PROJECT SUMMARY In November 2023, New York City (NYC), NY became the first jurisdiction in the US to adopt an added sugar warning policy, which requires restaurants to display a warning label on menus next to items high in added sugar. The adoption of this policy has the potential to reduce the burden of nutrition-related chronic diseases, including obesity, type 2 diabetes, and heart disease. However, almost no real-world studies have examined whether added sugar warning labels affect purchases of restaurant foods. Moreover, no real-world studies have examined for whom these warning effects are largest. Also unknown are the psychological mechanisms that could explain how added sugar warning labels change behavior. The Warning Impact Model has established that cigarette and beverage warnings change behavior by changing thoughts, feelings, and beliefs, but has never been applied to added sugar warnings. We will determine whether, for whom, and how added sugar warnings affect restaurant purchases. All Aims will use a rigorous quasi-experimental design (difference- in-differences) that assesses changes in purchases in NYC from before to after the warnings are implemented, net of changes in comparison sites without warnings over the same period. We will combine two complementary data sources. First, we will partner with MFour, an innovative mobile phone app with whom we have completed a rigorous pilot test. Using MFour’s ability to send location-based surveys to a panel of participants, we will prospectively collect receipts from customers exiting chain restaurants in NYC and matched comparison sites. We will collect 4 repeated cross-sections of data (2 before and 2 after warnings are implemented, total n=10,000 customers). We will also collect information on customers’ demographic characteristics and their thoughts, feelings, and beliefs about added sugar. Second, we will obtain a dataset of every purchase made at Taco Bell restaurants nationwide (including >3 million transactions per year in NYC alone) from 2023-2028. We will use these data to examine whether added sugar warnings reduce total sugar purchased from chain restaurants (Aims 1.1 and 1.2) and to determine whether this effect differs for customers with lower education, lower income, or Black or Hispanic identities (Aim 2). We will also use these data to determine whether added sugar warnings lead to more negative thoughts, feelings, and beliefs about added sugar (Aim 3). Our central hypothesis is that added sugar warning labels will reduce total sugar purchased from restaurants in NYC relative to comparison sites, with larger reductions among customers with lower education, lower income, and Black or Hispanic identities. Completion of these Aims could lay the groundwork for the adoption of added sugar warning labels across the US to reduce obesity, type 2 diabetes, and other nutrition-related diseases.