The purpose of the study is to investigate in vivo whether a high-dose vitamin D3 oral bolus (2000 micrograms) produces marked vitamin D receptor target gene expression response and whether there is large inter-individual variation. These effects are compared to in vitro treatment of peripheral blood mononuclear cells from these subjects with 25(OH)D.

Serum 25-hydroxyvitamin D3 [25(OH)D3] is a well-established marker for vitamin D status of the human body. In addition to the general importance of vitamin D for bone health, low serum 25(OH)D3 concentrations have been associated with increased risk of several health outcomes, such as autoimmune diseases, type 2 diabetes and cardiovascular complications. However, there is significant inter-individual variation in the average serum 25(OH)D3 concentrations and also in the response to supplementation with vitamin D. Genetic and epigenetic factors have been suggested to be responsible for a large part of the variation, but currently there is little information about the health effects of the variation.

In our previous studies VitDmet (Clinicaltrials.gov NCT01479933) and VitDbol (Clinicaltrials.gov NCT02063334) we showed that the participants can be classified into high, mid and low responders to vitamin D and defined the new biomarker "vitamin D response index". Some 25% of the population seem to be low responders and are under higher risk to suffer from insufficient supplementation with vitamin D. The current study will focus on low vitamin D responders (among the 40 healthy individuals recruited in the study, 20-60 years old), i.e. it will use the same oral vitamin D3 bolus (2,000 µg, i.e. 80,000 IU in one day) as in our VitDbol study, in order to identify low vitamin D responders.

By in vitro treatment of peripheral blood mononuclear cells (PBMCs) of low responders with 25(OH)D3 for 24 h (in comparison to in vitro stimulations with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and in vivo vitamin D3 supplementation of the same subjects) we will obtain samples that allow the transcriptome-wide investigation of changes in gene expression. The underlying hypothesis of this study is that a stimulation with 25(OH)D3 is more efficient than a treatment with vitamin D3, so that in future low vitamin D responders may be supplemented with 25(OH)D3 rather than with vitamin D3.

Inclusion Criteria:

• Non-smoking
• BMI 20-25 kg/m2.

Exclusion Criteria:

• History of kidney stones, renal failure or dialysis, hypercalcemia, hypo- or hyperparathyroidism, severe liver disease (cirrhosis), or sarcoidosis or other granulomatous diseases, such as active chronic tuberculosis or Wegener's granulomatosis.
• Continuous use of anti-inflammatory medicines.
• Regular use of supplements containing over 20 micrograms of vitamin D.