From Picts to Parish: Stable isotope evidence of dietary change at medieval Portmahomack, Scotland

•Significant new dietary insights of medieval Scottish Highland communities.•Human and animal bone collagen analysed for carbon and nitrogen isotopes.•Diachronic dietary change between Pictish and parish church communities.•Fish consumption did not play an important part in the Pictish monks’ diet.•Age and sex isotope differences suggest gender and labour divisions in diet.