Iodine: An Unfinished Story

In these days of high tech medicine it is easy to forget that some of the most effective and efficient health interventions are simple and cheap. One example is the addition of iodine to salt, an idea which began in the early 1900s with experimental trials in schoolchildren living in what was then known as the “goiter belt” of the USA. In that region surrounding the Great Lakes, many children developed enlarged thyroid glands called goiters.  A goiter is a sign of iodine deficiency.  So successful were the trials of iodine-supplemented diets that, by the 1930s, 90% of residents of the Great Lakes region used iodized salt and goiter rates in the region had plummeted.  Now, 70% of the world’s population uses iodized salt.

Iodine as an essential element

When iodine is in short supply, thyroid glands grow large in an attempt to harvest as much of the vital element as possible from the blood.  Iodine is necessary for making thyroid hormone and thyroid hormone is crucial for normal development and metabolism.  Pregnant women who have  low iodine levels and insufficient thyroid hormone often miscarry their babies or produce babies who are deaf, mentally-retarded  and stunted in growth.  In children and adults, iodine and thyroid hormone deficiencies cause fatigue, weight gain, lowered IQ levels, mental apathy and numerous metabolic abnormalities.  Regular intake of iodine is a simple preventive measure for a host of serious problems.

Unequal distribution

Iodine exists in an inorganic form in soil and water and makes its way into the plants and animals that we consume by combining with larger carbon-containing molecules.  In its inorganic form, iodine is a water-soluble salt which washes out of soil easily, especially in areas where the land is rocky and exposed. Where soil is iodine deficient, so are crops, unless supplemented with iodine containing fertilizers.  In contrast to its variable presence in soil, iodine is much more uniformly distributed in salt water seas.  Algae, kelp and other seawater plants, as well as saltwater fish and shell fish are the most reliable natural sources of dietary iodine, while iodine concentrations in land based plants depend on the amount of iodine in soil that supports them.  Terrestrial animals supply iodine proportional to the iodine in their food sources. Egg yolks are a good iodine source, because, like people, chickens develop goiters, and chicken feed is supplemented with iodine. Dairy products are also good sources. Cattle feed was originally supplemented with iodine to prevent hoof rot, and and because of the supplemented feed, iodine is secreted in the milk the cows produce.

Iodine and breast tissue

Milk contains iodine because mammary gland tissue, like thyroid gland tissue, accumulates iodine. The fact that iodine is found in human breast tissue, where it has no known function, has prompted studies of the element’s relationship to breast health.  Japanese women have low rates of breast cancer and fibrocystic breast disease compared to American women, and their regular iodine consumption via seaweed is high, perhaps 25x higher than the recommended daily iodine consumption in the US. Studies on the treatment of fibrocystic breast disease with iodine supplements have been promising but so far a direct relationship between breast disease and iodine consumption has not been proven.

Iodine supplementation?

Even if high dietary iodine content has something to do with low breast cancer rates among Japanese women, translating this information to attempts to prevent breast cancer is not a straightforward task. While it is clear that iodine supplementation prevents goiter, hypothyroidism and cognitive impairment, it is also clear that increasing iodine intake is not risk free, particularly in people who are accustomed to low levels of dietary iodine.  The thyroid gland, when faced with insufficient iodine in the blood, becomes a ruthless scavenger, extracting every last iodine molecule it can find. When iodine levels in the blood suddenly increase because of supplementary iodine intake, some thyroid glands will actually grow in size, pump out excessive thyroid hormone and even develop cancerous nodules. It may be that Japanese women can tolerate high amounts of iodine because it has never been in short supply for them. Caution and careful follow-up are always advisable when supplementing the diet with iodine in the form of tablets, drops or multivitamins.

Dietary iodine in the age of dietary angst

Obtaining enough iodine through the diet should be possible in almost all circumstances, especially because of the wisdom of public health policies regarding iodine.  Nevertheless, some eating trends in health in the closing decades of the 20thC have again raised public health concerns about iodine intake.   Assessments of body iodine content are made by measuring urinary iodine levels, since the body extracts as much iodine as it needs and excretes the rest in urine. But individual measurements are so variable that averages of all people tested are used to estimate the iodine status in a given geographic area.  Between 1971 and 2001, American iodine intake dropped dramatically then leveled off at half of the 1971 levels.

What happened over the last few decades?   Americans began getting much more of their salt in the form of the un-iodized salt in processed foods. Many people began avoiding salt altogether, some quite unnecessarily. Sea salt appeared on the grocery store shelves as part of the natural and organic food trends.   It is also possible that the 1971 levels of iodine consumption were artificially high. Studies in the 1970s showed that iodine-containing sanitizers were raising iodine levels in cows’ milk. Practices changed and milk iodine levels returned to normal.   Between the 1960s and 1980s, iodine was used in dough making and bread supplied 25% of the iodine consumed during that period.*  Perhaps the baseline measurements of iodine intake in the early 1970s were unnecessarily high. Perhaps intakes in 2001 and since  are adequate, at least to prevent goiters from developing. But the fact that Japanese people ingest far higher levels of iodine from whole food sources without ill effect suggests that we can tolerate more. Stay tuned.


*Note: Iodine in the Nuclear Age

In the wake of the atmospheric nuclear testing period, the government mandated the use of iodine containing oxidizing agents for dough conditioning in commercial baking. The iodine in the bread  competed  in the diet for uptake into the thyroid gland with radioactive iodine isotopes generated in  the wake of atmospheric nuclear testing. Saturating the thyroid gland with normal iodine is standard practice when radioactive iodine in the atmosphere  is a threat, as it was after the Chernobyl disaster. Taken within 8 hours after, or 48 hours prior to a nuclear disaster, iodine can prevent accumulation of radioactive iodine in the thyroid gland and thus prevent radiation damage to the gland. Pills to be taken in the event of a nuclear catastrophe are simply potassium iodide.

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