Associations and pathways between residential greenness and metabolic syndromes in Fujian Province

BACKGROUND: Greenness exposure is beneficial to human health, but its potential mechanisms through which the risk for metabolic syndrome (MetS) could be reduced have been poorly studied. We aimed to estimate the greenness-MetS association in southeast China and investigate the independent and joint mediation effects of physical activity (PA), body mass index (BMI), and air pollutants on the association. METHODS: A cross-sectional study was conducted among the 38,288 adults based on the Fujian Behavior and Disease Surveillance (FBDS), established in 2018. MetS was defined as the presence of three or more of the five components: abdominal obesity, elevated triglyceride, reduced high-density lipoprotein cholesterol (HDL-C), high blood pressure, and elevated fasting glucose. The residential greenness exposure was measured as the 3-year mean values of the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) within the 250, 500, and 1,000 meters (m) buffer zones around the residential address of each participant. Logistic regression models were used to estimate the greenness-MetS association. The causal mediation analysis was used to estimate the independent and joint mediation effects of PA, BMI, particulate matter with an aerodynamic diameter of 2.5 μm (PM(2.5)), particulate matter with an aerodynamic diameter ≤ 10 μm (PM(10)), nitrogen dioxide (NO(2)), and sulfur dioxide (SO(2)). RESULTS: Each interquartile range (IQR) increase in greenness was associated with a decrease of 13% (OR = 0.87 [95%CI: 0.83, 0.92] for NDVI(500m) and OR = 0.87 [95%CI: 0.82, 0.91] for EVI(500m)) in MetS risk after adjusting for covariates. This association was stronger in those aged < 60 years (e.g., OR = 0.86 [95%CI: 0.81, 0.92] for NDVI(500m)), males (e.g., OR = 0.73 [95%CI: 0.67, 0.80] for NDVI(500m)), having an educational level of primary school or above (OR = 0.81 [95%CI: 0.74, 0.89] for NDVI(500m)), married/cohabitation (OR = 0.86 [95%CI: 0.81, 0.91] for NDVI(500m)), businessman (OR = 0.82 [95%CI: 0.68, 0.99] for NDVI(500m)), other laborers (OR = 0.77 [95%CI: 0.68, 0.88] for NDVI(500m)), and non-smokers (OR = 0.77 [95%CI: 0.70, 0.85] for NDVI(500m)). The joint effect of all six mediators mediated about 48.1% and 44.6% of the total effect of NDVI(500m) and EVI(500m) on the MetS risk, respectively. Among them, BMI showed the strongest independent mediation effect (25.0% for NDVI(500m)), followed by NO(2) and PM(10). CONCLUSION: Exposure to residential greenness was associated with a decreased risk for MetS. PA, BMI, and the four air pollutants jointly interpreted nearly half of the mediation effects on the greenness-MetS association.