Living near greenery delays dementia

Alzheimer's disease and related dementias (ADRD) (a group of brain diseases causing memory loss and cognitive decline) may begin decades before symptoms appear, with cognitive decline being a key aspect.

Research into new risk factors for cognitive decline is critical for dementia prevention. Studies show that residential greenness can improve cognitive function by reducing stress, restoring attention, decreasing air pollution, increasing physical activity, and enhancing social connections.

However, most studies are cross-sectional. Middle-age risk factors such as psychological stress, vascular problems, and physical inactivity strongly influence dementia risk. The APOE-ɛ4 allele is a significant genetic risk factor.

Further research is needed to confirm the longitudinal link between residential greenness in middle age and cognitive decline reduction, as well as to identify the most effective interventions for different populations, including APOE-ɛ4 carriers and socioeconomically disadvantaged groups.

The Nurses' Health Study (NHS), started in 1976, included 121,700 female nurses aged 30-55 in 11 US states and now covers the entire country. Participants completed biennial health questionnaires with over 90% follow-up.

This study, approved by institutional review boards, involved women aged 70+ without a history of stroke. Cognitive function was assessed from 1995 to 2001.

Of 22,715 eligible women, 92% initially participated, and over 90% continued through three subsequent periods until 2008. The analysis included 16,962 women who completed at least one cognitive assessment.

Exposure to greenness in middle age was measured using the normalized difference vegetation index (NDVI) from 1986 to 1994 within 270m and 1230m buffer zones around homes.

Cognitive function was evaluated via telephone interviews using six tests with composite z-scores for global cognition and verbal memory. Covariates included socioeconomic status (SES), demographics, mental health, air pollution, physical activity, body mass index (BMI), and social engagement.

Linear mixed models assessed the association between middle-age greenness and cognitive decline, adjusting hierarchically for SES, age, mental health, and antidepressant use.

Study Results

Among 16,962 women in the study, the average age at the first cognitive interview was 74.2 ± 2.3 years. Areas with the highest greenness exposure had higher median income ($47,718 ± $17,990) but lower neighborhood home values ($133,738 ± $84,842) compared to areas with the lowest greenness exposure.

The mean global cognitive score was higher in women living in areas with the highest greenness exposure compared to those in areas with the lowest exposure (0.02 ± 0.57 versus -0.07 ± 0.61). Baseline characteristics were similar between the cognitive sub-study participants and the analytic sample.

Higher greenness exposure in middle age (per interquartile range) was consistently associated with higher baseline cognitive function, even after adjusting for individual and neighborhood covariates.

Participants with higher greenness exposure had a global composite cognitive score at baseline that was 0.03 higher, which slightly decreased to 0.02 after adjusting for depression and antidepressant use.

This association remained robust across various adjustments and buffer sizes, with one year of age correlating with a -0.05 unit difference in baseline cognition. This indicated more greenness corresponded to baseline cognition equivalent to being ten months younger.

Middle-age greenness exposure was associated with slower annual rates of cognitive decline. Fully adjusted models showed higher middle-age greenness exposure correlated with a 0.004 unit slower annual decline in global composite scores, equivalent to approximately eight months slower cognitive aging.

Participants with higher middle-age greenness exposure experienced slower cognitive decline compared to those with lower exposure, consistent across all buffer sizes and NDVI measurements. However, no association was found between greenness and verbal memory decline.

Stratified analysis showed higher middle-age greenness was associated with higher baseline cognitive scores in medium and highly populated areas, but not with cognitive decline.

Among non-carriers of the APOE-ɛ4 allele, higher greenness exposure was linked to higher baseline cognition and slower cognitive decline. For APOE-ɛ4 carriers, higher greenness exposure was associated with a 0.01 unit slower annual decline in global composite scores.

Mediation analysis showed that mental health explained 18% of the association between middle-age greenness and baseline cognitive function and 24% of the association with cognitive decline. No other mediators were significant for cognitive function or decline.

Sensitivity analysis confirmed these results, except the confidence interval included zero for cognitive decline among APOE-ɛ4 carriers and non-carriers.