This paper presents the results of new research into British Iron Age diet. Specifically, it summarises the existing evidence and compares this with new evidence obtained from stable isotope analysis. The isotope data come from both humans and animals from ten British middle Iron Age sites, from four locations in East Yorkshire, East Lothian, Hampshire, and Cornwall. These represent the only significant data-set of comparative humans (n = 138) and animals (n = 212) for this period currently available for the UK. They are discussed here alongside other evidence for diet during the middle Iron Age in Britain. In particular, the question of whether fish, or other aquatic foods, were a major dietary resource during this period is examined.The isotopic data suggest similar dietary protein consumption patterns across the groups, both within local populations and between them, although outliers do exist which may indicate mobile individuals moving into the sites. The diet generally includes a high level of animal protein, with little indication of the use of marine resources at any isotopically distinguishable level, even when the sites are situated directly on the coast. The nitrogen isotopic values also indicate absolute variation across these locations which is indicative of environmental background differences rather than differential consumption patterns and this is discussed in the context of the difficulty of interpreting isotopic data without a complete understanding of the ‘baseline’ values for any particular time and place. This reinforces the need for significant numbers of contemporaneous animals to be analysed from the same locations when interpreting human data-sets.