The theory behind zero calorie or negative calorie foods is that they contain such a scant amount of calories that the energy you expend eating them cancels out their calories. In essence the bottom line is that you don’t take on any excess calories.
Slimmingworldwithstyle: helors Slimming World Quiche -using Batchelor's Pasta 'N' Sauce (-free if made up with water). Used bacon here, but obviously anything else will do. Peppers, mushrooms, tuna etc etc!