Author
 |
Fries, George |
|
Submitted to: Chlorinated Dioxins and Related Compounds Symposium Proceedings
Publication Type: Proceedings Publication Acceptance Date: 10/10/1994 Publication Date: N/A Citation: N/A Interpretive Summary: Evaluations of human exposures to dioxins and related compounds include food chain pathways that require estimates of residue accumulation in food animals. Most experimental measurements of residue accumulation are short-term and a method is required to extrapolate these results to the full production cycle. A pharmacokinetic model was devised to examine the effects of changing feed intake and fat pool size during growth on the predicted concentrations of residues in body fat. Pigs were used as the animal model because more adequate growth and composition data bases are available for this species, and there are fewer variations in the management and feeding of pigs during growth than other food animal species. The model covers the period from weaning through the typical slaughter weight for pigs. The validity of the model was verified by comparing model predictions with observed values in the literature. An interesting finding of the simulations is that the predicted concentrations peaked at about 100 days and then declined even though feed intake increased and concentration in the diet remained constant. This observation reflects the shift in energy partitioning from protein deposition to fat deposition as the animals age. The maximum predicted concentration in fat was about seven times the concentration in the diet. In addition to providing information on human exposure under normal conditions, the model is useful for predicting the effect of changing genotypes and feeding systems, which are being adopted to meet consumer demand for meat with less fat, on residue transmission to human diets. Technical Abstract: Assessments of contaminants in food chain pathways require estimates of bioconcentration factors (BCFs) in food producing animals, but few BCFs have been measured experimentally at steady state. A pharmacokinetic model was devised to examine effects of changing feed intake and fat pool size during growth on predicted concentrations of lipophilic residues in pigs. Major assumptions are that lipophilic compounds are distributed uniformly in a single body fat pool, and that accumulation and elimination are passive processes with rates proportional to concentrations. A function for ad libitum intake of typical swine diets was obtained from the literature, and growth and body composition curves were derived from data in a published serial slaughter experiment. Simulated concentrations in fat peaked at about 50 days of continuous intake, and then declined even though feed intake increased and concentration in the diet remained constant. This observation reflects the shift in energy deposition from protein to fat as animals age. The maximum predicted BFC was 7.7 with 85 percent absorption. The model was verified for residue depletion by finding that declines in concentrations of recalcitrant compounds were accounted for by dilution in the enlarged fat pool. Predicted values were consistently greater than observed values when the model was used to predict residue accumulation. This suggests that improvement is needed in selecting absorption coefficients and elimination rate constants. If the model is correct, energy restriction or changing genotypes to produce leaner meat would cause greater total accumulations and concentrations in fat from a given concentration of environmental contaminants in the feed. |
