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In recent years, it has become evident that limitations exist in our ability to meaningfully assess palaeodiet using stable isotope compositions. These limitations in part arise because many of the fundamental assumptions about tissue-diet relationships are poorly understood. In order to redress this deficiency, a controlled feeding experiment was undertaken to define the impact of terrestrial- vs. marine-derived dietary protein consumption on consumer tissue carbon isotopic compositions (δ13C). Two generations of pigs were raised on one of five feeds with varying proportions of terrestrial (soy) and marine (fish meal) protein. A comprehensive range of tissues and fluids from 49 pigs was submitted for δ13C analysis. The observed tissue–whole diet and tissue–dietary protein carbon isotopic offsets were found to be highly dependent on the percentage of marine protein in diet. We suggest that the trend in δ13C offsets most likely derives from the increased routing of non-essential amino acids, especially glycine, with the increasing proportion of marine protein in the diet. These findings demonstrate that solely using bulk δ13C compositions not only masks considerable information about diet, but may also lead to erroneous representations of marine and terrestrial resource consumption in the past.

Compound-specific amino acid carbon-isotope compositions have shown particular promise for elucidating dietary behaviors in complex environmental contexts, and may also be able to mitigate the effect of many of the limitations inherent to palaeodietary reconstructions. Here, we investigate the efficacy of compound-specific amino acid isotopic proxies in characterizing the consumption of different dietary protein sources using amino acid carbon-isotope compositions for humans and fauna from Rössberga (Early to Middle Neolithic), Köpingsvik (Mesolithic and Middle Neolithic), and Visby (Medieval Period), Sweden. We also assess the explanatory capabilities of an isotopic mixing model when used with essential amino acid carbon-isotope compositions of humans and local fauna. All three isotopic proxies distinguished among humans from the three sites consistently and informatively, and were able to enhance the broad interpretations made using bulk isotopic compositions. The mixing model palaeodietary reconstruction revealed considerable diversity in relative protein source contributions among individuals at both Köpingsvik and Visby. Comparing the mixing model for bulk carbon- and nitrogen-isotope compositions to the model for essential amino acid isotopic compositions further demonstrated the likelihood of underestimation and overestimation of marine protein consumption for both aquatic-dominant and mixed marine-terrestrial diets when using bulk isotopic compositions.

Of central importance to palaeodietary reconstruction is a clear understanding of relative contributions of different terrestrial (i.e., C3 vs. C4 plants) and aquatic (i.e., freshwater vs. marine) resources to human diet. There are, however, significant limitations associated with the ability to reconstruct palaeodiet using bulk collagen stable isotope compositions in regions where diverse dietary resources are available. Recent research has determined that carbon-isotope analysis of individual amino acids has considerable potential to elucidate dietary protein source where bulk isotopic compositions cannot. Using δ13CAA values for human and faunal remains from Zvejnieki, Latvia (8th – 3rd millennia BCE), we test several isotopic proxies focused on distinguishing freshwater protein consumption from both plant-derived and marine protein consumption. We determined that the Δ13CGly-Phe and Δ13CVal-Phe proxies can effectively discriminate between terrestrial and aquatic resource consumption, and the relationship between essential δ13CAA values and the Δ13CGly-Phe and Δ13CVal-Phe proxies can differentiate among the four protein consumption groups tested here. Compound-specific amino acid carbon-isotope dietary proxies thus enable an enhanced understanding of diet and resource exploitation in the past, and can elucidate complex dietary behaviour.

Archeological hair from 14 adults from the Nasca Region, Peru (c. AD1-1000) was analyzed for carbon and nitrogen isotopic compositions and cortisol levels. We investigated the relationship between isotopic compositions, which reflect diet, and cortisol, which reflects biogenic cortisol production and chronic stress. Using a case study approach, we determined that there are consistent changes in cortisol production associated with the rapid dietary change characteristic of local mobility. Moreover, changes in nitrogen- and carbon-isotope compositions, when integrated with cortisol levels, enabled inferences to be made about nitrogen metabolism and carbon routing, and elucidated the nature of potential stressors in the months before death. The isotopic and cortisol data suggested a relatively high rate of exposure to stress that is consistent with what is known about the Nasca Region social and physical environments. Of the 14 adults included in this study, six likely suffered from illness/trauma before death, and a further three experienced stress without an observable associated change in isotopic composition. Five individuals also experienced increased stress related to local mobility, inferred from co-occurring changes in cortisol production and dietary shifting. The integration of cortisol and isotopic data revealed individual characteristics of hidden frailty and risk that would not be apparent using more traditional methods of evaluating health status. This approach will provide a powerful enhancement to the understanding of stress, morbidity, and well-being developed through skeletal analysis.