Biomarkers of Sarcopenia and Sarcopenic Obesity in Renal Transplant Recipients: A Systematic Review and Evidence Quality Assessment

<b>Background:</b> Sarcopenia and sarcopenic obesity are increasingly recognized in kidney transplant recipients (KTRs), yet their molecular underpinnings remain poorly defined. We sought to synthesize current evidence on biomarker associations with muscle loss and function in the post renal transplant setting. <b>Methods:</b> A comprehensive search of PubMed/MEDLINE and Cochrane databases was conducted according to PRISMA guidelines. Studies evaluating biomarkers related to sarcopenia or sarcopenic obesity in adult and pediatric KTRs were included. Quality assessment was performed with the NHLBI tool. <b>Results:</b> Seven studies were included, encompassing 548 KTRs. Myostatin levels predicted sarcopenia in KTRs (cut-off: 390 pg/mL) and inversely correlated with Metabolic equivalent of Tasks (METs), handgrip strength (HGS), and graft performance. Although adiponectin was negatively correlated with body fat, its high-molecular-weight isoform was linked to lower muscle mass and long-term graft decline. Leptin was associated with sarcopenic obesity and lower estimated Glomerular Filtration Rate (eGFR). Insulin like Growth Factor-1 (IGF-1) independently predicted HGS but not muscle mass. Brain-derived neurotrophic factor (BDNF) levels predicted sarcopenia (cut off: 17.8 ng/mL) and reflected physical activity levels. Visfatin showed no association with sarcopenia but it was positively correlated with eGFR. Lastly, certain polymorphisms of Alpha-actinin-3 (ACTN3) were shown to genetically predispose to post-transplant sarcopenia. <b>Conclusions:</b> These emerging candidate biomarkers provide promising mechanistic insight into post-transplant muscle decline and may ultimately support more personalized risk assessment. Further validation is needed, and functional measures remain the most reliable clinical tools at present.