Model building and model parameter identification for transfer of dioxin TEQs from feed to eggs.
van Eijkeren, J.C.H. (1), M.J. Zeilmaker (1), C.A. Kan (2), L.A.P. Hoogendoorn (3) and W.A. Traag (3)
(1) Centre for Substances and Integrated Risk Assessment, National Institute for Public Health and the Environment, Bilthoven, the Netherlands; (2) Animal Science Group, Wageningen University, Lelystad, the Netherlands; (3) Institute of Food Safety, Wageningen University, Wageningen, the Netherlands
Objectives: Residue levels of dioxin-like compounds (dioxins, furans, mono-ortho PCBs and non-ortho PCBs) expressed in ng of toxic equivalents (TEQs) per kg fat in eggs of notably biological farms tend to exceed the EU limit. For better understanding the transfer of dioxin TEQs from feed to eggs a carefully prepared experiment, exposing the hens to feed contaminated at different levels, was performed. To support legislative measures, a kinetic model, describing disposition of dioxin TEQs in hens and their eggs, was developed. Intended model application includes attuning limits in egg level and in feed level.
Methods: Based on physiological arguments, a classical two compartment model was developed. The one peripheral compartment contains the fatty tissue (abdominal, subcutaneous, inter and intra muscular fat, bone marrow, etc.), the central compartment contains all the other tissues. The model includes the absorption phase (fraction absorbed), the distribution phase (inter compartment transfer), metabolism (site assumed to reside in the central compartment) and excretion through egg yolk. Parameter identification analysis was performed to assess the possibility of identifying from the given data the two unknown model parameters for inter compartment transport, the rate of excretion through egg yolk, the rate of metabolism, the fraction of TEQs absorbed over the gut wall and the fat compartment weight.
Results: Identification analysis showed the structural unconditional identification of the two inter-compartment mass transfer rates. Excretion rate, metabolism rate, fraction absorbed and fat compartment weight could only be identified conditionally within a certain range. The model parameters were fitted to the experimental data consisting of TEQ residues in eggs and abdominal fat for five different levels of feed contamination. During fitting the metabolism rate was set to zero and so the maximum rate of excretion through egg yolk, the minimum fraction absorbed and the minimum fat compartment weight were found. Parameter ranges were obtained from setting the fraction absorbed to its maximum value of 1 and calculating the other extremes from the conditional expressions relating these parameter values. Thus, a very satisfactory model of the experimental data was obtained. The model showed also a successful description of the experimental data on the different group sums of TEQs, i.e. the sums for dioxins and furans, for mono-ortho PCBs and for non-ortho PCBs separately. Furthermore, the model could successfully be applied to data of a much higher level of exposure and with a quite different composition of dioxins and PCBs. Moreover, the model shows a mismatch of the EU-limit in egg residue level and the limit in feed contamination level.
Conclusion: Based on careful experimentation and model building a kinetic model for the transfer in laying hens of dioxin TEQs from feed to eggs was developed successfully. Identification analysis appeared to be an essential ingredient of model building.