Divergent associations of social isolation with body composition in Chinese middle-aged and older adults

Social isolation represents a critical challenge in the context of global population aging, yet its effects on body composition in older adults remain paradoxical. This study aimed to systematically examine, from a life-course perspective, the differential associations of social isolation on sarcopenia and obesity among middle-aged and older adults in China, thereby evaluating the proposed divergent associations hypothesis. Using data from 5,525 participants in the China Health and Retirement Longitudinal Study (CHARLS), we constructed life-course patterns of social isolation integrating both childhood and adulthood exposures. Outcomes were defined according to the Asian Working Group for Sarcopenia (AWGS) guidelines and the World Health Organization (WHO) criteria for Asian populations. Multivariable logistic regression models were employed to assess the primary associations, supplemented by subgroup, sensitivity, and exploratory mediation analyses. Persistent social isolation was significantly associated with increased odds of low muscle strength (OR = 1.72, 95% CI: 1.21–2.44) and possible sarcopenia (OR = 1.49, 95% CI: 1.03–2.14), reflecting a potential cumulative disadvantage. In sharp contrast, persistent isolation showed a significant inverse association with obesity (OR = 0.63, 95% CI: 0.47–0.85). Persistent social isolation is strongly associated with functional muscle decline without concurrent BMI-defined obesity, suggesting a potential high-risk pattern. These findings underscore the importance of early interventions targeting social isolation to preemptively address covert functional decline.