Growth and catabolism in isotopic incorporation: a new formulation and experimental data

Summary 1.  We propose a new model that uses an organism’s change in mass to estimate isotope incorporation rates. Our model is a re‐parameterization of a widely used one‐compartment‐first‐order kinetic model that estimates the rate of isotopic incorporation as a function of time. If animals are growing exponentially and fractional growth rates are measured, the time model allows partitioning the contribution of growth and catabolic turnover to isotopic incorporation. The model can be extended to include more than one‐compartment. 2.  Our model makes the same assumptions but uses mass change as an independent variable. It estimates the ratio of the contribution of catabolic turnover to growth. 3.  To test the model, we fed juvenile Nile tilapia ( Oreochromis niloticus ) at three different rations and measured their change in mass and tissue δ 13 C isotopic composition for 128 days. 4.  Fish grew exponentially and fractional growth rates differed significantly between rations. The contribution of catabolism (tissue replacement) to isotopic incorporation was 2·6 to 5 times higher than growth (tissue addition) in liver. In muscle tissue, the contribution of catabolism and growth were roughly equivalent for all rations. 5.  When we used the parameters of the mass model to re‐parameterize the time model we found a relatively good fit ( r 2  = 0·8 ± 0·1, mean ± SD) to the isotopic incorporation data. 6.  The mass model estimated a smaller, and more biologically realistic range of δ ∞ values than did the time model which suggests that it might yield better results when isotopic incorporation experiments are not carried on long enough to yield estimates of the asymptotic isotopic value of an animal’s tissues after a diet shift. 7.  Rather than being alternatives, the time and mass models are complementary in that they allow us to see the same phenomenon from different perspectives.