Objectives To evaluate the effect of increased vitamin D intake on vitamin D status, bone mass and bone turnover in young girls aged 12-13.

More specifically the effects on the following parameters will be assessed:

• Vitamin D metabolites.
• Parathyroid hormone.
• Bone turnover.
• Bone mineral accretion.
• Serum calcium and phosphate.
• Renal handling of calcium and phosphate.

This work package will consist of a randomised double blind one-year vitamin D supplementation study. Primary effect parameter is change in bone mineral content and therefore study duration will be one year. A total of 225 subjects, 75 in each of the three groups: one control and two intervention groups, will be included Two dosages of supplementation will be used: 5 µg per day and 10 µg per day.

Inclusion Criteria:

• Girls born in Denmark with Danish citizenship aged 10.0-11.0 years living in the municipals of Copenhagen and Frederiksberg

Exclusion Criteria:

• Any chronical disease with influence on bone mass or turnover
• Intake of drugs with influence on bone mass or turnover
• Intake of calcium or vitamin D other than the intervention