Higher fat burning during the session, similar over the day
Exercising fasted, for example before breakfast, tends to raise the share of fat used as fuel during that session, with studies often reporting acute increases in fat oxidation. This happens because low insulin and depleted overnight glycogen tilt the body toward drawing on fat stores during the workout.
The important caveat is that the body compensates afterward. When fat oxidation is measured across the full 24 hours rather than just the workout, the difference between fasted and fed exercise tends to even out. In other words, burning more fat during the session does not reliably translate into more total fat loss, which depends mainly on overall energy balance.
Tradeoffs that may favour eating first
Eating before training can support higher-intensity or longer efforts and may reduce muscle protein breakdown during exercise. Some people also feel lightheaded or underperform when training fasted. The best choice is largely individual and depends on the workout type, timing, and personal comfort.
This is general educational information about exercise metabolism, not medical or training advice. People with diabetes or other conditions should consult a qualified professional before exercising fasted.