Association of Residential Greenness Exposure with Depression Incidence in Adults 50 Years of Age and Older: Findings from the Cohort Study on Global AGEing and Adult Health (SAGE) in China

Abstract

Background: Depression is a social and public health problem of great concern globally. Identifying and managing the factors influencing depression are crucial for preventing and decreasing the burden of depression.

Objectives: Our objectives are to explore the association between residential greenness and the incidence of depression in an older Chinese population and to calculate the disease burden of depression prevented by greenness exposure.

Methods: This study was the Chinese part of the World Health Organization Study on Global AGEing and Adult Health (WHO SAGE). We collected the data of 8,481 residents $\ge 50$ years of age in China for the period 2007-2018. Average follow-up duration was 7.00 ($\pm 2.51$) years. Each participant was matched to the yearly maximum normalized difference vegetation index (NDVI) at their residential address. Incidence of depression was assessed using the Composite International Diagnostic Interview (CIDI), self-reports of depression, and/or taking depression medication. Association between greenness and depression was examined using the time-dependent Cox regression model with stratified analysis by sex, age, urbanicity, annual family income, region, smoking, drinking, and household cooking fuels. Furthermore, the prevented fraction (PF) and attributable number (AN) of depression prevented by exposure to greenness were estimated.

Results: Residential greenness was negatively associated with depression. Each interquartile range (IQR) increase in NDVI $500\text{-m}$ buffer was associated with a 40% decrease [$\text{hazard ratio}\left(\text{HR}\right)=0.60$; 95% confidence interval (CI): 0.37, 0.97] in the risk of depression incidence among the total participants. Subgroup analyses showed negative associations in urban residents ($\text{HR}=0.32$; 95% CI: 0.12, 0.86) vs. rural residents, in high-income residents ($\text{HR}=0.28$; 95% CI: 0.11, 0.71) vs. low-income residents, and in southern China ($\text{HR}=0.50$; 95% CI: 0.26, 0.95) vs. northern China. Over 8.0% ($\text{PF}=8.69\%$; 95% CI: 1.38%, 15.40%) and 1,955,199 (95% CI: 310,492; 3,464,909) new cases of depression may be avoided by increasing greenness exposures annually across China.

Discussion: The findings suggest protective effects of residential greenness exposure on depression incidence in the older population, particularly among urban residents, high-income residents, and participants living in southern China. The construction of residential greenness should be included in community planning. https://doi.org/10.1289/EHP13947.

Figure 1.

Annual average maximum NDVI across China during the SAGE study period (2007–2018). In the figure, the color shades of yellow and green represent the NDVI, the black ranges represent the provinces where the study was conducted, and the red ranges represent the cities and municipality where the study was conducted. The percentage in each selected province/municipality indicates the incidence rate of depression during the study period. The figure was produced using R software based on the background map (https://www.resdc.cn), NDVI data (http://www.nesdc.org.cn), and results from the study. Note: NDVI, normalized difference vegetation index; SAGE, Study on Global Ageing and Adult Health.

Figure 2.

The selection process of SAGE study participants in China. Missing key information includes drinking status ($n=190$), body mass index ($n=47$), household cooking fuel ($n=20$), annual family income ($n=7$), smoking status ($n=3$), and marital status ($n=1$). Note: SAGE, Study on Global Ageing and Adult Health.

Figure 3.

Exposure–response curve of NDVI with depression incidence among SAGE participants in China ($n=\mathrm{8,481}$). Adjustment for sex, age, marital status, urbanicity, education, annual family income, and region. A time-dependent Cox proportional hazards regression model with natural spline function ($\text{df}=3$) was used to analyze the nonlinear relationship. Likelihood ratio test was used to test the nonlinearity. The solid line is the HR, the dashed lines are the 95% CI of the HR, and the green shaded area represents the population density. (A) NDVI in $100\text{-m}$ buffer. (B) NDVI in $250\text{-m}$ buffer. (C) NDVI in $500\text{-m}$ buffer. (D) NDVI in $\mathrm{1,000}\text{-m}$ buffer. Note: CI, confidence interval; df, degrees of freedom; HR, Hazard ratio; NDVI, normalized difference vegetation index; SAGE, Study on Global Ageing and Adult Health.

Figure 4.

Hazard ratios (HRs) and 95% confidence intervals (CIs) for the effect of residential greenness at different buffers on depression incidence (per IQR increase) among SAGE participants in China ($n=\mathrm{8,481}$). Model 1 included crude models without controlling for any covariates. Model 2 included models adjusted for the covariates from the DAG, including sex, age, marital status, urbanicity, education, annual family income, and region. A time-dependent Cox proportional hazards regression model was used to analyze the linear relationship. The error bars in the figure are the 95% CI and the dots and triangles are the HR. Table S7 corresponds to this figure. Note: CI, confidence interval; DAG, directed acyclic graph; HR, hazard ratio; IQR, interquartile range; SAGE, Study on Global Ageing and Adult Health.

Figure 5.

The HRs and 95% CIs of depression incidence for residential greenness (each IQR increment in NDVI $500\text{-m}$ buffer) among SAGE participants in China ($n=\mathrm{8,481}$). Adjustment for sex, age, marital status, urbanicity, education, annual family income, region, smoking status, drinking status, and household cooking fuel. In subgroup analyses, other confounders except for the subgroup category variable, analyzed as an independent variable, were adjusted for. A time-dependent Cox proportional hazards regression model was used. $p$ For difference values were calculated by Z-tests. The error bars in the figure are its 95% CI, the dots are the HR. Note: CI, confidence interval; HR, hazard ratio; IQR, interquartile range; NDVI, normalized difference vegetation index; Ref, Reference group; SAGE, Study on Global Ageing and Adult Health.

Figure 6.

New depression cases prevented by residential greenness exposure at the provincial level in China in 2020. In the figure, the numbers on the map represent AN. Unit: in thousands. The figure was produced using R software based on the background map (https://www.resdc.cn) and results from the study. Table S10 corresponds to this table. Hong Kong, Macau, and Taiwan were not in this analysis. Note: AN, attributable number.