PROJECT SUMMARY High dietary sodium intake is linked to incident hypertension (HTN) and cardiovascular events. Conversely, lower sodium intake has been shown to reduce both blood pressure (BP) and prevent atherosclerotic cardiovascular diseases (ASCVD). American Heart Association guidelines advocate daily sodium intake of less than 2,300 mg, but the estimated average sodium intake for Americans is 3,400 mg per day. In recent years, consumption of pre-processed and restaurant foods has substantially increased, and more than 75% of sodium in the average U.S. diet now comes from these sources. Despite widespread knowledge of the harms of high dietary sodium intake, an ongoing challenge for clinicians is engaging patients in lifestyle-based treatments to reverse these trends as well as promote ongoing self- management and long-term adherence to low sodium diets. Although there is growing hope that digital health platforms may help to overcome these issues, few empirical studies have demonstrated their efficacy over time. Relying on a strong conceptual framework in health behavior theory, we recently demonstrated in early work that a mobile application-based just-in-time adaptive intervention (JITAI) reduces dietary sodium intake (R21HS024567). This JITAI uses geofencing technology to provide tailored and contextual push notifications at home, grocery stores, and restaurants with links to sodium-based nutrition information within the mobile application. While our preliminary results were exciting, the critical gaps in knowledge that remain to be tested are 1) whether a mobile application-based JITAI focused on lowering sodium intake will lead to a decrease in BP, 2) which dietary components mediate the intervention's effect on BP, and 3) how to optimize participant engagement to sustain both dietary and BP effects over time and prevent habituation. The long-term goal of our research program is to develop successful mobile application-based strategies, driven by adaptive technologies, that support patient self-management in common cardiovascular conditions. Our overall objectives are to evaluate the efficacy of a dietary sodium JITAI on reducing BP, establish the changes in diet that mediate improvement in BP, and optimize the JITAI for long-term patient engagement. Our central hypothesis is that JITAIs targeting dietary sodium will reduce BP over time and that these effects will be better sustained through optimization of JITAIs. Accordingly, we will perform a two-step sequential, prospective, randomized controlled trial in 400 HTN patients to meet our overall objectives. In the first step, we will investigate the effect of the JITAI on BP and perform a micro-randomized trial to optimize the JITAI for patient engagement. In the second step, we will study the sustained effects of an optimized JITAI on BP.