Back before I knew anything about anything, I would set out for a run in the morning on an empty stomach, thinking that my body would be forced to use my fat stores for fuel. Although it is possible to train the body to do this, it’s more complicated than simply not eating. All I was really doing was setting myself up for a disappointing workout.
Now I know that by fueling up before I train, I can push myself harder, forcing my body to adapt more, which means more strength, speed, etc. Usually this means half a low-sodium turkey sandwich with honey mustard, at least a half hour before I train:
That works for weights, the bike, and swimming. Running with a sandwich in me usually causes a side-ache, though, so instead I’ll have a banana. Before really long sessions I’ll have a bowl of pasta with chicken, a full 2 hours prior to setting out.
You have to experiment to find what works for you.