You know, even oxygen in excess quantities is poisonous to us.

I wouldn't say that sugar is a bad thing on its own, but the people who use it to enhance their products are not doing so to help our brains work better. They are doing that because they know that our species has an appreciation for this stuff and in their quest for short term profits they can externalize the long term costs associated with the effects of sugar overload which we ourselves do not have a way of detecting until it is too late.