Macronutrient-based model for dietary carbon routing in bone collagen and bioapatite

Carbon stable isotope ratios (??¹³C), measured in human bone collagen (??¹³C_collagen) and bioapatite (??¹³C_bioapatite), are commonly used indicators in ancient human diet reconstruction. The underlying assumption is that human tissues broadly reflect the ??¹³C signal of dietary food sources (??¹³C_diet) plus an isotopic offset. However, interpretation of results may be confounded by the differentiated routing of macronutrients (energy, that is carbohydrates and lipids, and protein) having associated different isotopic signals (??¹³C_energy, ??¹³C_protein). Multiple regression analyses were conducted on data from controlled animal feeding experiments compiled by Froehle et al. (J Archaeol Sci 37:2662?C2670, 2010). We derived a simple algebraic macronutrient-based model with ??¹³C_bioapatite?=?10.1?+???¹³C_diet (??) and ??¹³C_collagen?=?4.8?+?0.74 ??¹³C_protein?+?0.26 ??¹³C_energy (??). While the established relationship for ??¹³C_bioapatite is similar to previously known results, the model also suggests that ??¹³C_collagen signal contributions originate from surprisingly consistent proportions of protein and energy macronutrients. Given that feeding experiments explore extreme variations in the proportion of diet macronutrients, the applicability of the proposed model and its predictions were tested in a variety of well-known, wild animal and human, natural contexts. Possible biochemical mechanisms explaining these empirical results are discussed.