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 µg 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
µg; 1-3 yrs, 70 µg; 4-6 yrs, 90 µg; 7-10 yrs, 120 µg; over 11 yrs, 150 µg;
pregnancy, 175 µg; and lactation, 200 µg. Globally, the per capita iodine
requirement is 150-200 µg 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.
