Vitamin D supplementation is very effective in case of rickets (https://en.wikipedia.org/wiki/Rickets https://en.wikipedia.org/wiki/Rickets). It is very rare no

Vitamin D supplementation is very effective in case of rickets (https://en.wikipedia.org/wiki/Rickets https://en.wikipedia.org/wiki/Rickets). It is very rare now in developed countries because nutrition has considerably improved. Some countries even fortify certain foods with vitamin D so you have to be a very special case to not get at least some vitamin D. VitD has some other functions in the body too. However, it appears that once you have resolved issues with the bones, the levels are generally sufficient for everything else to function properly. I cannot imagine a situation where a person has problems with the bones due to vitamin D insufficiency and then gets supplements, it will somehow fail to improve these other functions too. Optimal levels are just a guideline and slightly suboptimal levels mostly will have only a marginal effect. Currently we don't routinely test for vitamin D levels in population except for people at risk because the benefit is small and not cost effective. There is a theory that these suboptimal vitD levels are very dangerous in case of covid but studies have been really controversial about that. I suspect that most of those studies that show big effect are biased in one way or another similarly to many positive ivermectin studies.