Stable isotopes of carbon and nitrogen in human fingernails for diet reconstruction: a comparison among mixing models

The stable isotope analysis of carbon and nitrogen in ancient bone remains is a method that is commonly used to decipher the dietary habits of ancient human communities. The isotopic approach is based on the fact that human and animal body tissues reflect different components of the diet. Over the past two decades, the use of mixing models to estimate the contribution of the various resources assimilated in the diet has become increasingly common; based on isotope data and statistical methods, such models offer advantages and limitations when they are used to simulate human diets, because humans are omnivorous, opportunistic and very flexible consumers. In this study, a comparison was performed among six commonly used mixing models; the different models were tested using the carbon and nitrogen isotope ratios of human fingernails measured in 22 living individuals with varying diets (i.e., omnivores, vegans and individuals affected by celiac disease who followed a gluten-free diet), to simulate the different components of the diet and to verify which models provide the most realistic estimate of the human diet. Mixing models represents a valuable tool in the interpretation of isotopic data, although the results have shown that they provide a moderately accurate estimate of different dietary contributions. Limitations and uncertainties might be higher for bio-archaeological remains; therefore, a multidisciplinary and critical approach is still essential to avoid misinterpretations of the data.