Two-thirds of the worldwide population will live in cities by 2050. Urban life is characterised by high-density business and residential buildings, more stressful conditions, lower access to green areas, and better exposure to substance use.

A recent study published in Nature Medicine explores the consequences of the urban environment on adult mental health.

Study: Effects of urban living environments on mental health in adults. Image Credit: Aleksandr Ozerov / Shutterstock.com

Study findings

In the current study, researchers investigate the consequences of urban environments on the mental health of adults between 41 and 77 years of age in the UK Biobank (UKB). The study included 156,075 participants, predominantly from urban areas. Participants were sub-stratified based on the provision of neuroimaging (NI) data.

Brain NI was performed in over 42,000 subjects, 14,988 of whom had complete NI, whereas the remaining 141,087 participants constituted the non-NI dataset. A complete of 128 urban environment variables across 53 categories and 21 psychiatric symptoms were assessed. A sparse canonical correlation was performed to find out associations between urban living categories and psychiatric symptoms.

A split-data evaluation was implemented for training and test datasets comprising 90% and 10% of information from the non-NI dataset, respectively. An urban environmental profile was significantly related to 5 psychiatric affective symptoms within the training dataset, which was also replicated within the test dataset.

The affective symptom group comprised frequencies of tiredness, unenthusiasm, depressive mood, feeling fed-up, and loneliness. Furthermore, these symptoms positively correlated with sound and air pollution, density and traffic of urban infrastructures, measures of street network accessibility, and socioeconomic indices of multiple deprivations.

The affective symptom group negatively correlated with green space proximity and distance to urban facilities. The team identified one other symptom set (anxiety symptom group) that included anxious feelings, feeling tense, worrying too long, affected by nerves, visiting a psychiatrist, and nervous feeling.

The anxiety symptom group was significantly linked to a second urban environmental profile that positively correlated with densities of mixed urban infrastructure and leisure places, coast proximity, mean terrain, and variation of normalized difference vegetation index (NDVI). Comparatively, anxiety symptoms negatively correlated with water proximity, distance to energy and waste facilities, and mean NDVI.

The third set of symptoms, which was categorized into the emotional instability symptom group, comprised mood swings, frequency of feeling miserable or highly strung, neuroticism rating, sensitivity and irritability, risk-taking, stress, grief, and hurt feelings. These symptoms negatively correlated with distance to food stores and densities of water, unused land, amenities, and open space.

The emotional instability group positively correlated with terrain variations and densities of educational facilities, accommodation, and medical or emergency facilities. These correlations were repeated by applying split-data evaluation for the NI dataset, which reproduced the three significant correlations identified in the first analyses.

Genome-wide association studies of the canonical covariates of the three symptom sets were performed in a sub-set of non-NI participants with the whole urban environment, psychiatric, and genomic data. Gene set enrichment analyses were performed to discover underlying genes related to symptom sets.

Over 3,400 significant associations with single-nucleotide polymorphisms (SNPs) were reported in genes for the affective symptom group. The strongest associations were for SNPs in a supergene candidate on chromosome 17q21.3 and the corticotropin-releasing hormone receptor 1 (CRHR1) gene.

The anxiety symptom group was significantly related to 29 SNPs across nine genes, with rs77641763 being the lead SNP in one in all the introns of the exonuclease 3′-5′ domain containing 3 (EXD3) gene. The emotional instability symptom group was significantly related to 10 SNPs, with the lead SNP of rs77786116 present within the intraflagellar transport 74 (IFT74) gene.

Multiple sparse canonical correlations on urban environmental profiles, psychiatric symptom sets, and brain volume were performed in an independent NI dataset. Significant associations were evident between 13 regional brain volumes, the affective symptom group, and the primary environmental profile. Eleven regional brain volumes were related to the anxiety symptom group and the second urban environmental profile.

Likewise, 12 brain volumes were related to the third urban environmental profile and emotional instability symptom group.

A moderated mediation evaluation was also performed to judge whether genetic differences moderated the associations mediated by brain volumes. To this end, CRHR1, EXD3, and IFT74 gene scores moderated the mediation pathway of the affective, anxiety, and emotional instability groups, respectively.

Conclusions

Specific urban environmental profiles correlated with distinct symptom groups. The primary urban profile, which was related to affective symptoms, was characterised by air pollution, deprivation, traffic, lack of green space, and a brief distance to urban facilities, reflecting a dense, poor inner-city neighborhood.

The second urban profile inversely correlated with anxiety symptoms and was characterised by green spaces, lakes, rivers, seas, and long distances to energy and waste facilities. The third urban profile related to emotional instability symptoms, which explained a lower variance than the primary two symptom groups. This profile was positively correlated with urban infrastructure and land use density.

Taken together, the study findings imply that distinct urban environmental profiles may influence specific mental health symptoms.

Journal reference:

• Xu, J., Liu, N., Polemiti, E., et al. (2023). Effects of urban living environments on mental health in adults. Nature Medicine. doi:10.1038/s41591-023-02365-w