It is true, apples are one of the foods that actually have negative calories!  There are other foods too, mainly fruits and veges.  Here is what I found on multiple websites:

Apples: Negative Calories at Their Best

"Do apples have negative calories? Apples are one of the most popular negative calorie foods out there. They are great fruit that are both cheap and delicious. Who hasn't grown up remembering snacking on apples as a kid (and sorry - apple pie doesn't count)?

To answer the question about whether or not apples are negative calorie food, you have to look at the official list of foods for this diet. That list says that, indeed, apples count as part of the negative calorie diet.

This might seem surprising, since when many people think about a fruit or vegetable that might be "negative calorie," they assume that it has to be tasteless, or exotic. Neither is the case, as there are few fruits that are as tasty as the apple.

Negative Calories, however, do not refer to a type of calorie, but to foods on the negative calorie diet list which, in theory, can create a caloric deficit.

So what does all that fancy language mean? Apples are one of many fruits in a diet that has been referred to as the negative calorie diet, or zero calorie diet, or any similar derivative of those two names.

The theory is that if an apple you eat has 100 calories in it, but it takes your body 120 calories to digest it, then that apple has a total calorie effect of -20, because you took more to burn it off than it gave you.

While some of the expected foods, like celery and carrots, appear on the list, many people are surprised by the numbers and variety of fruit that are acceptable for this eating plan, including apples.

Negative calorie foods are only fruits and vegetables, and while not everything makes the list (you'll never find potatoes or coconut, for example), a lot of even common foods from these two major food groups are an accepted part of a zero calorie diet.

Knowing this information is helpful for many people who are trying to make this unique eating plan work for them. It's one thing to look at celery, bell peppers, and onions and try to figure out how to make these a major part of the diet, but it's much easier to do this with an apple.

Negative calorie foods are somewhat controversial, in that people argue if a caloric deficit actually ever takes place, but the good news if you're looking at this diet is that every food on here is healthy."

http://negative-calorie-foods-diet.blogspot.com/2008/08/apple-negative-calories-apples-negative.html