Rationale: Protein from plant-based foods may be unable to stimulate protein synthesis due to a suboptimal essential amino acid content and a lower digestibility. However, a more optimal amino acid profile may be achieved by increasing portions sizes and blending multiple plant-based sources. Studies investigating the anabolic properties of vegan diets (total elimination of animal-based foods) rather than single plant-based foods in older adults are scarce. It remains unknown if a vegan diet can also affect protein synthesis rates in other musculoskeletal tissues than muscle.

Primary objective: To assess the effects of a 2-week preoperative vegan diet in comparison to an omnivorous diet on protein synthesis rates in Hoffa's fat pad, synovium, tendon, bone, ligaments, menisci, and cartilage in older adults with knee osteoarthritis undergoing total knee arthroplasty.

Secondary objective: To assess the effects of a 2-week preoperative vegan diet in comparison to an omnivorous diet on blood pressure and plasma amino acids in older adults with knee osteoarthritis undergoing total knee arthroplasty.

Study design: Multi-center, randomized, controlled trial with an intervention and a control group.

Study population: 40 older adults (60 - 80 years) with osteoarthritis of the knee undergoing total knee replacement.

Intervention: Controlled vegan diet versus controlled omnivorous diet, for a duration of 14 days.

Main study parameters/endpoints: Primary study parameters are protein synthesis rates of Hoffa's fat pad, synovium, tendon, bone, ligament, menisci, and cartilage. Secondary parameters include blood pressure and plasma amino acids.

Inclusion Criteria:

• Written informed consent;
• Patients scheduled for total knee arthroplasty;
• BMI between 20-32 kg/m2;
• Aged 60 - 80 years;
• Mentally competent, as judged by the treating physician;

Exclusion Criteria:

• Following a vegetarian or vegan diet or a diet that affects protein intake during the six months prior to the study;
• Participating in a structured progressive exercise training program in the past three months;
• Lost more than 4 kg body weight during three months prior to the study;
• Chronic use of medications that affect protein metabolism (i.e. systemic corticosteroids, or prescription strength acne medications);
• Being diagnosed with one of the following: diabetes mellitus, rheumatoid arthritis, peripheral artery disease Fontaine III or IV, COPD GOLD III or IV, neoadjuvant chemotherapy or radiotherapy, phenylketonuria, collagen disorders (e.g. Marfan and Ehler-Danlos);
• Alcohol abuse;
• Surgical intervention to the knee in the past four weeks;
• Total parenteral nutrition at day of surgery;
• Glomerular filtration rate (GFR) <20 mL/min/1.73 m2;
• Allergic or intolerant to any product included in the diets;
• Osteoarthritis of the knee secondary to septic arthritis, osteonecrosis, fracture, osteochondritis dissecans, or malignant processes;
• Any other medical condition that may interfere with the safety of the participants or the outcome parameters, in the investigators judgement;
• Not willing to stop nutritional supplements, with the exception of vitamin D and supplements on medical advice.
• Investigator's uncertainty about the willingness or ability of the participant to comply with the protocol instructions;
• Participation in other studies that may have an impact on the outcomes during the three months before the start of the current study.