The concept of the personal vitamin D response index.

Humans are able to synthesize vitamin D3 in their skin when exposed to UV-B, but seasonal variations, textile coverage and predominant indoor activities often make supplementation with the compound necessary. There is some dispute on the desired vitamin D status, measured via the serum concentration of the most stable vitamin D3 metabolite, 25-hydroxyvitamin D3, and the respective recommended daily supplementation. A possible answer may be provided by the concept of the personal vitamin D response index describing the efficiency of the molecular response to supplementation with vitamin D. The concept is based on the fact that vitamin D3 activates via its metabolite 1α,25-dihydroxyvitamin D3 the transcription factor vitamin D receptor and thus has a direct effect on the epigenome and transcriptome of many human tissues and cell types. Individuals can be distinguished into high, mid and low responders to vitamin D via measuring vitamin D sensitive molecular parameters, such as changes in the epigenetic status and the respective transcription of genes of mobile immune cells from blood or the level of proteins or metabolites in serum. Thus, we suggest that the need for vitamin D supplementation depends on the vitamin D status in relation to the personal vitamin D response index of an individual rather than on the vitamin D status alone.