[ErikCorry]

Most vitamins are a waste of time and money, some are even harmful[1], but there are a lot of people with D deficiency, especially in winter[2].

1 https://www.mdpi.com/2072-6643/17/17/2744#:~:text=highest%20...

2 https://www.abs.gov.au/articles/vitamin-d#edit-group-image--...

[jghn]

The issue I've found with these discussions is it appears there's mixed evidence on if vitamin D *supplementation* actually has a positive impact, regardless of vitamin D deficiency. In other words, is the deficiency causal or correlative.

I have no opinion on the matter, and am inclined to think there is at least some positive benefit. But YMMV

[theshrike79]

The problem is that vitamin D doesn’t absorb the same way for everyone.

If a 100 people take 50IU of Vitamin D, you get 100 different results.

Some get enough from minor sun exposure and maybe eating a fish now and then. Others need massive doses to get any results.

[nradov]

Most of the vitamin D supplement studies have been very low quality in that they give all subjects in each group a fixed amount (or placebo). Ideally they should periodically test blood levels and titrate the dose to hit a target range. This would get us closer to establishing causality (or lack thereof) including a response curve. The amount needed to hit a given target will be wildly different for many individuals based on factors that are still not well understood.

[LocalPCGuy]

Just my results (n=1) and I don't think this is exactly what you were saying, but just in case other read it the same way I did at first: having had (lab tested) vitamin D deficiencies, vitamin D supplementation can help to restore levels back into the desired range. So supplementation can have the desired effect of improving vitamin D levels (more below). It is a simple test that most doctors don't quibble about adding on to other blood tests (i.e. during annual checkup, for instance), but isn't generally checked by default. (note: insurers may want it to be "diagnostic" rather than "preventative" in order to cover the test.)

Whether it has a "positive impact" on overall health (which I believe to be your point), that would be even more anecdotal and also impossible for me to narrow down whether that one factor had any significant effect, so I won't posit that. And I agree that from different studies I've read, the actual science on it is pretty varied and I haven't seen anything conclusive. Even this study notes their conclusion was "... among adults with suboptimal baseline vitamin D levels".

[mwigdahl]

This is solely my own anecdote, but I used to get bad seasonal depression every winter. I tried a number of interventions short of medication; none moved the needle very much. I started supplementing with vitamin D probably 8 years ago and haven't had any issues with seasonal depression since.

I'm pretty personally convinced that it was the supplements that helped here.

[swalsh]

I can tell you supplementation works 100%.

I took a blood test several weeks ago, my Vitamin D level was 14 ng/ml. I was so fatigued there were times I had to lay on my office floor because I didn't even have the energy to sit in my chair. I started taking 50k IU's weekly and then 10k IU's daily, and the results were dramatic. I went from having 0 energy to nearly normal. I also had soreness in my legs which went away.

[layer8]

Vitamin D isn’t technically a vitamin in the strict sense, because unlike the other vitamins the human body can produce it itself (by exposure to sunlight).

[rhdunn]

The body can also synthesize vitamin A from beta-carotene which is effectively two vitamin A molecules joined together (one rotated 180deg relative to the other).

[slow_typist]

Dogs can synthesise vitamin C…

[bluSCALE4]

I don't agree. As with everything, it requires care. Taking a multivitamin and thinking you're good to go is delusional.

[bluGill]

For most people just eating a good balanced diet and they are good to go. There are a few with genetic/biological issues and they need more - ask your doctor. Vitamin D is one that modern lifestyles likely don't get enough of and so probably worth it - again talk to your doctor.

[supportengineer]

How do you feel about vaccines and Tylenol?

[IAmBroom]

Uncalled for. GP is pointing out that the fact the human body can produce Vitamin D means it is not a vitamin.

vi·ta·min /ˈvīdəmən/ noun any of a group of organic compounds which are essential for normal growth and nutrition and are required in small quantities in the diet because they cannot be synthesized by the body.