Before I had diabetes under control, the excess sugar caused leaky retinal capillaries in my right eye. Laser and injection treatments helped for a while, but eventually the optic nerve was destroyed.
Now, while under much better but not great control, my ophthalmologist sees a few problems in my left eye and I have to get the blood sugar even lower.
While it's fine to demonize all that sucrose in your lattes and Lucky Charms, all carbohydrates and sugars, simple and complex, eventually break down into glucose so the cells can use the stuff. Even proteins can break down to glucose at times, and if blood sugar gets low, the liver can create it.
A properly working body can sort all of this stuff out and a decent diet with moderate exercise will keep you going in most cases. You don't have to cut out all sugars, just keep it reasonable. And remember that those fruits and veggies are often sugar/carb heavy. (Just for fun, compare the carb counts and glycemic indices of a few slices of pineapple, a bunch of grapes, a potato and an ear of corn. That donut for breakfast doesn't look so bad after that.)
When things go wrong, though, they can really go wrong.
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