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Justus von Liebig (1842) introduced soil fertilization with nitrogen and improved the importation of Chile saltpetre, which contained iodine up to 1 g/kg and increased the iodine content of the food. World War I stopped nitrogen fertilization in Germany, resulting in iodine deficiency. After World War II, East Germany imported fish meal from Peru, which satisfies the iodine requirement of farm animals and - via milk and eggs - that of man. After the end of fish meal importation, iodine deficiency reappeared. The iodine contents of most rocks vary from 0.01 - 0.50 mg/kg in the ultramafic rocks to 2 - 6 mg/kg in shales. Worldwide, the iodine concentration in soils varies between < 0.1 and 10 mg I/kg dry matter (DM), with an average of 2.8 mg/kg DM. Ocean water is rich in iodine, which gets back to the continents with the rain as diiodine methane, 8-10 µg/L near the coast and < 1 µg/L in the European Alps. The vegetation of the Holocene riverside soils is relatively rich in iodine, whereas that growing on Triassic sediments is extremely iodine-poor. Iodine is delivered to the food chain of animals and man through contamination of feeds and foods. The iodine concentration of the flora decreases with increasing age of the annual plants. Flowers, fruits, seeds, pulps and all starch- and sugar-rich parts of plants are poor in iodine. All baked goods contain low iodine amounts.
Iodine is very quickly and almost completely absorbed from the skin and intestines. It is trapped and concentrated in the thyroid and converted there into organic iodine. During intensive follicle growth of laying hens (birds), the ovaries collect iodine same as the thyroid gland, it enters the placenta where it is incorporated in the fetus, and after birth iodine is accumulated in the milk. Thyroid, milk, egg, hair and blood vessels reflect the iodine status of animals (and man) best. Symptoms of iodine deficiency in ruminants, mono gastric species (and man) are considerably similar. A diet with < 100 µg I/kg food dry matter significantly lowered the feed intake of goats, depressed the growth rate, the success of first insemination and the conception rate, and significantly increased the abortion rate, the days of pregnancy and the mortality of the kids and their mothers. The weight of the thyroid increased 400 fold in the kids and < 250 fold in the adults. The thyroids of kids and mothers, blood serum, biestings, milk and hair decreased to a quarter or third of the normal iodine amount. All organs significantly reflect the iodine status of farm animals (milk, blood serum, thyroid, hair). After intrauterine iodine depletion, hairlessness is common. Adults suffer from infertility, and semen quality is low. Glucosinolates intensify the iodine deficiency symptoms same as selenium and zinc deficiency. Iodine overload exists in all species of farm animals (feed intake lowered, growth depression, sexual development slow-down, reduced ovulation and egg production, apathetic reactions, depressed immune status, increased thyroid). The toxic limits are > 5 mg I/kg feed DM in horses, > 50 mg in calves and sheeps, > 150 mg in chicken and 350 mg/kg feed DM in hens and pigs.
The urinary and faecal strontium outputs have been measured in healthy German adults all consuming a normal mixed diet. The results of this investigation showed that the faecal excretion (86%) is six times higher than the urinary (14%) for both men and women. The bioavailability of Sr from daily nutrition was found to be about 20%. The strontium balance was slightly positive for both sexes. Women excreted 2.1 and men 2.8 mg Sr per day, whereas the alimentary intakes were 2.4 and 3.0 mg Sr per day, respectively. All values were in good agreement with data from the literature.
The iodine intake by 7 ad least respectively omnivorous and ovolactovegetarian women and men was examined in 21 test teams in Central Europe and Mexico by the duplicate portion technique over 7 successive days. Women in Germany presently consume > lOOµg, men > 120µg I/day on average, or 1.3 and 1.5 µg/kg body weight/day respectively. In Mexico, iodine intakes by women and men amount to 150 and 200 µg I/day, or 2.5 and 2.6 µg/kg body weight and day, respectively. Although, on average, the normative iodine requirement of adults is met (1 µg, recommendation 2 µg/kg body weight and day), 25% of German adults take in less than 100 and 120 µg I/day, resp. Iodine consumption in summer is 40% lower than in winter. It increases by 30% with rising age (20 - 69 years) and by 45% with increasing body weight. In the endemic iodine- and selenium-deficient region of Central Europe, where iodine intake and serum T4 status were normalized by iodinated salt, it was not possible to normalize the free T3 level in the blood serum. The GSH-Px levels of women’s blood serum (170 U/L) were normal. Supplementation of 50 µg Se and 100 µg I/day normalized the serum-free T3 level. A normalization of iodine metabolism is only given if both the iodine and selenium requirements are met. An increase in the incidence of hyperthyroidism worldwide has been described after iodine supplementation in iodine deficiency regions. The intake by adults is limited to 7 µg/kg body weight. The main excretory routes for iodine are the kidneys and the breast, which compete with the thyroid for plasma iodine. Urine and milk are good indicators of the iodine status and intake, if fecal iodine excretion of humans (about 10 - 80 µg/day) is taken into consideration. On average, omnivores and vegetarians excrete 76 to 87% of the consumed iodine renally and 15 to 24% fecally. Their apparent iodine absorption rate is 83%, on average, and their iodine balance is 2 to 27%. Breastfeeding women without iodine-supplementation excrete 48% of their iodine intake with the milk, 45% with urine and 7% with feces. After iodine supplementation of the mineral mixtures fed to animals and of salt for humans, iodine concentrations in cows and breast milk increased from 10 µg/L to > 100 µg/L. At the start of the new millennium, the annual world production of iodine amounted to 19,600 t, which is partly used for iodine supplementation of animals and man.
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