We propose a theoretical model to study individual lifestyle choices related to calorie intake and physical activity, depending on personal fitness and body weight. The model builds on the rational eating literature and can generate a variety of behaviors that are consistent with the empirical evidence. In particular, we show that engaging in periods of a sedentary lifestyle can be a rational, utility-maximizing decision-a finding that is not present in the existing literature but is empirically widespread. Additionally, we show the possible existence of multiple equilibria and multiple indifferent lifestyles. The former justifies policy interventions to help individuals exit a self-reinforcing, but unhealthy equilibrium; the latter provides a theoretical basis for remediation plans that compensate for earlier unhealthy behaviors.