The nation's fight against fat has not reduced obesity, but it has had other worrying effects. Mental health researchers have raised the possibility that the intense pressures to lose weight have heightened the risks of developing eating disorders, especially among the young. Medical anthropology can help connect the dots between the war on fat and disordered eating, identifying specific mechanisms, pathways, and contextual forces that may lie beyond the scope of biomedical and psychiatric research. This article develops a biocitizenship approach that focuses on the pathologization of heaviness, the necessity of having a thin, fit body to belonging to the category of worthy citizen, and the work of pervasive fat‐talk in defining who can belong. Ethnographic narratives from California illuminate the dynamics in individual lives, while lending powerful support to the idea that the battle against fat is worsening disordered eating and eating disorders among vulnerable young people.