Nonlinear associations between exposure to natural environments and biological aging: A cohort study of middle-aged and older adults in China.

OBJECTIVE: To examine nonlinear associations between natural environments (green and blue spaces) and biological aging in middle-aged and older Chinese adults. METHODS: We analyzed longitudinal data from the China Health and Retirement Longitudinal Study (CHARLS). Environmental exposures were assessed using satellite-derived indicators, normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), green space coverage ratio, and blue space coverage ratio. Biological aging was quantified by frailty index (FI) and KDM-biological age acceleration (KDM-BAacc). Generalized additive mixed models (GAMMs) were employed to model nonlinear relationships; beneficial exposure ranges and optimal level were derived by differentiating the curves. Subgroup analyses examined effect modification by socioeconomic status, sex, and age. RESULTS: Significant nonlinear associations were observed between environmental indicators and biological aging. For FI, the maximum protective effect of green space occurred at 66.6% coverage, reducing FI by 0.006 (95% CI: -0.009, -0.004), the protective effect interval was (0.466, 0.784). Blue space showed maximum FI reduction of 0.014 (95% CI: -0.021, -0.008) at 10.9% coverage, with protective interval of (0.013, 0.140). When the blue space ratio was 10.9%, the protective effect on FI was the largest. For KDM-BAacc, optimal NDVI was 0.663, and optimal blue space proportion was 10.8%. Subgroup analyses revealed stronger protective effects among individuals with low socioeconomic status, women, and those aged 45-65 years. CONCLUSION: Moderate levels of green and blue spaces were associated with lower levels of biological aging. Nonlinear associations were stronger in socially vulnerable groups, suggesting that equitable nature-based planning could be a strategic public health intervention to promote healthy aging.