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...

Sugar specifically though. It is a cause of high calories due to it's effect of creating a high and then a crash making you hungry again. It is also addictive in itself.

WHO recommend getting it below 50g, ideally 25g per day. I looked on the side of some of the staple things I consumed (milk, bread etc.) and it is tough with modern processed foods to get that low.

If everyone followed the WHO recommendation then full sugar coke is out of business!

It's far, far easier to end up obese on a diet high in cheap carbs than it is one high in proteins, or even fats.

Food has no impact on the body's metabolism. All sources of calories affect the body equally, therefore weight gain is simply calories in and calories out.

I've said it out loud but it doesn't quite ring true.