Time and time again we see epidemiological studies posting findings that are clearly best explained by correlation than causation. For example, think of the people you've met with the worst teeth. They will almost certainly, on average, have worse overall health, lower socioeconomic status, lower intelligence, lower paid jobs, lower educational attainment, worse diet and so on. Surely these are just as likely, if not more, to be on the causal pathway for Alzheimers? And that's completely ignoring the fact that people with early Alzheimers which has not been diagnosed are much less likely to remember to keep up with oral hygiene rules, so the risk for reverse causation is HUGE.

Now I know you're going to argue that the epidemiological studies account for this using multiple regression and so on, but the fact is time and time again it has been demonstrated that this just cannot be done. Indeed, vitamin D in practically every epidemiological study to date has shown to reduce the risk of cancer, cardiovascular mortality, tuberculosis infect. And yet, just this month, someone FINALLY did a randomised trial of vitamin D in TWENTY SIX THOUSAND people (the VITAL trial) and found it made no difference at all (in fact, cardiovascular death was slightly but non-significantly more common in the vitamin D group).

Unfortunately, people's careers and publications rely on this stuff, so it'll keep coming. Please remain skeptical, hacker news.