True, so long as your vitamin d receptors (VDRs) have not been plugged by l-form bacteria and other pathogens to thwart the immune system. I didn't believe it, but it turned out to be the case for me and probably a lot of other people out there. when this happens, taking extra vitamin d actually has the opposite effect, it suppresses the immune system. best to be sure before you start taking any extra vitamin d, natural or otherwise.
John