Microbial communities are most commonly described mathematically using MacArthur’s consumer-resource model. One characteristic of such model is that the so-called “metabolic strategies”, the rates at which species uptake and metabolize resources, are constant parameters. However, microbes can adapt their metabolic strategies to the availability of different resources in the environment: when exposed to different sugars they often consume them sequentially resulting in population growth curves with distinct phases of growth rates, a phenomenon known as “diauxic shift”. In this work, we introduce adaptive metabolic strategies to consumer-resource models. We show that if the dynamics of metabolic strategies maximizes each species’ relative fitness, consumer-resource models can reproduce diauxic shifts in agreement with experimental observations. Furthermore we gain new insights on the coexistence of multiple species on a finite pool of resources. Introducing adaptive metabolic strategies allows consumer-resource models to violate the “Competitive Exclusion Principle”, a controversial theoretical argument according to which the number of coexisting species is limited by the number of available resources.