Do redheads synthesize vitamin D even without sun exposure?

The study Increased 25(OH)D3 level in redheaded people: Could redheadedness be an adaptation to temperate climate?  was conducted in 2020 in the Czech Republic on a sample of 203 individuals: 73 with red hair and 130 with different hair colours. Its aim was to investige the correlation between red hair and vitamin D, in particular the vitamin D precursor 25(OH)D3 (calcidiol) and folic acid.  

The results were quite interesting: 

Redheaded subjects had higher 25(OH)D3 concentrations and approximately the same folic acid concentrations as non-redheaded subjects. […] Redheaded subjects also reported that they used more intensive chemical and mechanical sun protection than their non-redheaded peers. In contrast to the situation in non-redheaded persons, redheaded persons' 25(OH)D3 concentrations seemed independent of the intensity of sun exposure or protection from solar radiation. […] This suggests that a factor other than eumelanin concentration (and skin fairness) is responsible for higher concentrations of 25(OH)D3 in redheaded individuals and that both traits, that is both red hair and fair skin, may well be two independent adaptations for life in environments with low UVB radiation.

 

Basically, they are saying that red-haired people are capable of synthesizing sufficient amounts of 25(OH)D3 even when their sun exposure is minimal. However, the authors warn that this observation cannot be generalised, and add:

This phenomenon was observed in two medium-sized samples of 93 men and 110 women who passed a relatively stringent self-selection process. Until this phenomenon is demonstrated in other, more representative populations, especially those living north of the 55° parallel, such as Scotland or Sweden, our conclusions must be considered merely preliminary.

In the years that followed, the study was not refuted, but it was not definitively confirmed either. There are several studies about skin pigmentation and vitamin D (like this one), but none of them focuses on the difference between red hair and other hair colours.

In any case, this Czech study is very interesting nonetheless, and we can verify these results by ourselves. If you have red hair and don’t expose yourself to the sun too much, you can have you level of vitamin D tested and see if you have deficiency of it or not. 

Also, should the study be correct, one wonders why red hair is not more common, at least in northern countries. If you have a hair colour that gives such an advantage, one expects for this hair colour to be selected by evolution over all the other hair colours.