Working out on an empty stomach, or around 3-6 hours after eating, is known as ‘fasted cardio’. The insulin in your blood is relatively low, which means that the energy you get from glucose in your bloodstream isn’t relatively available. Your body will then need to use stored energy, known as glycogen and fat, in order to get your muscles moving. However, if you don’t eat enough of the right foods during the day to fuel your workouts, it could mean that your blood glucose and glycogen levels are completely depleted. As a result, your body may start using muscle tissue for energy, and you’ll lose definition and tone in your figure. This is an extreme example, but it does mean you’ll need to be careful about how you fuel your body before and after a workout, so you avoid losing muscle mass.