I believe this already since my first step into the vitamin D field but only now a review shows that
The integration of all these genome-wide data facilitates the identification of the most important VDR binding sites and associated primary 1,25(OH)2D3 target genes. Expression changes of these key genes can serve as biomarkers for the actions of vitamin D3 and its metabolites in different tissues and cell types of human individuals. Analysis of primary tissues obtained from vitamin D3 intervention studies using such markers indicated a large inter-individual variation for the efficiency of vitamin D3 supplementation.
It is a continuous medical malpractice to supplement newborn children with vitamin D without taking into account variation of the inter-individual response, body weight, or any co-medication.