Environmental Exposures and Diabetes Risk

Abstract

With type 2 diabetes (T2D) reaching epidemic proportions globally and domestically, and mounting evidence for the role of environmental exposures in diabetes risk, the time is ripe for further epidemiologic investigation into these population-level modifiable risk factors. This dissertation set out to probe the following gaps in the literature: 1) the role of long-term exposures to air pollution; 2) the role of residential greenness; and 3) understanding biological pathways to T2D diagnosis. The first study (Chapter 2), addressing research gap 1, observed positive associations between long-term fine particulate matter (PM2.5) and nitrogen dioxide (NO2) exposure and T2D incidence, persisting even at levels below the U.S. Environmental Protection Agency (EPA) National Ambient Air Quality Standards (NAAQS). The second study (Chapter 3), addressing research gap 2, detected negative associations between long-term residential greenness exposure, measured by the satellite-based Normalized Difference Vegetation Index (NDVI), and T2D incidence in the same U.S.-based nationwide prospective cohorts. Both studies also made headway into identifying modifiers and exposure interactions. The third study (Chapter 4) targeted research gap 3, by focusing on glycated hemoglobin (HbA1c), an important biomarker of prediabetes and diabetes risk, as the outcome. Through its investigation of simultaneous environmental exposures using single and multi-exposure linear regression models, as well as models incorporating Principal Components Analysis (PCA) component scores, it considered the effects of multiple environmental exposures and how they might interact. While the third study did not uncover any associations in fully adjusted models, it underscores that more work is needed to understand the mechanisms linking environmental exposures and T2D diagnosis, using mixture methods as needed to address potential multi-collinearity issues. This dissertation contributes to the epidemiologic literature on environmental exposures and diabetes risk through the application of established U.S.-based nationwide prospective cohorts of women and men, with finely resolved exposure data and in-depth information on individual and area-level covariates. By identifying modifiable diabetes risk factors and improving understanding of mechanisms, this work helps pave the way for public health interventions -- in the form of air quality policy changes and urban planning decisions -- to decrease the heavy burden of diabetes felt not only in the U.S., but worldwide.