unexplained (REF-475, A-).
Pre-exercise carbohydrate lowers fat oxidation against fasting (Achten & Jeukendrup 2004) as insulin inhibits adipose lipolysis (Spriet 2014, full text); in 17 trained male cyclists a fasted start raised fat oxidation during submaximal work, with interval power unchanged (REF-157, B+).
Common misreading
"The fat-burning zone burns fat off"; "one high fat-oxidation test is my metabolic type": the mix moves with diet, sex and the days before (REF-475).
Backs
SYNTH-51 "a fasted or protein-only start raised fat oxidation during submaximal work" · SYNTH-51 "an individual test, not a percentage, locates it" · SYNTH-69 "fasting raised fat oxidation, and the protein meal kept it similarly high" · SYNTH-83 "Zone 2 is the fat-burning zone and trains fat oxidation best"