Dietary protein content and tissue type control 13C discrimination in mammals: an analytical approach

RationaleCarbon isotope measurements on collagen and other tissues are used in modern and paleodiet studies, and relating these patterns to the dietary protein fraction (f(dp)) and to C-13 values of dietary protein and energy (lipids and carbohydrates) remains important for determining diets. MethodsCarbon fluxes from dietary protein to tissues in feeding studies of mice, rats, and pigs were estimated to test how tissue C-13 values related to the C-13 values of dietary protein, dietary energy, f(dp), or log(e) f(dp). Values of f(dp), protein C-13, and energy C-13 were used in stepwise regressions that estimated the metabolic routing of dietary protein and the C-13 enrichment of tissues relative to dietary protein. ResultsThe C-13 enrichment of rat collagen and mouse muscle relative to dietary protein depended on log(e) f(dp) and the C-13 enrichment of dietary protein on energy: The coefficients of log(e) f(dp) reflected the higher proportion of non-essential amino acids in collagen (79%) than in muscle (61%). In rats, dietary protein contributed from 66% (liver) to 85% (hair) whereas, in mice, dietary protein contributed from 76% (liver) to 85% (body protein). In pigs, collagen derived primarily (83 +/- 5%) from dietary protein. ConclusionsThese equations provided analytical frameworks to interpret dietary patterns from isotopic measurements. Metabolic routing and C-13 enrichment varied with f(dp) and with the tissue amino acid composition. Thus, f(dp) and tissue amino acid compositions were key parameters to assess dietary sources from isotopic values. Incorporating these insights will improve our ability to interpret the isotopic paleorecord and modern diet studies. Copyright (c) 2017 John Wiley & Sons, Ltd.