The diet of the cancer patient is a major focus of prevention and treatment strategy of the recent plans that fight against cancer. It is sometimes reduced to a rapid series of more or less general advice, often interfered by other sources of information, more or less conventional. In this pathological situation where the nutritional status of the patient is paramount, it seems crucial to understand the different modalities of how the food behavior is implemented. This article describes the construction modalities of the cancer eater decisions. The goal of the socio-anthropological analysis proposed in this article is to initiate a reflection on the under nutrition problem by focusing on the approach of the eater diagnosed with cancer. The aim is to help identify ways of action to fight against under nutrition and improve the quality of life of the patient.