The purpose of D-pro is to investigate the combined and separate effects of milk protein and vitamin D on bone health, growth, muscle function, body composition and cardiometabolic health in 6-8-year-old children.

The D-pro study investigates the effects of high or medium intakes of milk protein, and vitamin D supplements or placebo, in healthy 6-8 year-old Danish children. The study is a 2×2-factorial randomized controlled trial with 4 study arms. The subjects will be randomly allocated to receive yoghurt products with either high or medium protein content (300g/day, 6 days/week) and to receive tablets with either vitamin D (20 μg/day) or placebo for 24 weeks. Measurements and biological sampling will be performed at baseline and at the end of the intervention.

The primary outcome will be bone mineral density (BMD). The investigators will also measure effects on children's muscle function, cardiometabolic health, body composition, growth and growth factors, nutrient status, immune defence and risk of acute respiratory tract infections, as well as appetite hormones. The investigators will also assess potential sex-specific effects and the potential modifying effect of genotype.

Inclusion Criteria:

Children must:

• Be 6-8 years of age at the start of the intervention
• Be healthy
• Have a frequent intake of milk and yoghurt products, corresponding to at least ¼ liter a day
• Like yoghurt and high protein yoghurt ("skyr")
• Be of white, Danish or European origin

Moreover:

• The family must not be planning a winter vacation south of a latitude of 50° North
• The child must speak and understand Danish in order to understand the study procedures
• At least one parent must read, speak and write Danish

Exclusion Criteria:

• Known or suspected allergy or intolerance to milk or milk components
• Chronic illness or disease that may affect study outcomes
• Chronic intake of medicine that may affect study outcomes
• Use of vitamin D-containing supplements >3 days/week the 2 last months and use of any vitamin D-containing supplements the last 1 month before intervention start
• Concomitant participation in other studies involving dietary supplements or blood sampling
• Living in a household with another participating child

D-pro will be double-blinded in respect to D vitamin supplementation or placebo, whereas the allocation of yoghurt with high versus medium milk protein content will not be blinded.