Associations of urinary nicotine metabolites and essential metals with metabolic syndrome in older adults: The mediation effect of insulin resistance.

Exposure to tobacco smoke and essential metals is linked with metabolic syndrome (MS). However, the joint effect of them on MS in older adults and the underlying mechanisms are still unclear. This large-scale study measured the urinary concentrations of 8 nicotine metabolites and 8 essential metals in 4564 older adults from Shenzhen, China. The biomarker of insulin resistance, triglyceride-glucose index (TyG), was also calculated. Restricted cubic splines (RCS), Bayesian kernel machine regression and quantile-based g-computation were used to access the single and joint effects of urinary nicotine metabolites and essential metals on MS and insulin resistance. Mediation analysis was performed to investigate the role of TyG in these relationships. Single urinary nicotine metabolite and essential metal had non-linear relationships with MS in RCS. The overall effect of urinary nicotine metabolites and essential metals was positively associated with MS. Urinary zinc (52.2 %) and copper (20.1 %) were the major contributors to MS, whereas molybdenum had a negative association with MS. TyG mediated 64.7 % of the overall effect of urinary nicotine metabolites and essential metals on MS. Overall, the mixture of urinary nicotine metabolites and essential metals had a dose-response relationship with MS. Insulin resistance was as a crucial mediated pathway in this association.