Latin America has experienced rapid demographic, epidemiological and nutrition changes that have successfully contributed to decreasing undernutrition, but concomitantly have resulted in an increase of obesity and associated conditions; in this paper we propose that policies to address undernutrition have not adapted at a sufficiently rapid pace to address the emerging challenges. Taking Chile as an example we show that health promotion policies, implemented only when the obesity epidemic was well advanced, have succeeded in establishing effective regulatory frameworks and in implementing national large-scale programs for treatment; however, they have been insufficient in preventing obesity. The main lessons learned are: (1) Failure to monitor existing malnutrition programs for changing needs and true effectiveness can have unintended consequences. (2) Institutions and professionals working in nutrition and health need to assess changing scenarios and redefine their priorities for action accordingly. (3) There is a need to provide updated information to decision makers, program planners and to the population at large on how to promote and achieve healthy food consumption and active living considering local context. Timely policies and interventions to address these issues may contribute to the prevention of the obesity epidemic in transitional countries, particularly among low socioeconomic groups.