A lot of people are critiquing the statistical methods and quality of the study, which is fine. But it's worth pointing out that you—the individual—should not be concerned with someone else's p-value. You should be concerned with maximizing your own utility. A safe, possibly effective, and cheap intervention is probably worth trying. If it was more expensive or less safe, it would require more evidence to try.

Take it from an academic like me that peer review in just over a month is rare and a sign of low-quality editorial work at the journal (the exceptions would be the most open, progressive journals like PCI and similar).

The formatting/style and peer review history alone are enough for me to doubt this. Of course, the other users' points about study design and lack of transparency make it even harder to trust the claims.

Do we really know what "optimal" vitamin D levels are? I've heard a wide range of answers on this, and it's not even clear to me that we know whether there is natural human variability in the amount needed.

I haven't seen convincing evidence that vitamin D supplementation is materially useful for anything but rickets. I get the impression that naturally high serum levels are an effect rather than a cause of other positive things, and that supplementation mostly increases serum levels without effecting positive things. It doesn't seem harmful either, so can't hurt might help?

Reminder that the Recommended Daily Allowance of Vitamin D found on all the labels (800 IU) is mistakenly too low, by a factor of 10x, due to a maths error (should be 8000 IU). It has not been corrected yet. Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC5541280/

Supplementing with vitamin D is honestly one of the easiest things you can do... it's cheap, available everywhere, and makes a real difference. Just make sure you're also taking magnesium citrate (or another good form of magnesium) with it, since your body needs magnesium to properly use vit D

What about the article that that talks about Vitamin D being the same chemical as rat poison and that the positive effects it has on our bodies may be due to the fact that we are low-level poisoning ourselves with it?

"Vitamin D3, also known as cholecalciferol, is used as a rodenticide because it is highly toxic to rodents when ingested in sufficient quantities. It functions by causing a life-threatening elevation in blood calcium and phosphorus levels, leading to severe acute kidney failure"

"Despite its use in rodenticides, vitamin D3 is safe for humans and pets when consumed in normal dietary or supplement doses. However, extremely high doses of vitamin D3 can be toxic to humans as well, potentially leading to hypercalcemia, kidney stones, and renal failure. The difference in susceptibility between rodents and humans is significant; rodents are much more sensitive to the effects of cholecalciferol, which is why it is effective as a rodenticide."

The theory is that we are just poisoning ourselves by taking it and that our bodies react to being poisoned with the positive effects that are well documented and observed.

Can confirm anecdotally. I used to get 2-3 colds a year for about 15 years straight, especially during season changes when the humidity levels changed. Started taking 5000 IU vitamin D3 daily about 2 years ago and I haven't gotten sick since.