The study of racial/ethnic inequalities in health is important to reduce the uneven burden of disease. In the case of colorectal cancer (CRC), disparities in survival among non-Hispanic Whites and non-Hispanic Blacks are well documented, and mechanisms leading to these disparities need to be studied formally. Body mass index (BMI) is a well-established risk factor for developing CRC, and recent literature shows BMI at diagnosis of CRC is associated with survival. Since BMI varies by racial/ethnic group, a question that arises is whether disparities in BMI is partially responsible for observed racial/ethnic disparities in CRC survival. This paper presents new methods to quantify the impact of a hypothetical intervention on BMI on racial/ethnic disparities in survival. We do this by fitting a Bayesian density regression model for BMI, coupled with an accelerated failure time model for survival. We perform a simulation that shows our proposed approach performs as well as or better than current methodology allowing for a shift in means only, and that standard practice of categorizing BMI leads to large biases when interactions are present and the distribution of BMI is not normal.