Human and domesticate animal bone collagen δ¹⁵N values in prehistory differ generally by 3‰ or more from Neolithic to post- Roman times in Northwest Europe, leading to an assumed dietary animal protein fraction of 60–80% using a standard interpretation of δ¹⁵N values. We examine the assumptions on which this model rests and the limitations of our knowledge in the analysis of δ¹⁵N values in archaeology. We have developed a set of models which, with small changes made in assumptions (on the order of 1‰), can produce substantially lower estimates of the dietary animal protein fraction for given δ¹⁵N values. We consider the implications of various dietary animal protein fractions on agricultural carrying capacities and human population densities in prehistory.