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Introduction: The use of stable isotopes in reconstructing past diet is becoming increasingly popular. One of the challenges in reconstructing palaeodiet in prehistoric mainland Southeast Asia has been the predominance of C<sub>3</sub> plants limiting the identification of potential differentiation or temporal changes in diet (Krigbaum 2005). Palaeodietary reconstruction is often used to highlight abrupt temporal changes from one primary food to another, for example in the Americas with a change from C<sub>3</sub> plants to maize with a C<sub>4</sub> signature or in Europe with a change in protein source from marine to terrestrial (Richards et al. 2003). In this chapter, we are not so much concerned with major dietary shifts in prehistoric Southeast Asia, but rather with more subtle dietary differentiation among a range of environmentally similar sites in the region. Several research project using stable isotopes have been undertaken in Southeast Asia (Bentley et al. 2005, 2007, 2009
Bower et al. 2007
Cox 2009
Cox et al. 2011
King 2008
King et al. 2013
Kringbaum 2001, 2003, 2005). The stable isotopes and tissues analysed from different assemblages vary depending on the research objectives of the respective projects. The focus of these studies has not solely been to investigate diet, but also as a tool to assist in understanding residence, migration and marital customs. Although the emphasis of these studies was not necessarily palaeodietary reconstruction, they offer an important suite of data that can be used to contextualise palaeodiet from a regional perspective. Previous research focused on understanding palaeodiet in prehistoric Southeast Asia has suggested that it is unlikely that the few C<sub>4</sub> plants in the area, for example millet or Job's tears, were an integral part of the diet
rather, there was a focus on C<sub>3</sub> food resources (King 2008
Kringbaum 2003, 2005). However, a contribution from C<sub>4</sub> to ancient Southeast Asian diets is becoming increasingly apparent (Bower et al. 2007
King 2008). Although rice, as a predominant staple in prehistoric Southeast Asia, has never been questioned, recent developments in archaeobotany and an intensified emphasis on macro remains retrieved from floatation have provided new information which suggests that millet was in fact cultivated before rice at several sites in Thailand (Castillo 2011
Weber et al. 2010). Moreover, recent work conducted at the Neolithic site of Rach Núi (<i>c</i>.1,605-1,435 BCE) in Cần Giuộc in southern Vietnam by Castillo has found evidence for millet, supporting the view it could have been incorporated into the diet (Oxenham et al. 2015). Sampling bias has been suggested as a possible reason why rice is so often cited as being present at these sites, because it is more easily and frequently identified, while smaller grains such as millet require a more intensive sampling strategy and analysis (Castillo 2011
Weber et al. 2010). There is no evidence to suggest that millet was a staple subsistence crop in prehistoric Southeast Asia, but the fact that it is being identified in the archaeobotanical record provides an impetus to consider its dietary role by way of isotopic analysis. Dietary studies focus on stable isotope values in humans and other animals, but ultimately these are derived from plants, as they contribute to the diet of both. The isotopic values of plants are dependent on humidity, temperature and soil nutrients
while, in animals, isotopic values are dictated almost entirely by their diet. Almost all plants acquire their energy through photosynthesis, the chemical process in which plants use energy from sunlight to convert atmospheric carbon dioxide (CO<sub>2</sub>) into organic compounds, particularly glucose. Depending on the photosynthetic pathway of the plant, the way that they fix carbon from the atmosphere (C<sub>3</sub>, C<sub>4</sub> or crassulacean acid metabolism (CAM)), the ratio of <sup>12</sup>C to <sup>13</sup>C isotopes is reflected differently in the tissues relative to the atmosphere due to fractionation, resulting in distinctive non-overlapping signatures. This chapter presents a palaeodietary reconstruction for An Sơn and Hòa Diêm from Vietnam using stable isotopes δ<sup>15</sup>N<sub>collagen</sub> and δ<sup>13</sup>C<sub>collagen</sub> from tooth dentine and δ<sup>13</sup>C<sub>apatite</sub> from tooth enamel. In palaeodietary reconstruction, δ<sup>15</sup>N<sub>collagen</sub> reflects animal protein consumption providing information on trophic level effects, δ<sup>13</sup>C<sub>collagen</sub> reflects the contribution of terrestrial C<sub>3</sub> or C<sub>4</sub> or marine animal protein and δ<sup>13</sup>C<sub>apatite</sub> reflects the whole diet. Using δ<sup>13</sup>C<sub>apatite</sub> and δ<sup>13</sup>C<sub>collagen</sub> provides an opportunity to identify whether there was any difference between the protein portion and the whole diet of individuals at these sites. New isotopic data from An Sơn and Hòa Diêm, and a growing appreciation for the contribution of archaeobotanical analyses in augmenting our understanding of past subsistence practices, provide an impetus to re-examine previous palaeodietary interpretations. This chapter aims to investigate the possible contribution and likely source of C<sub>4</sub> in the diet, using multiple methods to try to reconstruct and interpret palaeodiet in a local and regional context.
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