A high dietary iodine increases thyroid iodine stores and iodine concentration in blood serum but has little effect on muscle iodine content in pigs

There is still iodine deficiency in many populations, which justifies efforts to increase this trace element in food such as milk, eggs and meat by fortifying compound animal feeds with extra iodine. The iodine requirement of growing pigs is in the order of 100-200 mu g/kg feed (as a supplement) and the effects of this dosage range or higher on pork iodine concentration should be determined including the action of relevant iodine antagonists in feed, e.g., rapeseed. In three experiments on a total of 208 pigs [Pietrain x (Landrace x Large White)] the iodine concentration of meat (m. longissimus) - 71 samples -, blood serum - 100 samples - and of the thyroid - 100 samples - was analysed by intracoupled plasma-MS. In Experiment 1, 4 x 10 pigs received diets without or with rapeseed cake (0 and 3.2 mmol glucosinolates/kg diet) either with 125 or with 250 mu g iodine/kg. In Experiment 2, the three groups with 46 pigs each were fed high iodine diets (1200 mu g supplementary iodine/kg) without or with 100 or 150 g solvent extracted rapeseed meal/kg diet (0; 0.8 and 1.2 mmol glucosinolates/kg). In Experiment 3, 3 x 10 pigs received either 600 mu g iodine/kg feed (1) or the 5-fold dosage (600 + 2400 mu g iodine/kg diet) administered 7 days (2) or 18 days (3) before slaughtering. The group means of pork iodine content were in the relatively small range from 3 to 16 mu g/kg, which contrasted to the enormously varying dosage range from 125 to 3000 mu g iodine/kg diet. There was a certain iodine dosage effect in Experiment 3 when - in comparison to the control - a 3-fold higher meat iodine concentration resulted from a 5-fold higher diet iodine concentration. In Experiment 1 with the low iodine offer, rapeseed cake with glucosinolates decreased the serum iodine level whereas in Experiment 2 this did not happen due to higher iodine fed and lower glucosinolates exposure. The thyroid iodine reflected the dietary iodine better than blood serum iodine and the serum better than muscle. However, in Experiment 2, 1200 mu g iodine/ kg diet produced only half the serum iodine concentration than half as much dietary iodine in Experiment 3 (600 mu g iodine/kg diet), which may result from rapid elimination of blood iodine and a higher urinary excretion by longer duration of feed withdrawal before blood sampling. The muscle of pigs has to be classified as a low iodine food. Thus, there are no possibilities to concentrate this trace element reproducibly in amounts relevant for human nutrition in pork. (c) 2005 Elsevier Ltd. All rights reserved.