Encyclopedia of Food & Culture | Iodine

IODINE. Iodine is an essential dietary element necessary for normal development and function of all vertebrates. Its sole physiological function is as a constituent of the thyroid hormones, thyroxine and triiodothyronine. It is removed from the blood by the thyroid gland for storage in organic form where it is found as iodinated amino acids in peptide linkage in thyroglobulin, a highmolecular weight protein.

Iodine is widely but usually sparsely distributed in nature, so that in vast areas of the world the supply in customary diets is marginal or insufficient. It has been estimated that over two billion persons are at risk of disorders attributable to iodine deficiency. Among these disorders are goiter, impaired intellectual function, growth retardation, reduced fecundity, lowered work capacity, increased rates of fetal loss and infant mortality, deafness, and in extreme instances a well-defined but somewhat varied constellation of physical findings collectively known as cretinism. Cretins are recognized by severe mental deficiency, disturbances in gait, impaired or absent hearing, and other neurological defects, but the signs and symptoms in these individuals may be subtle. These features merge with those of the less impaired members of the same community or nearby countryside where they may appear in lesser severity.

The iodine content of edible plants is largely dependent on the iodine content of the soil on which they are grown. The iodine content of foods of animal origin depends on the iodine in their food. Iodine is concentrated in milk, and is found in relatively high concentration in sea fish, who are at the upper levels of the food chain that contains algae. Some sea fish concentrate iodine from sea water. The only structure among the vertebrates that contains a significant amount of iodine is the thyroid gland.

Role of Iodine in Disease

For centuries the disorders arising from iodine deficiency have been recognized in well-defined regions. These have been called "goiter belts." Switzerland was included in the goiter belt until the iodine deficiency in that country was corrected in the first half of the twentieth century. Until recent years iodine deficiency was a recognized disorder in the United States, especially the Midwest and West, where goiter was commonplace. Iodine deficiency has been a major public health problem in the Andean region and eastward, in large areas of central and north Africa, in the Middle Eastern countries, in India, and in eastern and central Europe, and even today in localized regions of western Europe. Fortunately, remarkable headway has been made in elimination of iodine deficiency through various methods of supplementing diets.

Goiter is only one of the many consequences of iodine deficiency, and is relatively trivial when compared with the damaging effects of iodine deficiency on the nervous system. From the human point of view, it is more correct to speak of "endemic mental deficiency" than "endemic goiter."

Endemic thyroid disease has traditionally been considered a feature of iodine deficiency in the mountainous regions of the world. Endemic thyroid disease is found in regions of high elevation, but has also been common where glacial run-offs occur and in floodplains where there has been chronic leaching of the soil. Such geographic regions include the Gangetic plain and much of India and southeastern Asia, the Himalayan region, and central Africa, where the iodine deficiency disorders are frequent and severe; the coastal regions of western Europe are marginally iodine deficient. Endemic iodine deficiency can be detected almost anywhere with currently available sensitive techniques. In the United States until recently the mean intake of iodine was excessive, but recently has been rapidly falling into a normal range. The recent precipitous fall in iodine consumption in the United States has led to concern that iodine deficiency may again become a problem if the present rate of decline continues. The need for monitoring iodine intake is apparent. This is customarily done by measuring the iodine content of urine from a fair sample of the population under observation.

The optimal daily adult iodine intake is about 150 μ g/day, about half that for children and infants. This figure rises to about 200 μ g during pregnancy, but under normal circumstances there is wide latitude in intake because of the ability of the normal thyroid system to compensate for varying levels of supply. The thyroid and pituitary through a feedback relationship provide a highly efficient regulatory system. If iodine intake falls below about 50 μ g/day the pituitary gland becomes stimulated to increase its iodine uptake and hormone production, and, if the iodine supply exceeds needs, the pituitary shuts down appropriately.

Iodine is readily absorbed by the stomach and upper gastrointestinal tract. Iodine in chemical combination is released in the gut and absorbed; it may be rapidly taken up by the thyroid gland or excreted in the urine. Only a small fraction appears in the stool. Exceptions occur when iodine is in chemical combination with such drugs used as radio-contrast agents and amiodarone, the widely used cardiac medication.

Iodine Deficiency and Disease

Certain chemical agents found in some foods interfere with the uptake or utilization of iodine by the thyroid. Among these are the cyanoglycosides found in cassava (manioc), a component of millet, and a variety of chemical agents and some unidentified substances found in the effluent water from rock formations and in factory discharges. It must be stressed that the inhibitory effect of these substances may be bypassed if there is an ample supply of iodine in the diet, but their effect may be critical if the iodine intake is marginal or lower.

When marginal or low iodine intake is identified in a geographic regions such as a district or country, an effort should be made to correct the deficiency. A variety of techniques have been employed. These include distribution of iodine solution to school attendees, candies containing potassium iodide, addition of iodine to drinking water, and the use of canisters containing iodine that is slowly released into sources of drinking water. None of these methods has proved to be widely accepted. In addition, it should be stressed that the primary target for the prevention of neurological damage due to iodine deficiency is the pregnant and nursing mother.

Prevention of Iodine Deficiency

The most effective and widely employed method for correcting iodine deficiency is salt iodization. The technique is simple, inexpensive, and effective. Potassium iodate rather than iodide salts is used because it is more stable when mixed with salt. Nevertheless certain problems must be corrected. Unscrupulous traders may sharply increase the cost of iodized salt to the consumer. If improperly stored the iodine may sublime and be lost from the salt. If addition of iodine by the manufacturer is not done carefully the salt may be overiodinated. In certain cases, especially those in which people have nodular goiters resulting from prolonged iodine deficiency, thyrotoxicosis may result, which may be subtle in onset and chronic, with unwanted or disastrous results. Careful and continued monitoring of dietary supplementation by iodized salt must be done, as with all food additives.

Promotion of salt iodization, especially in areas of particular need in the developing world, has been a health priority of many public and private agencies, including the World Health Organization, UNICEF, the International Council for Prevention of the Iodine Deficiency Disorders, and others. One of the principal problems with programs of salt iodization is that governments tend to lose interest, and the programs lapse, leading to recurrence of the iodine deficiency disorders. Again, constant monitoring is the key to continued success.

Injections of heavily iodinated poppyseed and other oils have been tried in mass campaigns, first in New Guinea; these methods have since been widely employed elsewhere. These are the same oils that have been widely used as radio-contrast agents. The results have been impressive. The iodine is slowly released from the oil and may be effective for two or more years. The oral route has also been used to administer the oils, but effectiveness is less prolonged. The disadvantages of programs using iodinated oil are principally cost and the requirement for sterile needles and trained personnel, which may be difficult to obtain in remote regions. Iodine-induced thyrotoxicosis may occur after administration of iodinated oil.

A unique and successful method of iodine distribution has recently been introduced. This method can be used in regions where iodine can be drip-added to irrigation water. It has been used in the desert regions of western China with salutary human benefit, and with a highly satisfactory effect on livestock production. The problems with this method are the need for skilled personnel to add the iodine to the irrigation system at the right time and rate, and the fact that it is only feasible when it is possible to add iodine to irrigation water. A somewhat similar technique that has proved beneficial is adding iodine to a municipal water supply. As with other methods of iodine supplementation, skilled maintenance of the program is essential, and the subsequent appearance of thyrotoxicosis is unknown.

Summary

Iodine is thinly distributed in the earth's crust, and much of the human population lives in regions that have marginal or insufficient iodine. Mountainous regions, flood-plains, and regions where there has been extensive leaching of iodine from the soil may not provide sufficient iodine for human needs. The result is the appearance of iodine deficiency disorders, which include neurological damage, goiter, increased fetal and infant mortality, deafness, and diminished human energy and resulting economic underproductivity. Iodine deficiency is a major public health problem for a large fraction of the world's population.

Wherever marginal or insufficient iodine exists, implementation of iodine supplementation is required. This may be done by supplementing table salt with iodine, administration of iodinated oil by injection or orally, or addition of iodine to the drinking water. It is essential that a monitoring system be in place to ensure that the population is receiving an adequate iodine intake. Care must be exercised to avoid an excess of iodine, which might induce thyrotoxicosis.

See also Body Composition; Fluoride; International Agencies; Malnutrition; Nutrition; Nutrition Transition: Worldwide Diet Change; Salt; Sodium; Trace Elements.

John Stanbury John T. Dunn