The theory behind zero calorie or negative calorie foods is that certain foods are so low in caloric count that your body does not take on excess calories when you eat them and your body actually burns calories when you eat them.
This theory is debatable.
In this video, Kara from DNews debates if any zero calorie foods even exists.
She also presents the argument for negative calorie foods as well.
Tara shows you how to calculate for yourself if any certain food has zero calories or negative calories using this calculation:
You take the thermic effect of that food which is the percentage of calories you burn by digesting it and divide that by the calories per serving of that food.
Watch Tara present the argument as she does the math: