Impact of Fat Mass and Fat-Free Mass on Locomotive Syndrome and Frailty in an Elderly Japanese Population: A Comprehensive Analysis

Background: The roles of fat mass index (FMI) and fat-free mass index (FFMI) in locomotive syndrome (LS) and frailty are not well understood. The current study explored the associations between FMI and FFMI and the prevalence of LS and frailty in an elderly Japanese population. Methods: This study examined 814 participants who underwent health checkups. Body composition including FMI, FFMI, and bone mineral content was measured via bioelectrical impedance analysis. LS was assessed using the Geriatric Locomotive Function Scale, and frailty was evaluated using the Japanese Cardiovascular Health Study criteria. Logistic regression analysis adjusted for age and sex was also performed. Results: A high FMI was significantly associated with LS, and a low FFMI was significantly associated with frailty. A U-shaped association was observed between FMI and frailty. Hence, both low and high FMI were associated with increased frailty risk. Participants diagnosed with both LS and frailty had a higher FMI and lower FFMI than those diagnosed with only one of the two conditions. Conclusions: FMI and FFMI have distinct roles in LS and frailty among elderly individuals. Monitoring and managing body composition via targeted interventions can improve musculoskeletal health and quality-of-life in aging populations.