The question of the cola's effect is not solved yet. There are many arguments for both sides. There are mant scientistific researches, but as usual, they doing them for interests of companies. But there are some possible suggestions:
We can't prove the harmlessness of cola, but we can prove that over-drinking causes problems with health. We can't show that over-drinking cola is junk food, but we can prove the harm of cola in case of over-drinking.