Wild organisms are usually infected with a whole range of parasites, however, the infection status of organisms is not usually taken into consideration in ecological and food web studies. The impact that parasites can have on their host organisms, affecting their food intake and internal resource use, might drive to differences in the isotopic composition or niche between infected and uninfected organisms, and thus to erroneous conclusions in stable isotope ecology studies. This is the first time that controlled diet-switch experiments and stable isotope analyses (whole tissue, bulk SIA, 13C and 15N) have been performed on a host-parasite system comparing infected and uninfected individuals. Our results highlight the importance of accounting for infection status when conducting ecological studies based on stable isotopes, also contributing to fill the gap of parasite contribution to food webs.