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.