Living near greenspace in midlife tied to slower cognitive decline in older age

By Vijay Kumar MalesuReviewed by Lily Ramsey, LLMJul 22 2024

In a recent study published in the Environmental Health Perspectives, researchers investigated the association between midlife residential greenness and cognitive decline later in life, considering the effects of apolipoprotein E (APOE)-ɛ4 status, socioeconomic status, and rural/urban residency.

Study: Midlife Residential Greenness and Late-Life Cognitive Decline among Nurses’ Health Study Participants. Image Credit: CandyBox Images/Shutterstock.com

Background 

Alzheimer’s disease and related dementia (ADRD) (a group of brain disorders that cause memory loss and cognitive decline) can begin decades before symptoms appear, with cognitive decline being a key aspect.

Research into novel risk factors for cognitive decline is crucial for dementia prevention. Studies suggest residential greenness may improve cognitive function through stress reduction, attention restoration, lower air pollution, increased physical activity, and social connection.

However, most studies are cross-sectional. Midlife risk factors like psychological stress, vascular issues, 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 association between midlife residential greenness and cognitive decline and to identify the most effective interventions for different populations, including APOE-ɛ4 carriers and socioeconomically disadvantaged groups.

About the study 

The Nurses’ Health Study (NHS), initiated in 1976, had 121,700 female nurses aged 30-55 across 11 United States (US) states and now spans 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 stroke history in a cognitive function study from 1995 to 2001.

Of 22,715 eligible women, 92% participated initially, with over 90% continuing through three follow-ups until 2008. The analysis included 16,962 women with at least one cognitive assessment.

Midlife greenness exposure was measured using the Normalized Difference Vegetation Index (NDVI) from 1986 to 1994, within 270-m and 1,230-m buffers around homes.

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

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

Interaction terms tested effect modification by SES, urbanicity, and APOE-ɛ4 status. Sensitivity analyses excluded participants with low baseline cognitive performance and recent movers and addressed changes.

APOE-ɛ4 status was determined from blood or buccal samples. Using SAS and R software, the mediation analysis estimated the effects of greenness on cognition via air pollution, physical activity, social connection, and mental health.

Study results 

Among the 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 a higher median income ($47,718 ± $17,990) but lower neighborhood home values ($133,738 ± $84,842) than those with the lowest greenness exposure.

The average global cognitive score was higher for women in areas with the highest greenness exposure compared to those in the lowest greenness exposure areas (0.02 ± 0.57 vs. -0.07 ± 0.61). Baseline characteristics were similar between participants in the cognitive function substudy and the analytic sample.

Higher midlife greenness exposure (per IQR) was consistently associated with higher baseline cognitive function, even after adjusting for individual and neighborhood-level covariates.

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

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

Exposure to greenness at midlife was associated with slower annual cognitive decline rates. In fully adjusted models, higher midlife greenness exposure correlated with a 0.004-unit slower yearly decline in the global composite score, equating to slowing cognitive aging by approximately eight months.

Participants with higher midlife greenness exposure experienced slower cognitive decline compared to those with lower exposure, consistent across buffer sizes and different NDVI measurements. However, there was no association between greenness and verbal memory decline.

Stratified analyses showed that higher midlife greenness exposure was associated with higher baseline cognitive scores in neighborhoods with medium and high population density, but not with cognitive decline.

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

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

Sensitivity analyses confirmed these results, except that the confidence interval included the null for cognitive decline among APOE-ɛ4 carriers without any APOE-ɛ4 allele.

Conclusions 

In this large prospective study of women, higher midlife exposure to greenness was associated with better baseline cognitive function and a slower rate of cognitive decline, based on global cognition scores.

The study found stronger associations in low SES and high-density neighborhoods, and among APOE-ɛ4 carriers. Mediation analysis indicated that mental health explained part of the association between midlife greenness and cognitive outcomes.

These findings suggest that midlife greenness may be a modifiable factor in reducing dementia risk, particularly for those with higher genetic risk and in disadvantaged communities.