“Consider a Spherical Cow ⋯” — on Modeling and Diet

Abstract

This paper explores how models may be developed to account for the relationship between the stable isotope composition of a body tissue of an organism and its diet. The main approach taken is to express this relationship as an explicit equation, of a "DIFF", and then to show how the values of such a DIFF can be evaluated from published experimental data. These values can be expected to have a much wider emaning that a simple encapsulation of a particular experimental design. As a main example, we show how the values may be used to construct a metabolic model in which the synthesis of non-essential amino acid for collagen construction can be treated. A second example is to show how the evaluation, in terms of diet, of the spacing between collagen and carbonate δ13C may be put on a rigorous basis. A second kind of model is briefly treated, based on isotopic mass balance arguments, and it is shown that large isotopic discrimination during methanogenesis in ruminants may account for data trends when comparing herbivores and carnivores. A third class of model is sketched at the elbel of biochemical flows, where some fundamental points are made concerning points were the isotopic composition of metabolites may be altered. The relevance of this to nitrogen isotopic enrichment is considered.