Table salt in the United States was iodized in the early 1900s to ensure that all Americans consumed enough iodine to function properly. According to recent research, adding iodine to salt improved health and raised the average IQ. But how did iodize salt help improve IQ?
According to a 2017 study, the introduction of iodized salt in 1924 increased the IQ of one-quarter of the population that was deficient in iodine.
What is Iodized Salt?
Iodized salt is so common in the United States that you may not have given it a second thought. However, new research indicates that common iodine significantly impacted cognitive improvements across the American population in the twentieth century.
Iodine is a critical micronutrient in the human diet, something our bodies cannot synthesize. It must obtain from food, and it has been added to salt, as potassium iodide, since 1924. Initially, iodization was used to reduce the occurrence of goiter, which is an enlargement of the thyroid gland. However, subsequent research has revealed that iodine also plays an important role in brain development, particularly during pregnancy.
Iodine deficiency is now the leading cause of preventable mental retardation worldwide. Nearly one-third of the world’s population is estimated to have an iodine-deficient diet, and the problem isn’t limited to developing countries; perhaps one-fifth of those cases are in Europe (pdf), where iodized salt is still not the norm. (Source: Discover Magazine)
How was Iodine Added to Salt?
A group of economists identified a natural experiment: comparing the intelligence of children born before 1924, the year iodization began, and those born after. James Freyer, David Weil, and Dimitra Politi used military data from the early 1900s to the 1920s, when millions of men and women enlisted in World War II.
All recruits were required to take a standardized intelligence test as part of their enlistment. Researchers needed access to the test results, but they devised a clever substitute: smarter recruits were assigned to the Air Forces, while less bright recruits were assigned to the Ground Forces. The researchers could infer test scores based on which branch a recruit was chosen for.
Because iodine levels in soil and water vary significantly from place to place, intelligence data were paired with birthdate and hometown. The researchers used nationwide statistics on the prevalence of goiter collected after World War I to estimate which regions were naturally high in iodine and which were low.
Researchers had enough information on approximately 2 million male recruits born between 1921 and 1927. (Source: Discover Magazine)
Improvements Because of Iodine
The economists discovered that introducing iodized salt significantly impacted the lowest-iodine areas — the bottom quarter of the study population. Men born in these regions in 1924 or later were significantly more likely to join the Air Force and had an average IQ 15 points higher than their forefathers.
The researchers report in a paper for the National Bureau of Economic Research that iodization results in a 3.5-point increase in IQ across the country.
The initiative had drawbacks; for example, sudden iodine supplementation in deficient people can result in thyroid-related deaths. The researchers estimate that salt iodization caused 10,000 deaths in the decades following 1924.
On the plus side, iodine deficiency and its symptoms were virtually eliminated. And the mental benefits of iodine help explain the Flynn Effect, which observed that IQ rose about 3 points per decade in developed countries throughout the twentieth century. Improved health and nutrition are the driving forces behind the Flynn Effect. Iodine was responsible for roughly a decade of that remarkable rise. All the more reason for the rest of the world to follow suit and consign iodine deficiency to history. (Source: Discover Magazine)