This paper explores the productive tensions occurring in an interdisciplinary research project on weight loss after obesity surgery. The study was a bio-medical/anthropological collaboration investigating to what extent eating patterns, the subjective experience of hunger and physiological mechanisms are involved in appetite regulation that might determine good or poor response to the surgery. Linking biomedical and anthropological categories and definitions of central concepts about the body turned out to be a major challenge in the collaborative analysis. Notably, the conception of what constitutes ‘appetite’ was a key concern, as each discipline has its particular definition and operationalization of the term. In response, a material-semiotic approach was chosen which allowed for a reconceptualization of appetite as a ‘fractional object’, engaged in multiple relations and enacted differently in each instance. This perspective produced creative contrasts and offered alternative explorations of both scientific knowledge production and anthropological practices. The paper thereby explores the interfaces between anthropology and medical science by attending to the challenges and opportunities that result from destabilising an assumed fixed and well-defined concept associated with the body.