In this placebo controlled study the investigators aim to investigate the effect of a 3 month vitamin D supplementation on parameters of the immune, endocrine and metabolic system in healthy women and men. Vitamin D. In a pilot study the investigators showed an increase of certain immune cells, which regulate immunotolerance, the investigators therefore try to confirm these results with a placebo group. Furthermore the investigators assess the effect of vitamin D on the glucose metabolism and hormone levels.

This is a randomized double-blind placebo controlled trial, with the aim to investigate the effect of a Vitamin D supplementation on parameters of the immune, endocrine and metabolic system in healthy humans without diabetes and obesity.

The primary aim is to assess the effect of vitamin D supplementation of the number and function of peripheral regulatory T-cells (Tregs). The investigators will measure the Tregs at baseline and monthly for 3 months. Furthermore the investigators assess additional immune cells from the adaptive immunosystem.

Furthermore the investigators will perform a standard mixed meal tolerance test at baseline and 3 months following the supplementation in order to assess the insulin secretion in vivo.

Inclusion Criteria:

• Age >= 10 years
• healthy subjects without chronic disease

Exclusion Criteria:

• Hypercalcemia >2,65 mmol/L
• pregnancy and lactation period
• acute infections
• chronic disease requiring treatment