alt Hacker NewsI don't think it invalidates a study as long as you do things on relative terms and have a control group. Another study can see if the same delta effect is reproducible in an e.g. homogeneous Asian population and report on it.
It is probably a logistical nightmare to do a study of this sort in multiple countries and regulatory systems simultaneously.
It doesn't invalidate the study at all! On the contrary, if you're measuring vitamin d levels from blood tests, it is easy to adjust the dosage to match.
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_.