Knowledge without effect: the controversial space of nutrition.

Summary This thesis focuses on the development of a public health policy on nutrition in France. By means of interviews, observations and archival work, this investigation approaches 1) the elaboration and the uses of prevention instruments as close as possible to the actors who conceive and implement them . 2) the controversies related to the regulation of food marketing . 3) the introduction of behavioral sciences (social marketing, nudge and social neuroscience) in nutrition. At the crossroads of the sociology of public action, the sociology of organizations and the sociology of expertise, this thesis shows that the implementation of a public health prevention program, the National Nutrition and Health Program (PNNS), does not lead to the stabilization of a definition, even if it is contested, of the nutrition problem. The elaboration of prevention instruments reveals several tensions, between the will to promote good nutrition without challenging the market and the will to individualize messages without stigmatizing individual behaviors. The end of the 2000s was marked by the publicity of knowledge presented as effective for "changing nutritional behaviors": social marketing, nudge and neuroscience. However, despite political announcements and enthusiastic expert opinions, this knowledge was not finally translated into public action. Behavioral knowledge is of interest not so much for its content as for the various strategic uses to which it is put. Heterogeneous actors (members of health agencies, private actors, scientists) use it mainly to challenge other forms of knowledge and actors and to legitimize themselves, rather than to transform public action.