Eat, Before Cardio.

[Redd Foxx]

It completely depends on what you're trying to do with your cardio. If you're trying to lose fat, then an empty stomach will help. If you're conditioning (building up your endurance, capacity) then eat something around 2 hours to an hour before (depending on what it is and how quickly you digest).

[Journeyman]

usually when i run in the morning, i chug a tall glass of orange juice and get my tired ass out the door

[mushahadeen]

the difference in fat % burned isn't significant enough to apply to real life.

-- so if you can do cardio without eating, do cardio without eating.
-- if you need to eat first, then eat first.

in terms of actual POUNDS LOST, the difference is not significant enough either way.

[oztheuzi]

*2TOUGH* is correct, cardio on an empty stomach will lead to catabolism (muscle wasting). What you want to do is work on endurance, and burn body fat if necessary, not muscle. Eat something that will give you sustained energy throughout - oatmeal, eggs and berries an hour before will help you.
If you want to burn serious amounts of fat, do circuit resistance training and when it comes to cardio, so HIIT (high intensity interval training) which will rev your metabolism for up to 12 hours.