Writing on social movement learning and environmental adult education invokes particular views on knowledge that need further examination and development in relation to food social movements. Although food social movements take different forms, the paper argues that the politics of food knowledge is at the centre of many of these movements. Contributing to the discourse of social movement learning, this article focuses on the film "Food, Inc.", an important activist resource and documentary film about a particular food movement. We analyse how it legitimates certain forms of knowledge about food production and consumption and de-legitimates others. Whilst a useful case study on knowledge and film activism in itself, the article seeks to challenge what it sees as some key tenets about knowledge in social movement learning literature. One key tenet is that it is self-evident whose interests are served by "ordinary people's knowledge" and "scientific knowledge." Instead, it is argued that when it comes to collective action for food there is ambiguity, messiness and contestation about what constitutes knowledge and, in particular, anti-capitalist knowledge. But realisation of such ambiguity, messiness and contestation should not lead to paralysed inaction, but to informed and nuanced action. A question then for social movement learning practitioners is how they can mobilise social change through a broader sense of knowledge and its effects.