That's it. Nothing.
There's no such thing as "negative calorie foods". The idea that some foods require more energy to digest and metabolize than the calories they contribute is nonsense.
At least in the time I've spent dieting and losing weight, and educating myself about nutrition, that's what I've learned. Even as something as low calorie as celery, it still doesn't suck calories out of you. The idea that the more you eat, more you lose weight, is just baloney.
Evidence shows that the people who originated the idea that some foods contain negative calories are those associated with PETA (People for the Ethical Treatment of Animals). PETA is a vegan group, trying to convince people to stop eating meat.
As it turns out, these people published lists of "negative calorie foods", and the list contained only fruits and vegetables. So, it was an attempt to capitalize on the diet craze, hoping they would lure more people away from eating meat.
If fruits and vegetables were negative calorie foods, then people who ate only fruits and vegetables would have lost ALL their body mass by now, and died.
It seems like the lower the calorie food, the less energy it takes to digest and metabolize. You can just never find a food that takes you into the negative calorie range.
But that doesn't make a celery any less effective a diet food. It's still a great snacking alternative. It's ultra low in calories. I like to buy the celery sticks cut into five-inch lengths, and sold in water-filled plastic containers. Each one of those sticks is just 1-3 calories each.
When you consider how easy it is to burn off celery, you could just as well think of it as a negative calorie food. A few stalks of celery can burned off just by standing for 10 minutes, or brushing your teeth.
But don't limit your diet to lots of celery. It contains so little nutrients, that you'll harm your body by eating too much of it.
Either way, when you hear someone talk about negative calorie foods, realize that it's just propaganda perpetuated by the vegan movement.
Here are some more articles that disspell the myth of negative calorie foods...