Multimorbidity, social activity diversity, and functional limitations among Chinese adults aged 45 years and older: exploring the buffering role of social participation

Background Multimorbidity is a major public health challenge among aging populations. While social participation has been recognized as a potential protective psychosocial resource, evidence regarding its buffering role against functional decline in multimorbid older adults remains limited and inconsistent. Methods We analyzed cross-sectional data from 18,746 participants aged ≥45 years in the 2020 China Health and Retirement Longitudinal Study (CHARLS). Multimorbidity was defined as having ≥2 chronic conditions. Social activity diversity was categorized as low (<2 types) versus high (≥2 types). Basic ADL limitation was measured as a binary outcome (restricted in ≥1 of five basic self-care items; the sixth continence item was excluded due to differing response format). Sociodemographic covariates included age, sex, education, hukou (urban/rural residence), number of living children, and smoking status. We employed chi-square tests, robust logistic regression with interaction terms, average marginal effects (AME), dose–response models, and linear probability models (LPM) for sensitivity analysis. Results The mean age was 61.87±9.85 years; 41.67% reported ADL limitations, 49.9% had multimorbidity, and 20.3% engaged in ≥2 types of social activities. Multimorbidity was independently associated with higher odds of ADL limitation (adjusted OR=1.104, 95% CI: 1.033–1.180, P=0.004). Social activity diversity showed no significant independent association (adjusted OR=1.022, 95% CI: 0.924–1.131, P=0.673). The interaction term was in the expected negative direction but did not reach conventional statistical significance (fully adjusted OR=0.946, 95% CI: 0.818–1.094, P=0.454). Results were consistent across dose–response, subgroup, and linear probability model analyses. Conclusions Multimorbidity is an independent risk factor for functional limitations among middle-aged and older Chinese adults. Social activity diversity showed no significant direct association with ADL limitations. The interaction term was in the expected buffering direction but did not reach statistical significance, suggesting that the potential protective role of social participation may be more nuanced than previously assumed. Further longitudinal research with more refined social activity measures is needed to clarify the buffering hypothesis.