I’ve always wondered that also, about a lot more than vitamin D. I look at the Costco sized mega vitamins my wife got me (which I don’t take) and wonder how much of it is really needed, and how much of it is just junk science. But, that’s a different issue, and one that is argued routinely by people. With this stuff though, eh they say vitamin D is a plus, ok that works. Advil may mess up the immune system, ok I’ll take asprin occasionally. The more you go out, the higher your risk. Ok, I’ll live in my back yard and wear a mask in public. My natural inverted personality was made for this