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

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_.