Causal association between PM2.5 and metabolic syndrome in the Chinese elderly population-insights from a cohort study of CHARLS

Existing evidence suggests that the components of metabolic syndrome (MS) are sensitive to PM_2.5, especially in the elderly population, and related results targeting different regions and populations are inconsistent. This study aims to quantify the risk of association between PM_2.5 and MS components in the elderly population, as well as the moderating effect of physical exercise (PE) for this association. Biochemical data, demographic data and health behavior data were obtained from CHARLS dataset in 2011 and 2015, and the individual information was matched to obtain the two-wave panel data. We match meteorological data by region to obtain population exposure indicators. Subsequently, the directed acyclic graphs was used to control confonding, then instrumental variable method and fixed effects model were employed to evaluate the causal relationship between PM_2.5 and MS components and the moderating effect of PE. A total of 6125 individuals were included. The prevalence of MS was 34.5% and 32.9% in 2011 and 2015 respectively. The instrumental variable probit regression indicated that high concentration PM_2.5 exposure (coefPM_2.5 = 0.007, P < 0.001) may increase the risk of MS, and PM_2.5 had a significant impact on the components of MS, with a positive impact on waist circumference (WC) (coef = 0.052, P < 0.001) and systolic blood pressure (SYS) (coef = 0.214, P < 0.001), and with a negative impact on high-density lipoprotein cholesterol (HDL-C) (coef = - 0.030, P = 0.021), Triglyceride (TG) (coef = - 0.275, P = 0.048) and diastolic blood pressure (DIA) (coef = - 0.030, P = 0.007). Specifically, for each 1 SD increase in PM_2.5 exposure, HDL-C decreased by 0.57 mg/dL, TG decreased by 5.29 mg/dL, DIA decreased by 0.57 mmHg, waist circumference increased by 1.001 cm, and SYS increased by 4.11 mmHg. Additionally, low-intensity physical exercise may alleviate the effect of PM_2.5 on WC and SYS, while the high intensity exercise may increase the effect of PM_2.5 on WC. Exposure to PM_2.5 is associated with the occurrence of MS in the elderly population, and has a significant impact on the components of MS in different directions. The moderating effect of physical activity on PM_2.5 and MS varies by component. These results may provide scientific support for the prevention and treatment of MS in the elderly.