Date of Award

January 2026

Document Type

Thesis

Degree Name

Master of Public Health (MPH)

Department

School of Public Health

First Advisor

Nicola L. Hawley

Second Advisor

Katherine Daiy

Abstract

Background: The transition from human milk to solid food feeding fosters gradual diversification of the infant gut microbiome. Emerging evidence suggests that diversity of the microbiome is involved in long-term health outcomes associated with chronic disease. Environmental factors, such as urbanization, may influence infant gut microbiome diversity, however, evidence from rapidly urbanizing populations is limited. Aim: We aimed to examine the association between urbanization and gut microbiome diversity among 4 to 8 month old infants in the Pacific nation of Samoa, the period during which solid food is introduced to the infant’s diet and weaning occurs. Approach: This study assessed fecal samples from 64 infants collected prospectively at 4- and 8-months of age. Richness (number of observed amplicon sequence variants [ASVs]) and Shannon diversity were used to measure alpha diversity. Census region was used as a proxy for urbanicity, with regions classified as urban, peri-urban and rural. Covariates included parity, infant sex and maternal age. We used parametric and nonparametric tests to explore the data, and linear regression models, linear mixed-effects models and likelihood ratio tests to test relationships between census region and alpha diversity. Results: Census region was not significantly associated with measures of microbiome diversity (Shannon diversity or richness) at 4 months or 8 months, or with magnitude of change across time. Additionally, where data were modeled longitudinally, census region did not improve model fit beyond time alone. Conclusions: Gradients of urbanization may not lie on the biological pathway that influences the infant gut microbiome diversification during weaning. The effects of urbanization may not be detectable at this age. These findings might reflect a relatively weak urban-rural gradient found in Samoa, compared with other settings. Additionally, human milk exposure and dietary factors related to weaning may have further limited our ability to detect urbanization-related differences in this cohort.

Comments

This thesis is restricted to Yale network users only. It will be made publicly available on 09/16/2028

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