One of the mechanisms proposed in recent years to account for Vit D correlations, but not causations, is that Ultraviolet A (from sun exposure), also helps prod

One of the mechanisms proposed in recent years to account for Vit D correlations, but not causations, is that Ultraviolet A (from sun exposure), also helps produce nitric oxide (NB, this is also a very good reason to be a nose breather, as this is a major source of your NO). This won't account for everything either, but we're now fairly sure that the sun exposure -> VitD -> disease link is not as straight forward, and that VitD supps are not necessarily an answer to the low VitD correlations. I believe the only VitD supplementation causative link we have strong evidence for is to preventing/treating rickets.