It’s a link specifically between those with the gene for Alzeimers also having increased insulin resistance in the brain. The risk factors between Alzeimers and Type 2 diabetes are still sketchy - mainly because Alzeimers tends to still be seen in lower numbers in Asia despite people being predisposed to Type 2 diabetes / insulin resistance due to genetic, metabolic, and lifestyle factors.
It’s the opposite to Motor Neurone disease which seems to prefer healthy thin to lower BMI people with ‘strong metabolisms’.
Vascular dementia is very common amongst people with low blood pressure / severe varicose veins / some types of clotting disorders and heart issues.