There are many claims about vitamin d because
people that have higher levels do better in some ways.
Do they do better because they have higher vitamin d?
We can test that. We make two groups of people and give vitamin d to one group, and see if it benefits them. In most cases, the benefit people thought this group would get, did not.
If a person has a vitamin d level of 32 ng/ml, increasing it almost never produces a benefit.
There are more kidney stones among people with a vitamin d level of 50 ng/ml.
Yes, I’ve always thought RCTs are better than someone saying on a forum “I do (or don’t do) x and therefore have (or don’t have) y”.