I think the link is insulin resistance. People with type 2 who have IR in liver and muscle probably also have IR in the brain, and it's the IR that lowers the response to insulin and causes the dementia. Nasal insulin (which gets into the brain) seems to help people with Alzheimer's.
Here is one of many links to "insulin resistance" and "Alzheimer's."
However, that doesn't mean that dementia is inevitable if you have IR. I agree that controlling BG undoubtedly reduces your risk.
I have never really heard of any connection but it seems like it makes sense and is scary as was mentioned before.
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Right on!