Abstract Background Diet is an essential determinant of health. Among the health determinants, we find access problems that are summarized as food insecurity. While such food insecurity has been studied in other countries and correlated to several health problems, it has been scarcely assessed in Belgium. The purpose of this work was to determine the factors of food insecurity existing within the Belgian population. Method From November 2016 to February 2017, a qualitative study using semi-structured interviews with 19 adults present attending the waiting rooms of six Public Social Action Centers in French-speaking Belgium, analyzed by grounded theory. Results In Belgium, for given food preferences and needs, food insecurity could be summarized as inadequacy between necessary and available resources within two dimensions: at the access level, for financial, temporal, informational, and freedom of action, and at the food use level, for temporal factors, material, knowledge, and skills. In these situations of inadequacy, participants reported finding either strategies to restore balance, or being forced to alter the quality or quantity of their diet. Conclusion While several factors of food insecurity may exist in Belgium, it appears essential that the first line of care these factors into consideration, since they could interfere with care and health, and because the first line of care is ideally placed to inform and refer the patients in question. Several courses of action are proposed in this work, which must still be confirmed by other studies.