The perception of primary care physicians of the ability to influence the lifestyle and eating habits of patients is an important factor in nutrition guidance practices. This perception is based on assumptions about the kind of influencing process that is effective or not and on the capacity of primary care physicians to play an effective role in these processes. The first elements is dealt with in this article. Three models are distinguished. The first model is the prescription model, based on a medical optimum and on information transfer as a metaphor. The second model is the persuasion model, based on a medical optimum, but presupposing blockades that have to be cornered by persuasive communication. The third is the interaction model. It is not based upon a medical but on an efficacy optimum, and on sharing of information and continuous involvement of the client in the interaction. Behind these three models we can perceive different views on communication and knowledge. Moreover, these three models are more or less appropriate with regard to different circumstances. The current stress on the psychological, social and cultural meaning of food and the new information context in which clients live, asks for more attention to the interaction model.