Make a commitment in the fight against obesity and physical inactivity in France. The construction of a hybrid and located expertise by the associative networks of prevention and care. While the State tries to increase its capacity of control in the field of the prevention of the obesity through nutritional recommendations in France, the associative mobilizations criticize its policy. Succeed in rendering these criticisms visible in order to make their voices heard and weigh on public action sanitary plan is the main problem. By its ambition and the delegation of its implementation in the regional health authorities and the civil society, we argue that the National Nutrition and Health Program provides an opportunity for associative healthcare networks studied of making a commitment in the public action to fight against obesity and physical inactivity. This process comes along with, according to us, the rise of a particular form of expertise. To exist in the debate, these last ones have to gather, enlist, which implies simultaneously an ability to hybridize the knowledges of the various actors involved and articulate various levels of analysis and action between the national level and the more located level. In considering this dynamic, we would like to participate in the debate reflecting the reduction of asymmetries between the promoters of prevention campaigns and the project owners associations in the definition of public health policies. For this study, we wish to test the hypothesis that this process is the result of a "sanitary democracy" at work, in other words an opening of the public decision at the civil society which is often described as "laypeople" versus the classical model of the institutional expertise.