As they stated, it is because the population of Denmark is very homogenous, as opposed to the US. If you are trying to make a generalization that applies to a range beyond just white people, having Denmark as your sole sample is clearly flawed.
Along the same lines, picking Japan for the purpose of generalizing to wider racial/ethnic groups would also be a bad idea. Not because their research is untrusted/considered non-reputable (it is quite the exact opposite), but because their population is too homogenous.
> As they stated, it is because the population of Denmark is very homogenous
If you know about vitamin D, you'll note that sun exposure is one of the primary reasons location matters for this study. It would be similarly relevant if they only studied students in Miami or southern California.
Essentially: sun exposure helps you create vitamin D, and so you shouldn't naively generalize this study to other lines of latitude
It is probably a logistical nightmare to do a study of this sort in multiple countries and regulatory systems simultaneously.
It's just an important factor - if you live much further south or spend a lot of time outdoors, your target dosage will be different than someone in _Denmark_.
Studies everywhere are now being scrutinized for the participant cohorts because it is now widely recognized that biological differences exist between different groups. Some medications for example aren’t sufficiently studied for effects on women vs men and are being reviewed.
Plus, studies in US are less scrutinized because researchers are aware of the need for a diverse cohort and you are more likely to get one in the US vs elsewhere.