This article shifts focus from an individualised and anthropocentric perspective on obesity, and uses a new materialist analysis to explore the assemblages of materialities producing fat and slim bodies. We report data from a study of adults’ accounts of food decision-making and practices, investigating circulations of matter and desires that affect the production, distribution, accumulation and dispersal of fat, and disclose a micropolitics of obesity, which affects bodies in both ‘becoming-fat’ and ‘becoming-slim’ assemblages. These assemblages comprise bodies, food, fat, physical environments, food producers and processing industries, supermarkets and other food retailers and outlets, diet regimens and weight loss clubs, and wider social, cultural and economic formations, along with the thoughts, feelings, ideas and human desires concerning food consumption and obesity. The analysis reveals the significance of the marketisation of food, and discusses whether public health responses to obesity should incorporate a food sovereignty component.