Background: Epidemiological data show significant associations of vitamin D deficiency and autoimmune diseases. Vitamin D may prevent autoimmunity by stimulating naturally occurring regulatory T cells.

Objectives: To elucidate whether vitamin D supplementation increases Tregs[1] frequency (%Tregs) of circulating CD4+ T cells.

Methods: We performed an uncontrolled vitamin D supplementation trial among 50 apparently healthy subjects including supplementation of 140,000 IU at baseline and after 4 weeks (visit 1). The final follow-up visit was performed 8 weeks after the baseline examination (visit 2). Blood was drawn at each study visit to determine 25-hydroxyvitamin D levels and %Tregs. Tregs were characterized as CD4+CD25++ T cells with expression of the transcription factor forkhead box P3 and low or absent expression of CD127.

Results: Forty-six study participants (65% females, mean age ± SD 31 ± 8 years) completed the trial. 25(OH)D[2] levels increased from 23.9 ± 12.9 ng/ml at baseline to 45.9 ± 14.0 ng/ml at visit 1 and 58.0 ± 15.1 ng/ml at visit 2. %Tregs at baseline were 4.8 ± 1.4. Compared to baseline levels we noticed a significant increase of %Tregs at study visit 1 (5.9 ± 1.7, P < 0.001) and 2 (5.6 ± 1.6, P < 0.001).

Conclusions: Vitamin D supplementation was associated with significantly increased %Tregs in apparently healthy individuals. This immunomodulatory effect of vitamin D might underlie the associations of vitamin D deficiency and autoimmune diseases. Hence, our finding provides a rationale for further studies to investigate vitamin D effects on autoimmunological processes.