Crazy -
My impression on the whole "negative calorie" business is that digestion is already accounted for in your BMR - and that while celery and such foods may have negative calories, it's not much.
From Snopes.com "Yet as enticing as all this [negative calories] sounds, the dietary bankroll built by this approach would be very small, probably amounting to no more than a few dozen calories a day. In a world where it takes 3500 calories to work off a pound of fat feasting on celery would only make the mearest difference."
I'm curious as to what you are eating for 500 negative calories in a day.