Encourage one another and build one another up. Pray without ceasing. Test everything. Keep what is good. Avoid all evil. -1 Thess 5:11,17,21,22
Are we talking vitamin D in general or vitamin D3?
As far as your body is concerned, once it reaches the liver (where it is stored) D2 or D3 don't make much difference. D# is a bit better as a supplement, but any D is better than none.
D3 is what is generally measured in the bloodstream as it is +95% of the form of D floating around. It has a longer half life and, supposedly, is the more active form of D.I went with D3 because that is what the first paper was measuring.
I'm going to take a contrarian position. I believe that most of the US vitamin supplement industry is the modern equivalent of the traveling snake oil salesman. It all started with Linus Pauling claiming that people wouldn't get cancer and could live to be 100 by taking massive amounts of Vitamin C. There's no evidence of that and, in addition, almost no evidence that Vitamin C does any good if you have a cold. (The majority of studies show zero effect, some studies show a modest shortening effect. The latter studies are the only ones ever touted). But there are studies that show that some vitamins will literally kill you. For example, they have tried experiments to see if so-called "antioxidant" vitamins will protect smokers from cancer but they had to halt the experiments because the smokers started dying of cancer at such a statistically higher level than the control group that it would have been unethical to continue the experiment. Some B vitamins, for example, will cause an increased risk of cancer in smokers as will beta carotene. Many vitamins cause stuff to grow. I'm in my 50s - if stuff is growing in my body, it's probably not good stuff. My advice - Eat good food, get outdoors and get some physical activity. Work on your personal relationships and have something you want to live for. Keep vitamin supplements to a minimum. This advice is from an anonymous guy on the internet and worth every penny you paid for it.
If everybody thinks the same thing, than most people are not thinking.Incidentally, Vitamin D in massive doses is an effective rat killer.The difference between a therapeutic drug and a poison is the dose.