Did the introduction of iodized salt raise IQs by 15 points in the U.S. Goiter Belt?

Did IQ scores go up for some men after the introduction of iodized salt in 1924?  Researchers analyzed data from World War I and World War II to try to find out.

Iodine is so important in the diet for normal brain development that infants from iodine-deficient mothers may be born mentally retarded. In the past, these unfortunate souls were labeled “cretins.”  In fact, iodine deficiency is the leading cause of preventable mental retardation in the world today.

A dietary deficiency of iodine can also cause a goiter, an enlargement of the thyroid gland that can lead to hyperthyroidism. Early in the 20th century, goiter was a serious health problem in the Great Lakes area and in the Northwest, called the Goiter Belt, where the soil is deficient in iodine. Some 4,000 potential recruits were rejected for military service in World War I because their goiters were too large to even button their shirts.  The prevalence of goiter in those areas dropped dramatically after the introduction of iodized salt in 1924.

Researchers James Feyrer of Dartmouth College and his colleagues at Brown University and in the UK wondered whether the introduction of iodized salt in the Goiter Belt may also have had an impact on the mental capabilities of people living there. But how could they possibly determine that? The U.S. military gave intelligence test to new recruits during World War I (before iodized salt) and World War II (after iodized salt). The scores on those tests were not available, but the researchers could estimate how well the individual men did because those who scored higher were assigned to the Air Force and those who scored lower to the Army.

From the military records, the researchers could also tell whether the individual men came from areas that were naturally rich or naturally poor in iodine. What they found was dramatic. Men from iodine-deficient areas were much more likely to be assigned to the Air Force during World War II than during World War I. The improvement in their performance on the intelligence tests from World War I to World War II was equivalent to an average increase of 15 points in their IQs!

But the introduction of iodized salt also had a tragic downside. Feyrer and his colleagues calculated that after iodized salt was introduced the death rate from hyperthyroidism doubled and tripled in iodine-deficient areas, resulting in an additional 10,000 deaths. Older people and women were the most susceptible. “We have found little discussion in the literature of what appears to be a short-term price the country paid for long-run benefits resulting from this public health intervention,” the researchers noted.

Source: James Feyrer, Dimitra Politi, and David N. Weil: The cognitive effects of micronutrient deficiency: evidence from salt iodization in the United States. National Bureau of Economic Research Working Paper 19233, 2013. (full-text)

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