HDL is not nearly as well understood as LDL. Directly raising HDL levels (e.g. with Niacin) does not change CHD risk, which suggests that HDL levels are a 'marker' for something else. I remember one study that showed about 30% reduced mortality for people who'd already experienced an infarction, but the niacin was combined with... fibrates? I don't remember any that looked at niacin and dementia. On a mobile, time to dig in when I get home...