Viking Haithabu and its successor, the medieval town of Schleswig, were important international trade centres. Human skeletal finds spanning a period of approximately 400 years represent the bodily relics of the former inhabitants, who witnessed the rise and fall of these trade centres. Analysis of δ13C and δ15N from bone collagen was performed to reconstruct and detect changes in dietary preferences over time. A comparison with the respective isotopic data obtained from a large archaeofaunal sample resulted in a classic ‘mixing muddle’ that could only be deciphered using isotope mass balance mixing models applied on an individual basis. It was found that the overall subsistence economy shifted over time from a focus on fishing to one based predominantly on farming. The move to utilizing a new main source of protein did not impair overall protein supply. In addition, changing living conditions experienced by the inhabitants of Schleswig may have led to a change in infant nursing strategy.