Fasted cardio is exercising on an empty stomach
People say it means you burn more fat. What does the research say?
Fasted cardio increases fat burning compared to fed cardio while exercising.
However, a 2014 study showed no difference in the long run
If you burn more fat during exercise, your body makes up for it later by burning less fat during the rest of the day.
Training fasted state can double protein loss during cardio compared to a fed state, meaning you lose more muscle.
Fasted cardio can reduce performance.
Fasted cardio seems to improve glucose tolerance (almost 30% higher), which might reduce the risk of diabetes.
Overall most of these effects are small.
If you enjoy cardio before breakfast, go for it.