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.