For the majority of your runs, especially long runs or interval sessions, you are likely to perform better if you start with your carbohydrate stores topped up. If you run before breakfast, eat a banana or drink 250ml of diluted fruit juice before heading out to provide sufficient energy. If you are running at lunchtime or after work, have a carbohydrate snack 30-60 minutes beforehand, eg oatcakes with nut butter or an energy bar.
That said, the latest research suggests that there is good reason to do occasional runs in a “fasted” state. These would typically be done before breakfast or your evening meal, having not consumed a snack first. The reason for incorporating some fasted runs into your training is that it promotes adaptations in your muscle tissue which can help you to become a better runner. Your muscle cells contain tiny organelles called mitochondria, which is where energy is produced through a process involving various enzymes, such as PGC-1 alpha. Studies have shown that running in a fasted state helps to increase both the activity of these enzymes and the number of mitochondria. These changes enable your muscles to be able to produce more energy.
Another benefit of sometimes running on an empty stomach is that your body has to turn to your fat stores as its primary source of fuel, and this may help you to reduce body fat. But you may not be able to run as far or as fast, so you probably won’t burn more calories overall. It’s also important not to lose the ability to use carbohydrates as a fuel source if you want to be able to race well. A good strategy would be to do one fasted run per week, then to do the others after a meal or snack.