Vitamin D, part 3 – The Evidence

There are thousands of reasonable hypothesis, and it is extremely costly (and in some cases, nearly impossible) to test.

e.g. our bodies need sunlight and cholesterol in order to synthesize vitamin D; This process also apparently has some other outputs like NO which are much harder to measure since they get used quickly. It is possible that vitamin D is merely (mostly) a measure of the prevalence of this process, and it is the other (harder to measure) outputs are much more important; Or that it's the consumption of cholesterol that's more important than the production of vitamin D; Or that sunlight has other functions we don't measure that are the important ones. Or that it's all just a correlation to some other process we're not even aware of.

That said, my own experience and anecdotally other people I know (and gwern has some blind n=1 experiments on his site to back this up) is that taking vitamin D early in the morning improves sleep at night, and taking vitamin D in the afternoon/evening reduces sleep quality. For that reason alone, I take vitamin D in the morning, and anecdotally, I burn less when I'm in the sun without sunscreen as well.

I'm still on the fence about this. Until someone from my health care system comes out and states that the research looks good I am not going to eulogise it. Do I take vitamin supplements yes, would I say it produces the affect on covid patents to other people, no.

I would frame it in the opposite direction: having a serious illness is correlated with low levels of Vitamin D. People with a serious illness may stay inside more, and go outside less. It's possible that "low levels" of Vitamin D are still within the healthy range, and they merely are an indication of how much time the person has spent outside during the day.

In other words, it's possible that Vitamin D is pure symptom. It's not even necessarily the case that going outside is healthy, but that healthy people tend to go outside more.

TL;DR: skip to the part "The areas in which meta-analyses have identified benefits from Vitamin D"

Also "Vitamin D failed to prevent cancer, stroke, heart attack, or cardiovascular death. "

Which is fair enough. But at least for the cardiovascular diseases mentioned, I don't think it was expected.

And of course, this does not mean anything against the role of Vitamin D in Covid prevention

Basically: don't dismiss X for disease D1 if you tested it with disease D2

That is a reasonable hypothesis. Also, people who are very ill are less likely to get outside and get enough sun, so they will develop low Vitamin D. Vitamin D is likely a marker of poor health. (Once the Vitamin D gets low enough, we do see direct harmful effects from it (think rickets in children), and this may contribute to poor health at that point.)