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Iodine (I2) is a nonmetallic element belonging to the halogen family. Iodine is converted to iodide in the gut, efficiently absorbed in the digestive tract and carried into circulation by serum proteins. Most iodide is actively trapped by the thyroid gland where, as I2, it forms an essential component of the thyroid hormones, thyroxin (T4) and triiodothyronine. Thyroid hormones regulate cell activity and growth in virtually all tissues and are, therefore, essential for both normal embryonic and postnatal development.

Deficiencies: Iodine deficiency impairs growth and neurological development, which can damage the brain. Depending on its severity and stage of development at which it occurs, iodine deficiency can lead to a wide spectrum of health problems, ranging from mild intellectual impairment to severe mental retardation, growth stunting, apathy, and impaired movement, speech or hearing. Cretinism, in which many of these abnormalities occur, represents the extreme of early iodine deficiency. It is rare. Much more widespread is an intellectual blunting that may afflict as many as 50 million of the estimated 1.6 billion "at-risk" people living in iodine deficient regions, making iodine deficiency the most common preventable cause of mental retardation in the world. Because of decreased production of thyroid hormones, iodine deficiency causes compensatory hypertrophy of the thyroid gland as it attempts to make more thyroid hormone, resulting in a goiter - a disfiguring condition that is common in high risk areas. Collectively, health problems arising from a lack of iodine are known as iodine deficiency disorders (IDD).

Prevention and treatment: Universal salt iodization provides the most effective and affordable means to prevent IDD throughout the world; over 90 countries presently iodize their salt. Typically, the iodine concentration is from 30 to 100 g per g of salt, levels that take into consideration anticipated losses during transport and storage. At salt intakes of 5-20 g per day, this fortification range is sufficient to meet requirements in most countries. Iodization costs between 2-7 US cents per kg of salt. Other iodizable vehicles include water, grain and dairy products. Iodized oil supplements may be periodically administered in remote areas where salt iodization is not feasible. Damage to the fetus resulting from maternal iodine deficiency is irreversible. Most goiters can be cured or reduced in size with iodine therapy.

Diet recommendations: The Recommended Dietary Allowance for iodine in the United States is as follows: infants, 40-50 g; 1-3 yrs, 70 g; 4-6 yrs, 90 g; 7-10 yrs, 120 g; over 11 yrs, 150 g; pregnancy, 175 g; and lactation, 200 g. Globally, the per capita iodine requirement is 150-200 g per day.

Food sources: Iodine content of food and water depends primarily on the supply of iodine in the soil. Glaciated, mountainous or heavy rainfall areas are likely to be low in iodine placing human and animal populations at risk. Thus, individuals can not usually improve iodine intake by diversifying their diet. Some plants, including cabbage, Brussels sprouts, legumes, and cassava contain goitrogenous substances that interfere with iodine absorption. This is of public health concern only where these foods are regularly consumed in large amounts and dietary iodine is low.

Toxicity: A wide range of iodine intake is tolerated by individuals. For this reason, long term consumption of iodine through iodized salt is considered safe. Chronic, excessive iodine intakes may occasionally lead to goiter and hypothyroidism. A small increase in thyrotoxicosis (<0.1%) may occur from increasing the iodine intake of a population which has had a low intake for many generations.

Recent research: Epidemiologic advances continue: Indicators of population status are increasingly correlated with health outcome measures. In China, Indonesia, Ecuador, Switzerland and Zaire, goitrous communities, or populations with low urinary iodine excretion, have poorer cognition than non-goitrous ones, a condition that has been responsive (in terms of I.Q.) in some studies to iodized salt. New vehicles are being tested, such as irrigation water in China and sugar in Sudan. In Indonesia, supplementing infants with iodized oil reduced mortality < 4 mo of age.

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