Colorectal cancer (CRC) is the 2nd leading cause of cancer death in the United States. The American Cancer Society (ACS) recommends normal body mass index (BMI), regular physical activity, and a healthy diet for cancer survivors. In 2018, we estimated that 38% of deaths within 5 years of diagnosis could be prevented in stage III colon cancer if all patients followed the ACS guidelines. Yet, <10% of CRC patients closely follow these lifestyle guidelines. Investigators have yet to optimize a lifestyle intervention, capitalizing on effective scalable components, to improve lifestyle behaviors in CRC survivors. Critical research gaps include: 1) whether specific intervention components (e.g., text messaging, etc.) are effective, overall or in sub-groups (men vs. women, etc.); 2) insufficient focus on improving diet; and 3) few studies with remote interventions have measured biological outcomes. To address these gaps, we propose to use the multiphase optimization strategy (MOST) framework to identify effective intervention components to increase the ACS guideline score (a standardized measure of physical activity, diet, and body size) among CRC survivors. The MOST framework is an engineering-based approach to efficiently optimize behavioral interventions while managing limited resources. Our team at the University of California, San Francisco; Dana-Farber Cancer Institute; and Northwestern University have strong expertise conducting lifestyle interventions in cancer survivors, including using MOST. Building on this experience, we propose a 12-month (mo.) randomized factorial experiment among 400 CRC survivors to determine the effect of 4 candidate intervention components [text messaging, digital health tool kit (physical activity tracker, apps), health coaching, buddy training (e.g., friend, family)] on change in the ACS guideline score from 0 to 12 mo. Changes in the ACS score (our primary outcome) have high potential to impact CRC survival, and it is modifiable and measurable remotely. Our Specific Aims are to: Aim 1) Identify which of 4 candidate intervention components increase the ACS guideline score at 12 mo. among CRC survivors. We will determine the individual and interaction effects of each component. Secondarily, we aim to: Aim 2) Examine mediators and moderators of the intervention components’ effects on change in the ACS guideline score from 0 to 12 mo. We will examine social cognitive theory constructs as primary target mediators and sociodemographic, clinical, and psychological/behavioral factors as potential moderators. This aim will help us understand how and for whom the intervention components affect lifestyle behaviors. Aim 3) Examine the ACS guideline score in relation to levels of fasting insulin, glucose, HOMA-IR and inflammatory markers at enrollment and 12 mo. The data from all three aims of this proposal will guide our next step to conduct a definitive randomized controlled trial to evaluate the effect of the optimized interve...