Galpin's reframe of "calories in, calories out" for fat loss specifically: body fat is oxidized and leaves the body as exhaled CO2, so the only two levers are ingesting less carbon or expelling more carbon — "carbon in, carbon out." All metabolism reduces to breaking carbon bonds to produce ATP; oxygen is a required cofactor, not a fuel, and the neural breathing trigger is a CO2 threshold in the brainstem, not oxygen depletion.
Which fuel is burned during a given workout (fat vs. carbohydrate, tracked via RER — see Metabolic Flexibility) does not determine fat-loss outcome; total caloric/carbon deficit does, so training modality is largely interchangeable for fat loss given equal adherence. Low-intensity "fat-burning zone" training is therefore not privileged for fat loss over higher-intensity work.
Cyclic hyperventilation technically increases fat loss (more CO2 expelled) but is flagged as unsustainable — it triggers adrenaline, jitteriness, and anxiety within seconds — a proof-of-concept for the carbon mechanism, not a practical protocol. EPOC ("afterburn") is real but smaller than commonly claimed; don't rely on it as a major fat-loss lever.
For a fat-loss-oriented session, hypertrophy/muscular-endurance training (6-30 reps) plus HIIT best deplete muscle glycogen; low-rep strength and pure skill/speed/power work contribute little. Added muscle mass raises BMR only ~6-10 kcal/day per pound (not the popularly cited ~50), so a single ~200-kcal food choice can erase a muscle gain's daily metabolic benefit.
See Andy Galpin — Training & Metabolism System for the underlying energy-system model and Nine Adaptations Framework for which training protocols to combine for fat loss.