In recent years, chronic diseases, such as high blood pressure, diabetes, heart attack, and cancer, have increased around the world. Obesity is a common factor that makes individuals susceptible to these diseases. One reason for excessive weight gain is the frequent consumption of fast food. This study examined the impact that fast food has on obesity by analyzing the influence of peer pressure on fast food consumption and the role that exercise plays in weight gain. Two mathematical models were proposed to study each factor separately. The models demonstrated the dynamics of individuals transitioning from one weight class to another according to their body mass index (BMI). The models were governed by systems of nonlinear ordinary differential equations. A qualitative approach for analyzing the systems was utilized, and the numerical simulations supported the qualitative results. The results of this study may contribute to the development of strategies for controlling obesity. In particular, the models showed the importance of resisting peer pressure that drives individuals to eat fast food meals, as well as maximizing the role that quitters can play in convincing obese individuals to stop consuming fast food meals. In addition to the two previous strategies, the research revealed that physical activity has a significant role in reducing weight.