Proportions of the association between visceral adiposity and circadian syndrome statistically accounted for by dyslipidemia and muscle mass: A cross-sectional analysis of middle-aged and older Chinese adults

Objective To investigate the associations between visceral adiposity, dyslipidemia, muscle mass, and Circadian Syndrome (CircS) in a middle-aged and elderly Chinese population, and to quantify the proportions of the observed cross-sectional association statistically accounted for by dyslipidemia and muscle mass. Method In this cross-sectional analysis of 2,074 participants from the 2015 CHARLS, visceral adiposity (VAI), dyslipidemia (NHDL), and muscle mass (ASM) were assessed. Associations were evaluated using logistic regression, threshold effect models, and mediation analysis under a counterfactual framework (used solely for cross-sectional association decomposition), with adjustment for confounders. Multiple comparisons were controlled using the Benjamini–Hochberg false discovery rate (FDR) procedure. Sensitivity was analyzed via E-values. Result Participants with CircS exhibited significantly higher VAI (7.24 vs. 3.38), NHDL (138.35 vs. 54.37 mg/dL), and ASM (18.54 vs. 17.67 kg) (all P < 0.001). After FDR correction, strong associations with CircS persisted for VAI (OR = 1.536, Q < 0.001), AIP (OR = 161.585, Q < 0.001), NHDL (OR = 1.015, Q < 0.001), ASM (OR = 1.383, Q < 0.001), frailty index (OR = 1.134, Q < 0.001), and hypertension (OR = 3.464, Q < 0.001). In contrast, residual cholesterol, Castelli indices, hemoglobin glycation index, and age showed only marginal significance (Q ≈ 0.05), while EGFR and frailty status were no longer significant after correction. Nonlinear analysis revealed critical inflection points; for instance, the increase in odds of CircS associated with VAI plateaued beyond 7.414. Mediation analysis showed that NHDL statistically accounted for the largest proportion (60.3%) of the total observed VAI–CircS association, while ASM accounted for 12.6%. Sensitivity analyses using E-values revealed that associations for AIP (E-value = 24.91) and remnant cholesterol (E-value = 6.59) were robust to unmeasured confounding. However, those for NHDL (E-value = 1.09), ASM (1.63), and frailty index (1.33) were more susceptible, indicating that even a modest unmeasured confounder could fully explain these associations—a caveat that tempers the certainty of their independent role. Conclusion Visceral adiposity is strongly associated with CircS, a relationship for which dyslipidemia (particularly NHDL) and altered muscle mass statistically accounted for substantial proportions. The identified threshold associations offer potential reference points for clinical risk stratification, though they require prospective validation. These findings underscore the importance of lipid profile management and muscle quality preservation in CircS prevention, while also highlighting the need for longitudinal studies with more rigorous confounding control to confirm the observed patterns.