The negative calorie diet is debated at length because the theory is not scientifically proven. There is really no such food that has negative calories. However, those that believe in the negative calorie diet argue that one can truly eat their way to losing weight.
The reason that some people may lose weight with the so called negative calorie diet is they may actually just be increasing their metabolism.
When on the negative calorie diet, if one eats what are called ‘negative calorie foods’ (an orange for example), the body is left with less calories than was in the food. So if you eat an orange, it takes about 25 calories to digest.
The negative calorie diet requires one to eat frequent healthy meals and doing so is actually increasing metabolism. Increased metabolism can result in weight loss.
So the answer is that there are really no negative calorie foods, but by eating healthy and burning more calories than is ingested, one can lose weight.