Iodine Excretion and Accumulation in Seaweed-Eating Sheep from Orkney, Scotland

Environmental Context. Iodine is an essential element of mammals and iodine deficiency of mammals has been recorded in more than 100 countries worldwide. Additionally, radioactive iodine is a major threat from nuclear fallout and so-called ‘dirty bombs’. Iodine supplementation is able to counteract deficiency and to reduce the potential for uptake of radioactive iodine. Seaweed, one of the best natural sources of iodine, has often been advocated for use as feed for livestock in order to increase the iodine concentration of our diets. The danger of excess iodine, however, has not been studied extensively. Here we investigate the bioavailability of iodine from brown kelps (Laminaria digitata and Laminaria hyperborea) and the adaptation of sheep to excess iodine intake.

Abstract. Iodine concentrations in tissue and urine samples of 11 seaweed-eating sheep from North Ronaldsay in Orkney, Scotland, were measured during a feeding experiment. Two groups of six sheep (control was kept on grass five months before experiment, while the trial group were caught at the beach) each ate 0.5 ± 0.1 kg dry mass of seaweed (Laminaria digitata and Laminaria hyperborea) each day in an 11-day feeding trial and had a body burden of around 2300 mg iodine daily, almost entirely as iodide. This iodine intake of 124 mg I kg bodyweight^–1 is more than 60 000 times higher than the recommended daily intake for humans. The iodine concentration in the urine within 4 h after the seaweed meal was 1295 ± 369 mg I kg^–1. Thus, more than 66% of the total iodine ingested by the sheep was excreted within 24 h. The iodine excretion of the control group was slightly slower (T_1/2 = 15.9 ± 7.2 h) than that of the trial group (T_1/2 = 9.1 ± 1.6 h). In the same experiment, the excretion of iodine was faster than that of arsenic (T_1/2 = 24.3 h). Tissue samples from feral North Ronaldsay sheep taken directly from the beach showed elevated iodine levels (liver: 2710 ± 505 μg I kg^–1 > kidney: 1827 ± 618 μg I kg^–1 > neck muscles: 404 ± 117 μg I kg^–1 based on fresh weight), pointing to insufficient homeostatic control of iodine uptake, so that iodine concentrations were 17 times higher than those of non-exposed sheep. A trend of continuous accumulation of iodine in the liver and kidney throughout the life of the sheep was observed. In contrast to arsenic, iodine was higher in the liver and kidney than in the muscle.