Variability in the routing of dietary proteins and lipids to consumer tissues influences tissue-specific isotopic discrimination

RationaleThe eco-physiological mechanisms that govern the incorporation and routing of macronutrients from dietary sources into consumer tissues determine the efficacy of stable isotope analysis (SIA) for studying animal foraging ecology. We document how changes in the relative amounts of dietary proteins and lipids affect the metabolic routing of these macronutrients and the consequent effects on tissue-specific discrimination factors in domestic mice using SIA. We also examine the effects of dietary macromolecular content on a commonly used methodological approach: lipid extraction of potential food sources. MethodsWe used carbon (C-13) and nitrogen (N-15) isotopes to examine the routing of carbon from dietary proteins and lipids that were used by mice to biosynthesize hair, blood, muscle, and liver. Growing mice were fed one of four diet treatments in which the total dietary content of C-4-based lipids (C-13 = -14.5) and C-3-based proteins (C-13 = -27) varied inversely between 5% and 40%. ResultsThe C-13 values of mouse tissues increased by approximately 2-6 parts per thousand with increasing dietary lipid content. The difference in C-13 values between mouse tissues and bulk diet ranged from 0.1 +/- 1.5 parts per thousand to 2.3 +/- 0.6 parts per thousand for all diet treatments. The mean (+/- SD) difference between the C-13 values of mouse tissues and dietary protein varied systematically among tissues and ranged from 3.1 +/- 0.1 parts per thousand to 4.5 +/- 0.6 parts per thousand for low fat diets and from 5.4 +/- 0.4 parts per thousand to 10.5 +/- 7.3 parts per thousand for high fat diets. ConclusionsMice used some fraction of their dietary lipid carbon to synthesize tissue proteins, suggesting flexibility in the routing of dietary macromolecules to consumer tissues based on dietary macromolecular availability. Consequently, all constituent dietary macromolecules, not just protein, should be considered when determining the relationship between diets and consumer tissues using SIA. In addition, in cases where animals consume diets with high lipid contents, non lipid-extracted prey samples should be analyzed to estimate diets using SIA. Copyright (c) 2015 John Wiley & Sons, Ltd.