Wasnt this study made in the UK where most people lack vitamin D, which is also associated with dementia and depression?
Deficiencies seem like such a frequent occurrence but most patients end up needing to self-medicate.
Thing is, intervention usually doesn't prove very effective. That's why doctors aren't that hysterical about it.
Then some vitamin supplement studies have shown actually harmful, eg. vitamin A as antioxidant. Vitamin D is a hormone and regulates many processes in the body, e.g. a large dose suppresses the immune system similar to steroids... caution is definitely warranted, instead of blanket recommendations of the "Just take 5000IU/d! #YOLO" you find online frequently.
Also, I found your reply really jarring. Perhaps it's the immediate ad-hominem attack, but I just wanted to note that for your reference, it really didn't motivate me to want to engage any further with you.
I threw your "vitamin D supplementation proved beneficial" into Google and this Nature review article on vitamin D supplementation was the first result:
* https://www.nature.com/articles/s41574-021-00593-z
Here is some additional reading you may find interesting: