PROJECT SUMMARY As Kamala Harris proclaimed in June 2022 when she launched the White House Action Plan on Water Security, the global water crisis is among the most urgent issues of our time. Water insecurity, or the lack of stable access to safe and acceptable water, has been identified as a threat to human well-being that is plausibly on par with that of food insecurity. Further, emerging evidence suggests that the impacts of water insecurity are surprisingly far-reaching, including suboptimal nutrition, physical health, and mental health. We are, however, early on in our understanding of the role of water insecurity in human health, in part because we have lacked precise measures of water insecurity. Therefore, the core scientific objective of this R03 application is to understand the roles of water and food insecurity experiences on key health outcomes among a nationally representative sample of Mexican adults from Mexico's 2021 National Health and Nutrition Survey [Encuesta Nacional de Salud y Nutrición (ENSANUT)]. Our central hypothesis is that water insecurity will be negatively associated with health measures, even when controlling for known key covariates. In Aim 1, we will investigate the associations between water insecurity and indicators of nutrition (intake of sugar- sweetened beverages, ultra-processed foods, fruits, vegetables), physical health (adiposity, blood pressure, diabetes), and mental health (depressive symptomatology). In Aim 2, we will examine if concurrent water and food insecurity have a synergistic (multiplicative) relationship with health measures, i.e., if those experiencing both insecurities have worse health than expected from the additive associations with water and food insecurity. Increasing our knowledge about how water insecurity shapes health is highly significant because global problems with water quantity (both flooding & shortages) and quality are occurring with increasing frequency and severity due to climate change, population growth, and crumbling infrastructure. This project is innovative for its precise measurement of water insecurity. Before 2019, global indicators of water captured only the physical availability of water or water infrastructure; these are far more distal predictors of human health. It is also innovative because ENSANUT 2021 is the first national health and nutrition survey in which experiences of water insecurity have been measured. Finally, the concurrent measurement of water and food insecurity will bring unprecedented insights into their potential synergisms in North America. Expected outcomes include a clearer picture of the epidemiology of water insecurity in Mexico and a more precise understanding of how experiences with problematic water access, use, and stability can act as barriers to optimal health. This information will likely reveal novel ways to intervene to reduce resource inequities and the risk of poor health outcomes among vulnerable populations. This work will als...