Some Biochemical Aspects of Carbon Isotopic Paleodiet Studies

Abstract

Carbon isotope ratios of preserved human tissues (collagen and carbonate in bone/tooth apatite) are used as indicators of the sources of nutrients in human populations. We discuss the possible roles of some biochemical phenomena in determining the partitioning of isotopically labeled nutrients in these tissues. Respiration converts all nutrient-derived carbon into CO ₂ which is irreversibly fractionated with respect to bicarbonate at the time of formation. Together with poorly-known fractionation between apatite and solution, this leads to the observed c. 11‰ fractionation between apatite and diet. Experimental studies show that exogenous amino acids (AAs) are preferentially “routed” to collagen, even though enzymes for synthesis of non-essential (ne) AAs (making up c. 80% of collagen carbon) exist in all cells. Assimilation of protein apparently leads to inhibition and suppression of endogenous synthesis of neAAs. Some natural populations of humans with lower protein intakes may not experience this inhibition, leading to more endogenous neAA synthesis from carbohydrate and lipid carbon sources, approaching linear mixing. This is suggested by high δ¹³C values in some C ₄ -consuming populations. Correction of their diets for routing would lead to nutritionally inadequate (maize-rich) diets. Biochemical pathways for synthesis of some neAAs from lipids are partially blocked; this may account for the apparent difference in δ¹³C spacing of apatite and collagen between carnivores and herbivores