The problem I have with this article is that we are not discussing "all-cause mortality" but rather optimal immune function. I don't think that one can extrapolate one from the other which is a problem with many research studies - too limited scope and focus - in my opinion. There is a lot of controversy on this topic as usual. All of the doctors I have seen in the past 3 years are now recommending levels of 80-100 ng/mL for their patients with immune dysfunction. For me to get my levels up over 50 ng/mL requires 5000-10000 IU/day. So I think everyone must determine their own personal optimal level along with their doctor and then test after supplementation to determine what it takes to get it there.