Irisin-to-leptin ratio as a novel biomarker of favourable body composition and insulin sensitivity in elderly residents: A prospective pilot study

Background Sarcopenic obesity poses a growing concern among older adults due to its complex interplay between muscle loss, adiposity and metabolic dysfunction. Whereas irisin and leptin are key regulators of muscle–fat cross-talk, their combined predictive utility in ageing populations remains unclear. This study aimed to investigate the association between the serum irisin-to-leptin (I/L) ratio and body composition, insulin sensitivity and adipokines profiles in a super-aged Taiwanese population, and to assess the impact of nutrition education with or without protein supplementation on these metabolic markers. Methods A prospective pilot study was conducted among 36 elderly residents with a mean age of approximately 78 years, residing in senior homes in Northern Taiwan. Sarcopenia was defined using the Asian Working Group for Sarcopenia 2019 criteria. Anthropometric parameters were measured via bioelectrical impedance analysis. Fasting serum levels of irisin, leptin, adiponectin and insulin, and Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) were measured. Participants were stratified by sarcopenia status and median I/L ratio. A subset of participants underwent a 3-month nutrition intervention with or without high-protein supplementation. Multivariate linear regression was performed to assess associations between I/L ratio and metabolic indices. Results A higher I/L ratio was associated with significantly lower fat mass, fat mass index, per cent body fat, HOMA-IR and insulin, triglycerides, very-low-density lipoprotein cholesterol and high-sensitivity C-reactive protein levels (all p  < 0.05), and with higher adiponectin levels and adiponectin/leptin ratio. Multivariate analysis confirmed that a higher I/L ratio independently predicted lower fat mass index (B = −0.164, p  = 0.004), lower HOMA-IR (B = −0.036, p  = 0.030) and higher adiponectin levels (B = 0.144, p  = 0.002). Interestingly, despite a higher prevalence of sarcopenia, individuals with higher I/L ratios exhibited more favourable metabolic profiles. Nutrition education was associated with increased irisin levels ( p  = 0.033), while protein supplementation selectively increased adiponectin levels ( p  = 0.006) without affecting body composition. Conclusion A higher I/L ratio was associated with lower adiposity, improved insulin sensitivity and reduced inflammation, suggesting a distinct metabolic phenotype among super-aged adults. The I/L ratio may serve as a novel integrative biomarker of metabolic resilience in ageing populations. Nutrition education alone may enhance irisin levels, highlighting the potential of dietary strategies in modulating endocrine function and metabolic health in older adults. Trial registration: Not applicable.