This article is from the Misc Bicycles FAQs, by various authors.
Amazingly, lots of (non-elite) athletes train before breakfast or
skip breakfast altogether. Skipping the morning meal is an
unproductive habit which should be "unlearned" at the earliest possible
Think about it: the average human uses more than six hundred
calories during any twelve hour period (including sleep); the
recovering athlete uses considerably more. Beginning a workout
hundreds of calories in glycogen debt does not lead to a productive
workout. Why be a slave to an unproductive habit? Get used to eating
breakfast. Start with a glass of juice; when you're comfortable with
that, add some toast or oatmeal or even a breakfast bar. After a few
days of eating breakfast any discomfort should go away and those
morning workouts will improve noticeably.