There is no real food that has negative calories. This is just a myth. However, eating something like concrete does have negative calories. Body can't digest it, but still burns energy trying to digest it and when it fails more energy to expel it. For a better effect, you can try eating nails or something. Lot's of calories will go to the healing process from the damage they will cause.. But probably you will end up dead from gastrointestinal perforation so I don't think it's worth it.