My father in-law was the epitome of health... until he got dementia. But he was 90 when he got it for crying out loud. Something is gonna kill a 90 year old man. No way around it. He was so healthy, that when they tried to do a stress test on him, they couldn't raise his pulse high enough on the treadmill to make the determination. So they had to do the chemical stress test. Crazy.
Then there was a VP at my wife's employer. He was also the epitome of health. He played sports all his life. Thin. Active. Smart. He was on the fast-pitch softball team. Maybe late 50s. Good looking guy. All the ladies liked him. Happily married. Started forgetting how to play softball. Had to step down from his job. Died of dementia two years later.
While I'm sure excess sugar ain't good for you (yet I keep eating cookies), I think dementia is a complicated issue. I'd not wish it on anyone, that's for sure. Or their loved ones.