In other words: establishing causation (or lack of it) is more useful than merely correlation. Yes. But hardly breaking news, or even specific to medicine.
I mean yes, but OP's comment is especially notable with Vit D because it's deficiency is very common and often a marker for other common age related diseases.
It is highly likely that Vitamin D isn't saving these people's lives, it's that the folks without vitamin D are older and/or have disease.
It is highly likely that Vitamin D isn't saving these people's lives, it's that the folks without vitamin D are older and/or have disease.