Lower-extremity osteoarthritis (OA) is a prevalent musculoskeletal disease in elder population. The main symptom of OA is pain which leads to muscle weakness and physical disability. Recently, most studies indicated that muscle weakness, function limitation, and severity of disease are closely associated with ageing-related muscle attenuations. Therefore, elder individuals with OA have potential sarcopenia risks. Under multifactor risks of OA, it is important to identify effectiveness and efficiency of multidisciplinary management for such elder population to achieve healthy status. Previous studies have indicated that protein supplementation (PS) enhances exercise efficacy by additionally increasing muscle mass and strength for elder individuals with high sarcopenia and frailty risks who were undergoing exercise training (ET). However, it remains unclear whether PS in combination with ET augments benefits in lean mass, strength, and physical function of older adults with lower-extremity OA. In addition, few systematic reviews and meta-analysis studies regarding the effects of PS plus ET have emphasized on elder people with OA. Therefore, this study aimed to investigate whether PS plus ET exert beneficial effects on functional outcomes in older individuals with lower-extremity OA by using the methods of systematic reviews and meta-analysis.
Osteoarthritis (OA), especially in joints in the lower extremity, is one of the most prevalent musculoskeletal diseases in older adults . Its prevalence increases rapidly from the sixth to ninth decades of life, and it is a major health issue at individuals and population levels . In particular, hip or knee OA is associated with impaired functional activity, potentially leading to disability .
Deficits in muscle volume and function have been observed in older adults with mild to moderate hip OA  as well as in people with mild to severe knee OA . Such deterioration of muscle mass occurring with disease progression has been attributed to sarcopenia, a condition associated with muscle attenuation in older adults . Recently, it has been reported that older adults with knee OA are at a high risk of sarcopenia . This finding of high risk is based on previous observations that older adults with OA have lower appendicular lean mass in the lower limbs relative to healthy controls , and that low skeletal muscle mass is independently associated with radiographic knee OA . Because lower-limb lean mass is closely associated with muscle power in OA  and because low skeletal muscle mass is closely associated with physical difficulty and poor health status among older adults , sarcopenia may lead to physical decline through muscle weakness during OA progression. Therefore, the maintenance of muscle strength and the prevention of sarcopenia are crucial for enabling older adults with hip or knee OA to successfully perform physical tasks.
OA has been recognized as a major musculoskeletal disease . The management of mild to moderate OA comprises multidisciplinary interventions, including pain medications and nonpharmacological treatments; however, for end-stage OA, total joint replacement is recommended . Several recent studies have indicated physical activity and nutrition as nonpharmacological and preventive treatments for OA and sarcopenia . Among treatment interventions for OA, exercise training (ET) has been recommended as the first-line treatment . In addition, conservative ET—administered in combination with a variety of training tools, such as kinesio tape  and whole-body vibration (or electromyostimulation) —has been effectively employed to improve muscle mass, muscle strength, and physical function . Additionally, dietary interventions, such as dietary protein or protein supplementation (PS), have been incorporated into the multidisciplinary management of OA . Studies have noted that 30.3%–65.1% of older adults with knee or hip OA had a daily protein intake lower than the recommended daily allowance of 0.8 g/kg/day . PS and protein-based diet interventions are believed to additionally increase the efficacy of ET in older adults . However, inconsistent results have been obtained regarding the effectiveness of PS combined with ET (PS + ET), specifically in older adults with OA or in individuals who recently underwent total joint replacement. Previous studies have shown that PS + ET significantly improved muscle mass  and strength  in older adults with OA, but other studies have not . Because older adults with OA have a high risk of sarcopenia , which may further affect postoperative outcomes in those who have recently undergone total joint replacement , the skeletal muscle plays an important role in functional recovery after total joint replacement and has thus been targeted in the management of OA . Therefore, determining the effectiveness of PS + ET in preserving muscle mass in older adults with OA is crucial, especially in those who have recently undergone total joint replacement.
Evidence regarding the effects of PS + ET on healthy, sarcopenic, and frail older adults has been well established by previous systematic reviews and meta-analyses ; however, few studies have focused on older adults with OA or those who underwent total joint replacement. Thus, this study examined the effects of PS + ET on muscle mass and functional outcomes in older adults with OA in the lower extremity.
This meta-analysis demonstrated that PS is effective as a nutritional intervention; it yields improvements in muscle mass and strength during postoperative rehabilitation (mostly in multicomponent ET regimes) for older adults with lower-extremity OA who have undergone total joint replacement. Postoperative PS further reduces pain, increases physical mobility, and improves global functioning after 2−7 weeks of rehabilitative ET. Considering the small number of RCTs included in this meta-analysis, more future studies are required to more robustly determine the efficacy of PS + ET in this specific population. In addition, only one RCT enrolled patients without prosthesis in our meta-analysis; therefore, we could not definitively conclude that PS + ET is effective in older adults with OA who did not undergo total joint replacement. Thus, future studies on the effectiveness of PS + ET should focus on people with OA who have not undergone total joint replacement. The results elucidate nutritional and exercise interventions that benefit older adults with OA, particularly those who have undergone total joint replacement. The results also facilitate the formulation of practical and interdisciplinary approaches to counteracting muscle loss and functional decline. Practitioners in geriatric care and in rehabilitation settings, such as clinics, hospitals, institutions, and communities, can use our findings as a reference. Nevertheless, to better identify more optimal supplementation protocols, our results must be further validated by additional studies with relatively large samples.