[filoleg]

The grandparent explained exactly why it is an issue though. It isn't because US is somehow just magically more legitimate than Denmark.

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.

[wrsh07]

> considering Denmark doesn’t get a lot of sun

> 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

[dheera]

I 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.

[wrsh07]

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_.