Negative calories foods are not foods that don't have calories-they're foods that cause the body to burn more calories than ingested to process it. For example, if a slice of carrot had 10 calories, but took 12 to burn, that food would have -2 calorie effect on your body.
The idea with negative calorie foods, which are always certain types of fruits and veggies and are usually very high in fiber, and have a low number of calories to begin with. The theory with negative calorie foods is that the energy (calories) the body burns in order to process the nutrients of these foods is more than the number of calories that are actually in the food.
If true, this makes these foods excellent choices for a diet, since they keep you full, but don't add any calories whatsoever.
So once again, I repeat this because it's important: no foods have negative calories. The lowest you can get is zero, which is basically water. But ice water requires the body to work to warm it up for use, meaning although the calorie amounts burned are paltry (an estimated 50 calories for the recommended 8 glasses of ice water a day), that still creates a negative calorie effect since your body burned 50 calories to process ice water, which gave back 0 calories.
This is what is meant by "negative calorie foods." A more accurate description might be "Foods low in calories that produce a negative calorie effect because of how much energy it takes to burn them," but that doesn't sound nearly as snazzy or sexy as "negative calorie foods," which gets the same idea across.
There is a heated debate, but the evidence seems to indicate that yes, there are foods that create a negative calorie effect. For people who automatically yell, "That's impossible!" without looking at the facts, well consider this very well known piece of survival knowledge: if you eat nothing but wild hares and wild rabbits for a large amount of time, you will starve to death. There aren't enough calories or nutrients because of the rabbits' diet, and the body takes too much energy to break down the protein that isn't giving anything back.
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