PROJECT SUMMARY/ABSTRACT Unreliable access to safe and sufficient water for all household uses (i.e., water insecurity) is a common but understudied public health issue. Although water is itself a critical nutrient that is essential for agricultural production, food preparation, and hygienic practices that reduce exposure to enteric pathogens, few studies have examined the role of water insecurity in nutrition; most consider water quality only. A more holistic conceptualization of water insecurity could reveal important pathways by which water accessibility, reliability, and adequacy impact diet and health. Water insecurity presents substantial barriers to cooking nutrient-dense foods, which often require considerable water to be made palatable and safe for consumption. Qualitative studies have reported that water insecurity limits breastfeeding duration and undermines the ability of caregivers to prepare preferred complementary foods. Individuals experiencing water insecurity are also more likely to substitute water with sugar-sweetened beverages, plausibly contributing to weight gain and increased risk of overweight and obesity. The proposed project seeks to expand upon this formative work by examining the extent to which differential exposure to water insecurity impacts diet quality and body composition throughout the life course. A mixed-methods approach is required to comprehensively understand how water insecurity manifests at the household level and subsequently impacts nutritional well-being. The proposed project will therefore use qualitative data to complement quantitative data from 2 existing cohort studies that comprehensively measured water insecurity and diet among infants, children, adolescents, and adults on San Cristóbal Island, Ecuador and in Cebu, Philippines. Specifically, the project aims to qualitatively describe how issues with water availability, accessibility, and use impact food production, purchase, and preparation (Aim 1). Multi-level modeling will be used to assess whether household water insecurity undermines child diet quality (Aim 2) and longitudinal structural equation modeling will be applied to evaluate how changes in household water insecurity throughout early development impact adiposity (Aim 3). The project will address a recognized knowledge gap within the field by using advanced statistical procedures to model water insecurity as a multidimensional, time-varying exposure. Findings from this work have the potential to reveal new targets for addressing the growing burden of malnutrition. Additionally, the proposed rigorous curriculum in nutrition and epidemiology, as well as mentorship from thought leaders in the water and nutrition sectors, will prepare the applicant for an academic career in global nutrition.