Ecology of Iodine Deficiency

There is a cycle of iodine in nature. Most of the iodine resides in the ocean. It was present during the primordial development of the earth, but large amounts were leached from the surface soil by glaciation, snow, or rain and were carried by wind, rivers, and floods into the sea. Iodine occurs in the deeper layers of the soil and is found in oil well and natural gas effluents, which are now a major source for the production of iodine.

The better known areas that are leached are the mountainous areas of the world. The most severely deficient soils are those of the European Alps, the Himalayas, the Andes, and the vast mountains of China. However, iodine deficiency is likely to occur to some extent in all elevated regions subject to glaciation and higher rainfall, with runoff into rivers. It has become clear that iodine deficiency also occurs in flooded river valleys, such as the Ganges in India, the Mekong in Vietnam, and the great river valleys of China.

Iodine occurs in soil and the sea as iodide. Iodide ions are oxidized by sunlight to elemental iodine,

Figure 1 The iodine cycle in nature. The atmosphere absorbs iodine from the sea, which then returns through rain and snow to mountainous regions. It is then carried by rivers to the lower hills and plains, eventually returning to the sea. High rainfall, snow, and flooding increase the loss of soil iodine, which has often been already denuded by past glaciation. This causes the low iodine content of food for man and animals. (Reproduced from Hetzel BS (1989) The Story of Iodine Deficiency: An international Challenge in Nutrition. Oxford: Oxford University Press.)

Figure 1 The iodine cycle in nature. The atmosphere absorbs iodine from the sea, which then returns through rain and snow to mountainous regions. It is then carried by rivers to the lower hills and plains, eventually returning to the sea. High rainfall, snow, and flooding increase the loss of soil iodine, which has often been already denuded by past glaciation. This causes the low iodine content of food for man and animals. (Reproduced from Hetzel BS (1989) The Story of Iodine Deficiency: An international Challenge in Nutrition. Oxford: Oxford University Press.)

which is volatile so that every year approximately 400,000 tons of iodine escapes from the surface of the sea. The concentration of iodide in the seawater is approximately 50-60 mg/l, and in the air it is approximately 0.7 mg/m3. The iodine in the atmosphere is returned to the soil by rain, which has a concentration of 1.8-8.5 mg/l. In this way, the cycle is completed (Figure 1).

However, the return of iodine is slow and the amount is small compared to the original loss of iodine, and subsequent repeated flooding ensures the continuity of iodine deficiency in the soil. Hence, no natural correction can take place and iodine deficiency persists in the soil indefinitely. All crops grown in these soils will be iodine deficient. The iodine content of plants grown in iodine-deficient soils may be as low as 10 mg/kg compared to 1 mg/kg dry weight in plants in a non-iodine-deficient soil.

As a result, human and animal populations that are totally dependent on food grown in such soil become iodine deficient. This accounts for the occurrence of severe iodine deficiency in vast populations in Asia that live within systems of subsistence agriculture in flooded river valleys (India, Bangladesh, Burma, Vietnam, and China).

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