In this study we focus on the integration of mathematical learning content (i.e., proportional reasoning) in game-based learning environments (GBLE). More specifically, two kinds of GBLEs are set up: an extrinsically integrated GBLE and an intrinsically integrated GBLE. In the former environment, the mathematical content is not part of the core mechanics and structure of the gaming world. In the latter environment, the mathematical content is delivered through the parts of the game that are the most fun to play and embodied within the structure of the gaming world and the players' interactions with it. Sixty-four vocational track students participated in the study, all of them working in either version of the self-developed GBLE "Zeldenrust". The results of this study suggest that the way the content is integrated in a GBLE (i.e., intrinsically or extrinsically) matters: contrary to our expectations, students who played the extrinsically integrated game showed higher learning gains, motivational gains and perceived usefulness than students who played the game in which the content was intrinsically integrated.