It's actually an awesome thing to do! It's basically the best way to exercise for fat loss, and to keep it off. 

This concept is called Fasted Cardio.

This vid explains it bette:
https://www.youtube.com/watch?v=O8rN5_Ace-8

Summary added by andy256:

* the video implicitly supports fasted cardio.

* *claims* it's the safest way of losing fat.

* claims 30 minutes of pre-breakfast cardio is worth 90 minutes of post breakfast cardio. 

* says you'll only throw up once!