In the present study, the effects of 5 days of lower limb immobilization with or without twice-daily protein supplementation on muscle mass and muscle fiber characteristics will be determined. A young group will be included to compare the changes in muscle mass and muscle fiber characteristics between young and old men.

Inclusion Criteria:

• Male
• Aged from 18-35 years and from 65-75 years
• 18.5 < BMI < 30 kg/m2

Exclusion Criteria:

• Smoking
• Performing regular resistance training in the previous 6 months
• Hypertension (according to WHO criteria) [52] and/or cardiovascular disease
• Any back/leg/knee/shoulder complaints which may interfere with the use of crutches
• Systemic use of antibiotics within 3 weeks prior to the study visit
• Current systemic use of corticosteroids, growth hormone, testosterone, immunosuppressants or insulin
• Type 2 diabetes mellitus
• Any history of thrombosis
• Any family history (1st grade) of thrombosis
• All co-morbidities interacting with mobility and muscle metabolism of the lower limbs (e.g. arthritis, spasticity/rigidity, all neurological disorders and paralysis).
• Myocardial infarction within the last 3 years
• Use of anti-coagulants
• Any (history of) gastrointestinal disease that interferes with GI function
• Indications related to interaction with the study product:
• Known allergy to milk or milk products
• Known galactosemia