In fact there is substantial evidence that a sugar calorie is not the same as any other calorie -- that fructose is metabolically toxic in large doses. I recommend watching the well-known Robert Lustig video "Sugar: The Bitter Truth" [0]; if you prefer reading, here's an article [1].

To give you a TL;DR I'll quote from [1]:

In animals, or at least in laboratory rats and mice, it’s clear that if the fructose hits the liver in sufficient quantity and with sufficient speed, the liver will convert much of it to fat. This apparently induces a condition known as insulin resistance

[0] https://www.youtube.com/watch?v=dBnniua6-oM

[1] http://www.nytimes.com/2011/04/17/magazine/mag-17Sugar-t.htm...