1. They measured maternal vitamin D before supplementation began. They explicitly adjusted for these preintervention levels.
2. the two groups started at essentially the same vitamin D levels.
3. They specifically tested whether baseline vit D status changed the effect of supplementation
For my own point: in this study they have like 22 test values but still use the 95% confidence interval. Even on random data there will be a significant result like a third of the time so I think it's easy to interpret these result as more definitive than they are. Not that it's a bad study though (no study will be everything, baby steps like this are important in science).
> Covariate-adjusted analyses of standardized scores (mean [SD], 0 [1]; higher values indicating better performance) showed positive associations of high-dose vitamin D3 with verbal memory (β = 0.17 SD; 95% CI, 0.03-0.32 SD; P = .02), visual memory (β = 0.24 SD; 95% CI, 0.06-0.42 SD; P = .01), and flexibility or set shift (β = 0.19 SD; 95% CI, 0.01-0.37 SD; P = .04); however, high-dose vitamin D3 was no longer associated with flexibility or set shift after multiple test correction.
Generally, when a study is done in the US - no one will ever question the location. The moment the study is outside the US, "not US so not generalisable" questions always arise.
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.
Spain have lower levels of vitamin d than Denmark.
In places like Spain only some “premium” milk gets this treatment.
That same mutation made them vulnerable to the levels of sunlight at lower latitudes, susceptible to sunburns, etc.
First, that's only true for about 4 months of the year. Second, people cooped up in offices in China, India, and the US don't get a lot of light either. In fact I'd bet the better work-life balance in Denmark means people actually do get more light there because they spend more of their evenings and weekends outside instead of in the office. Office buildings in Denmark also tend to have much better sunlight by design.
You can probably have the same results in the New England area of the USA, no? Even NY with 10mil people?
3. Okay, and?