Investigations Into the Effect of Diet on Modern Human Hair Isotopic Values T.C. O’CONNELL* AND R.E.M. HEDGES Research Laboratory for Archaeology and the History of Art, University of Oxford, Oxford, OX1 3QJ, United Kingdom KEY WORDS carbon; nitrogen; isotopic analysis; animal protein consumption; diet; humans ABSTRACT Carbon and nitrogen isotopic analysis of body tissues is one of the few techniques that can furnish quantitative information about the diet of archaeological humans. The study of the effects of various diets on modern human isotopic values can help to refine palaeodietary theories, and such work also enables the testing of palaeodietary theories independent of archaeological remains and interpretations. This report discusses the use of modern human hair as a sample material for isotopic analysis. The biogenic carbon and nitrogen isotopic signal is well preserved in hair, and the isotopic values of the keratin can be related to diet. We show that atmospheric and cosmetic contamination of hair keratin does not appear to affect the measured isotopic values. In a small study of Oxford residents, we demonstrate that the magnitude of the nitrogen isotopic values of hair keratin reflects the proportion of animal protein consumed in the diet: omnivores and ovo-lacto-vegetarians have higher  15 N than vegans. There was an observed relationship between the reported amount of animal protein eaten (either meat or secondary animal products) and the nitrogen isotopic values within the two groups of omnivores and ovo-lacto-vegetarians, indicating that an increasing amount of animal protein in the diet results in an increase in the  15 N of hair keratin. This provides the first independent support for a long-held theory that, for individuals within a single population, a diet high in meat equates to elevated nitrogen isotopic values in the body relative to others eating less animal protein. The implications of such results for the magnitude of the trophic level effect are discussed. Results presented here also permit a consideration of the effects of a change of diet in the short and long term on hair keratin isotopic values. Am J Phys Anthropol 108:409–425, 1999.  1999 Wiley-Liss, Inc. The ability to make quantitative state- ments about ancient diets has fundamental importance for archaeology, since diet re- flects the interaction between demography, economy, environment, and food-production technology. However, direct archaeological evidence of diet is often elusive. Since ‘‘you are what you eat,’’ the biochemical analysis of body tissues is an indirect analysis of the food consumed. The natural distribution of stable isotopes in biological systems enables us to quantify aspects of food uptake into the body, and carbon and nitrogen isotopic analy- Grant sponsor: SERC/NERC. *Correspondence to: T.C. O’Connell, 6 Keble Road, Oxford, OX1 3QJ, UK. E-mail: tamsin@ermine.ox.ac.uk Received 31 October 1997; accepted 16 December 1998. AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 108:409–425 (1999)  1999 WILEY-LISS, INC.