Chocolate (or something sweet) has 80 calories, the same as an apple, will...

[Meryl]

...you gain the same amount of weight? If a chocolate bar has 80 calories, will you gain the same amount of weight as an apple?

[Barbara A]

Calories in no way really determine how fat you get.
Even though two food have the same calories it is their nutritional facts that differentiate them (carbs, protein and fat composition).

Choose the apple. It has fibers.

[kaitthegreat]

Right on! Your body sees 80 calories as 80 calories, it doesn't discriminate. The reason why chocolate is discouraged is because it is very dense in calories, so you can eat far more calories without feeling full, whereas an apple has a low calorie density. So 80 calories of apple will be more filling than 80 calories of chocolate, which will help to prevent overeating, which leads to weight gain!

And another thing, the glycaemic index is in no way an indicator of weightloss potential. And the energy used to absorb calories is... well not a lot so that shouldn't play a role in your choice of apple v. chocolate.

While it is true that your body will absorb different nutrients in different ways, the overall energy intake is the main determinant in weight loss, not if it predominantly carb, fat or protein. I could explain, but it is very complex biochemistry and would take me hours to explain all the metabolic pathways, in fact it will take me four years at University until I understand it completely myself! But I did have a lecture on fad diets, and they explained it to us then.

Hope this helps