Back in August of 2009 – before Security Now was netcast on video – SN 209 discussed Vitamin D. I’m a long time listener, but I don’t remember that episode. @Leo rebroadcast it as SN 540 during the 2015 holiday season; I paid attention that time (TWIT TV archive, YouTube Archive, GRC transcript, GRC VitD health archive).Thanks to both of you for sharing this more than once! 
Knowledge has expanded tremendously since 2009. I’ll cite just one paper: Association of Serum 25-Hydroxyvitamin D Concentrations With All-Cause and Cause-Specific Mortality Among Adult Patients With Existing Cardiovascular Disease (2021) . This paper has the following graph:
The units of Vitamin D blood concentration are in nmol/L; we use ng/ml in the US. To convert, divide the nmol/L value by 2.5. The right edge of this chart is 60ng/ml. On the left side, 25nmol/l is 10ng/ml. Lowering the hazard ratio is a good thing; the P-value of this study is extremely low; that means the data quality is high. Please read the science paper before coming to any conclusions! I also recommend viewing John Campbell’s reading of parts of this paper on YouTube. In general, all of Campbell’s Vitamin D videos are excellent. There’s a standing joke for VitD enthusiasts: what if some [patented pharmaceutical] medicine had this much of a profound positive impact on our health?
My recommendations: get tested. Pester your doctor; your doctor should be able to order a VitD blood test. Alternatively, use private blood-testing services (either mail-order or walk-in) that process your sample in a lab and give you a report. Find a level of supplementation that works for you. The US RDA is 600IU/day for adults. That seems low/anemic/rickety for most people, but – again – please get tested to know for sure. During the pandemic Anthony Fauci noted that he supplements with 6000IU/day; your mileage may vary. The Vitamin D Council recommends test results in of 50ng/ml. Other sources recommend in the range of 40-80ng/ml. The VitD Council also curates a great variety of science papers showing the benefit of Vitamin D for particular health concerns. Who other than the VitD Council knew that raising VitD levels had a positive health impact on Acne?
During my VitD education, a burning question has emerged. Nature has been using Vitamin D for approximately one billion years; it’s been used for complex life functions (including immune regulation) since the Cambrian Explosion ~530 million years ago. Why is life – all plant and animal life – dependent on this molecule? Nature made all species on earth in the last 530 million years; isn’t the production of Vitamin D activated through sunlight – a tiny range of frequencies of UVB light – a rather fragile thing? In all that time, couldn’t the blind watchmaker have hacked together a better solution?
To make VitD, there must be a cleavage of the chemical bond between carbon molecules 9 and 10 of the B-ring of 7-dehydrocholesterol. No other bond must be altered! This change gives the molecule the ability be a “key”: to flex/distort and enter the nucleus of our cells acquire DNA sequence to express certain proteins. This VitD geometry has to be something fundamental to the geometry of our cell/nucleus walls, because nature never found anything else to do the job. If anyone knows more details – or has found a precise explanation on an AI – I’d love to hear about it.
A recent book discussed a frightening prospect: what happens when an enormous volcanic blast or an asteroid collision winds up blocking much UVB radiation for a period of months or even years? How would we maintain our health? How could plant/animal life in general survive? Wouldn’t viruses and other diseases ravage that immune-compromised world? Are any of the planet’s past extinction events linked to a global shortage of Vitamin D? I’m just starting to read; I’ll note what I find.
You two planted some seeds. Are you happy now, Steve/Leo?