One important factor in vitamin studies is the prevalence of vitamin deficiency in a particular population. E.g. vitamin D in the US might not be as effective because it is already added into many food products, e.g. milk, which is not the case for other countries.
My understanding is that even the definition of what constitutes deficient isn't settled due to how it's measured. For example, many African Americans show deficient even though it's believed they aren't.
Vitamin D is added to milk and butter in Sweden, even obligatory for some kinds! But that's because of our sad lack of sunshine for many months on end of course, so I expect the other nordic countries does something similar.
