> It is an essential hormone, definitely, but not a magical pill that will ward off disease. There are conditions that benefit from Vitamin D supplementation - for hypoparathyroidism, a condition that can cause dangerously low calcium levels, Vitamin D is lifesaving. For otherwise healthy adults, the benefit is less clear.
Takeaways: it's not clear what preventative benefits it has.
Even to use your vitamin C example, it's also widely touted to take vitamin C for general well being, such as helping colds. But its preventative benefits aren't clear [1]. Nobody has ever said vitamin C is useless, but it's just not a panacea (much like the article claims about vitamin D).
On vitamin C - I stated dietary vitamin C to make it clear I was not referring to supplemental vitamin C. I was hoping to avoid this exact rabbit hole; that example was just to highlight an issue with the argumentation.
As for my argument, I would never claim that Vitamin D is "a pretty useless vitamin". It is extremely useful, and is lifesaving for people with hypoparathyroidism. But that doesn't mean we should all be taking it. Note that the studies you mention are looking at treating something, not preventing something. Many of the people supplementing are doing it because they believe it will keep them healthy - and this does not appear to be the case. That is more the point of my article. The articles you cite are interesting and hopefully will produce larger trials.
I agree with much of your thinking - just not this particular conclusion.
1. As you mention, we know that low-to-moderate levels of vitamin D supplementation is safe.
2. We also know that a solid chunk of the population does not get enough sunlight / is vitamin D insufficient.
3. There are no large-scale trials yet for some items where vitamin D is suspected to have good effects. But there are a few nevertheless, which you share within the article itself - cancer, mortality, respiratory disease particularly stand out.
4. There are also a pile of smaller but promising, statistically significant, double-blinded, randomised controlled trials in a bunch of useful areas.
5. So this seems to be a Pascal's Wager type of situation, where supplementation is all upside and no downside, as long as people don't get crazy with it.
As you say it's not a panacea, and to cite an old comment, I am aware [1] that there are other benefits you get from sunlight. But even if it doesn't do as much as people think, I don't see why it is bad advice for most people to supplement vitamin D to keep themselves healthy!
Speaking solely as a layman, that study seems like it might be extremely relevant to the discussion here. But at the same time I don't have the background necessary to critique or dissect it.
HN Post on the study: https://news.ycombinator.com/item?id=24366006
TFA isn't saying that you don't need vitamin D, or that it can't be used to treat Ricketts or osteomalacia (which, like scurvy, are relatively rare conditions). It's simply saying that widespread supplementation on people who have normal levels or minor deficiencies isn't supported by the evidence.
What about these [1] [2] randomised controlled trials that show vitamin D supplementation has an effect on IBS symptoms?
Or this [3] double-blinded trial showing supplementation improved sleep quality in people with sleep disorders?
Or this [4] one showing improvements in depressive symptoms in overweight and obese subjects? The study cited in TFA is large but only uses participants aged 50 and above, with a relatively low dose of vitamin D.
Also, is it just me or is the line of argument used a little bit weak? You could use the same kind of argument to dismiss the need for dietary vitamin C, as follows: (a) vitamin C deficiency is associated with a bunch of other health conditions (as it's a marker of poor nutrition or socioeconomic status), so (b) let's look at the effects of supplementing vitamin C, which (c) naturally finds that supplementation does nothing for said conditions, and therefore (d) conclude that it is a pretty useless vitamin, while placing minimal focus on the things that it actually does fix, such as scurvy?
I am not an academic, so perhaps I am missing some context though.
[1] https://pubmed.ncbi.nlm.nih.gov/27154424/
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5900470/
[3] https://tumj.tums.ac.ir/article-1-8272-en.html
[4] https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2796...