I'm not saying you are wrong; I personally believe vitamin D3 is important, too. (I'm also not a medical professional)
I'm just saying that the importance of vitamin D3 is not something that can be figured out by common sense; it's the sort of thing that requires a double-blind study.
The importance of vitamin D3 can also be seen in the Somalia diaspora in Nordic regions. The veiled Somali women, with their dark skin, burka clothing and home-dwelling habits, have very low vitamin D levels, leading to health issues in themselves and leading to autism spectrum disorders in their offspring.
Yes, I understand that there is some scientific evidence that D3 is important. I'm not contesting that evidence.
My point is that scientific evidence and "common sense" are very different sorts of things; in common use, "common sense" means something that is observable without education or technical training.
In general, I think "common sense" is less useful that the knowledge you can gain through dedicated work of educated scientists and statisticians.
