The hormonal setting of a fasted state
Exercising after an overnight fast changes the hormonal backdrop. Insulin sits lower, and catecholamines tend to be relatively higher, a combination that favors releasing and burning fat as fuel during the session. This is why fasted cardio measurably increases the proportion of fat oxidized while exercising compared with the same effort after a meal. The diagram captures this acute tilt in fuel selection, which is real and reproducible at the level of a single workout.
The shift is most relevant at low to moderate intensities, where fat can supply a larger share of energy; harder efforts still lean heavily on carbohydrate regardless of feeding state.
Why daily energy balance overrides the session effect
Burning more fat during a workout does not guarantee greater fat loss over time. The body compensates across the rest of the day, and studies comparing fasted versus fed training generally find similar fat-loss outcomes when total calories and protein are matched. In other words, the substrate used in the moment is largely a bookkeeping detail; the daily and weekly energy balance is what determines change in body fat.
This is general educational information, not medical or training advice. Fasted exercise can affect performance, blood sugar, and how some people feel, so consult a qualified professional before adopting it, particularly if you manage diabetes.