The body doesn't process vitamin d supplements the same way as it does when it produces it via sunshine(full spectrum) or UVB tech etc. So supplement only in winter if your above/below a latitude that is impossible to produce vit d in winter. If your within a latitude (from memory 35 or less) then you should not need supplements unless your lifestyle doesn't allow you to be outside often. We have evolved with and to use the sun, we are bioelectrical beings.