The main fuel used by your body for activity is glycogen, broken down from carbohydrates and stored in the liver and muscles. After sleeping, your glycogen stores are depleted by as much as 80%, so a filling and nutritious breakfast is the best way to re-fuel and help you meet the demands of your work and training schedule.
By exercising on an empty stomach you risk losing muscle and decreasing your metabolic rate, as your body begins to use muscle tissue when there is no available glycogen. However early you exercise, try a light snack to raise your blood sugar before you leave (such as some fruit, a smoothie or a piece of toast), followed by something more nourishing and substantial when you return.
Your breakfast should provide you with around a quarter of your daily nutrients, and a fortified breakfast cereal is a great way to help achieve this. Wholegrain cereals such as Weetabix, Oatibix, Shredded Wheat or low-sugar muesli are good options, especially when combined with some chopped fruit and a source of protein (such as chopped nuts or seeds).