Basically nothing. You'd be wasting your money and the drug.

Xenical bonds with fat in your digestive system and moves it out of your body in your bowel movement. Without fat in your system to bond with, it would just process out of your body like anything else...just not bonded with the fat.

When I first started using Xenical was when I realized I eat a naturally low fat diet. I kept waiting for something dramatic to happen (based on stories) and it never did. If I eat a LOT of fat (a lot for me) I can tell a little bit that the drug was doing it's job, but not enough to keep taking it. So basically, by the same way of thinking, it was just a waste for me to try and work with.