I've been thinking and thinking on this question for a week.

For starters... no one really knows... yet.

Here are the possibilities I've come up with:

For the body to process refined sugar, it uses up almost every vitamin and mineral in the body but offers the body nothing. It is a biochemically, very expensive food, for the body to digest.

Another thing is that because it is a long chain sugar, it feeds bad bacteria in the gut which can lead to leaky gut et al.

Lastly, this is just a theory I've been throwing around...

It may be equal to the purest of water... like distilled water. Water that is so very pure as distilled water can do all kinds of damage to your body. It can even kill you because it is so nutritionally deficient. Refined sugar may possibly be in that same category.