During the Middle Ages, the Islamic-ruled area of Iberia was known as Al-Andalus. Present day Catalonia (NE Iberia) included one of the stable borders of Al-Andalus, the Upper Frontier, where Christian and Muslim communities lived together, bringing us the perfect opportunity to address lifestyle changes in form of diet modifications during this period. Human skeletons found in Balaguer (n = 13), an urban Islamic necropolis, and Santa Coloma d’Àger (SCA, n = 16), a rural Christian necropolis under Islamic rule (Mozarabs), are studied for isotopic composition of bone collagen together with three animal samples in order to reconstruct diet (δ13Ccol δ15N), and preliminarily address mobility (δ34S). No differences regarding sex or age have been found. δ13Ccol suggests a diet mainly based on C3 plants food chain. Significative differences between both sites regarding δ15N seem to indicate a preferential access to meat, eggs and/or dairy in the studied individuals from Balaguer in contrast to SCA. Despite their geographical proximity, the humans from Balaguer, tend to show slightly higher δ34S values than those from SCA. The homogeneity observed intra-site suggests that the individuals were local, but Balaguer could be more connected to the sea due to consumption of food imported from the coast, or transhumant animals; a fact that is coherent with the higher δ34S in the two analysed caprines found in Balaguer. However, more analyses are needed to address mobility. The comparison of dietary proxies with other Christian and Islamic sites reveals that the rural/urban character of the site was probably more key in dietary preferences than the predominant religion; in this sense urban communities seem to present a higher access to animal protein than rural sites. Despite being under Islamic rule, the Christian community of SCA has similar isotopic values than other Christian rural sites, a fact that does not support any kind of marginalisation on the former.