Influence of dietary iodine on the iodine content of pork and the distribution of the trace element in the body

Background Millions of people worldwide still suffer from iodine deficiency disorders. Besides salt iodination, iodine is added to animal feed to concentrate it in food of animal origin (milk, eggs, meat). Otherwise possible adverse effects of high supplementation should be avoided. Aim of the study: The objective of the study was to evaluate the iodine content of pork at various feed iodine concentrations to estimate its contribution to human iodine supply. Furthermore the handling of low and high iodine dosages by the organism should be investigated using the pig as a model for the human. Methods Seventy pigs (live weight period 27-115 kg), divided into five groups, were fed diets supplemented with 0 (group 1), 0.5 (group 2), 1 (group 3), 2 (group 4) and 5 (group 5) mg iodine per kg diet. Iodine was determined in the thyroid and in the fractions innards/blood, bones and muscle/fat of four pigs of each group by ICP-MS. Results Rising iodine supplementation of feed significantly increased (P < 0.05) the iodine content of the muscle/fat fraction [3.9 (group 1), 6.0 (group 2), 8.5 (group 3), 10.8 (group 4) and 17.1 (group 5) mu g I/kg]. Carry over (of supplemented iodine) into muscle/fat varied between 0.10 and 0.24%. The highest tested iodine dosage (5 mg I/kg diet) caused a 3.6-fold iodine concentration of the total body (calculated from the contents of the fractions), and a significantly increased thyroid weight compared to the group without supplementary iodine. Iodine supplementation increased iodine content in thyroid and bones significantly (P < 0.05) but not in innards/blood. On an average of the groups, the thyroid contained 80% of the body's iodine, innards/blood 14%, muscle/fat 5% and bones 1%. Conclusions: The iodine content of pork, and consequently its contribution to human iodine supply (similar to 1%), is very low, even at high supplementation of feed. The total body iodine content (empty body) is determined by the iodine intake. Irrespective of the iodine dosage, the thyroid contains about 4/5 of the body iodine. Bones represent a very low iodine concentration, even at a strongly increased iodine intake. The increase of the thyroid weight as an adverse effect of iodine supplementation requires further research with high dietary iodine