Today, there is an important lack of knowledge on the effects of dietary protein on protein metabolism after physical activity in children. Here, the investigators will measure changes in whole body protein metabolism using minimally invasive techniques and stable isotope methodology at rest and after physical activity with the intake of milk protein.

Inclusion Criteria:

• Healthy and physically active (as determined by medical and activity questionnaire)
• 10-12 years of age
• Within approximately 2 years of estimated age of peak height velocity (PHV)
• Minimum aerobic fitness of 35 ml/kg/min as assessed by the McMaster All-out Progressive Continuous Cycling Protocol
• Having obtained his/her informed assent
• Having obtained his/her legal representative's informed consent

Exclusion Criteria:

• Post-pubertal
• Taking any medication
• Smoker
• Undergoing medical treatment or investigations
• Body mass index > 24
• Suffer from chronic diseases (with regular intake of drugs, medical history)
• Vegetarians
• Vaccination in the last 4 months
• Any know food allergies or intolerance
• Pregnancy
• Special diet or weight loss program (e.g. Atkins diet)
• Acute illness
• Fever, cold, flu
• Participation in a cholesterol management program with functional foods like food supplement enriched in omega-3 fatty acids, dairy products enriched with phytosterols (margarines, yoghurts).