Maybe the relation is not as direct as we might think at first. Maybe the vitamin D deficiency is keeping people from going out more, which makes them feel iso

Maybe the relation is not as direct as we might think at first. Maybe the vitamin D deficiency is keeping people from going out more, which makes them feel isolated, not engaged fully with their friends and family which, in turn, pushes them to depression if they're prone to it. And that may explain why it seems that, as the writers of this paper mentioned, when some of the subjects get vitamin D the depression goes away. This can be caused by the effect of the intake on their ability to gain their more active way of life.