Iodized salt and the public health bargain

Source: The easiest way to make the world smarter, Rabbit Hole, 29:56, uploaded 2026-03-18, playlist index 39.

The salt in the kitchen is often carrying a small public health system. Iodine is poisonous in its pure form, yet a tiny amount of it in table salt prevents the thyroid from failing when the local soil and water contain too little. Rabbit Hole follows that system from the discovery of iodine through the goiter epidemics of Switzerland and Michigan, then into the less visible effect that gives the video its title: better iodine supply appears to have raised cognitive scores across whole populations.

The arrangement is easy to miss because the dose is so small. In the United States, only 53% of table salt is iodized, and the video says that the country’s iodine levels are falling. The practical lesson is that a cheap, routine measure can disappear from daily life before anyone remembers what it was doing.

A poisonous element in a useful dose

Iodine is element 53. It turns starch blue or black, which is why iodine appears in counterfeit-money pens, and a concentrated solution is poisonous and reactive. In its common charged form, iodide, it remains strong enough to serve as an antiseptic. The video demonstrates an emergency water treatment from the US EPA’s instructions: five drops per litre, a thirty-minute wait, and a drink that looks and tastes worse than the water it replaced.

The useful amount is tiny. The ocean contains an estimated 90 billion tons of iodine, yet its concentration there is only 0.06 parts per million because the ocean contains so much water. More than half the world’s iodine comes from caliche, mineral layers extracted in Chile. A trace element can be both scarce in the diet and abundant in the wider environment.

Bernard Courtois discovered iodine in 1811 whilst making saltpetre from seaweed for gunpowder. He cleaned his seaweed containers with sulphuric acid, added too much one day, and watched purple fumes condense into dark crystals. The name comes from the Ancient Greek word ioeides, meaning violet-coloured. Doctors soon noticed that iodine could treat goiters, the large lumps that form when the thyroid grows.

Jean-François Coindet suspected that iodine was the active ingredient in a traditional remedy made from burnt sea sponges. Diluted iodine made the goiters disappear in some patients within eight days. Later work connected goiters with low iodine in soil and water, and found iodine inside the thyroid itself. The geography made the problem harder to explain: mountain villages, river valleys, and inland cities could be full of goiters whilst nearby regions had almost none.

The thyroid, the mother, and the child

The thyroid produces the hormones T3 and T4. Their names describe the number of iodine atoms in each molecule. These hormones set the pace for how the body uses energy and support the growth of tissues and organs. The pituitary gland regulates the thyroid through thyroid-stimulating hormone, or TSH. When the blood contains enough T3 and T4, the pituitary releases less TSH. When iodine is missing, the signal remains high, the thyroid keeps trying to extract iodine, and the gland grows into a goiter.

The stakes rise during pregnancy. A foetus does not have an active thyroid until about halfway through pregnancy, so its early supply of thyroid hormones comes from the mother. A severe lack of T3 and T4 can produce cretinism, which the video describes as physical stunting, muteness, deafness, and intellectual disability. Goiter is the visible symptom. The damage to development can remain hidden until much later.

Switzerland became the clearest historical case. Mark Twain’s joke about having seen “Mont Blanc and the goiter” rests on a real prevalence: Rabbit Hole reports that up to 94% of men in some remote areas had goiters and that as many as 10% of babies showed signs of cretinism. The family doctor Heinrich Hunziker proposed that the thyroid expanded because the environment left it short of iodine. His idea faced the ordinary objection to adding a poison to food. A national diet would have to deliver the same minute dose to everyone.

Salt supplied a practical answer. It is cheap, available throughout the year, and does not spoil. People need a baseline amount of sodium, whilst taste gives them a rough upper limit on how much they consume. The quantity of salt eaten is therefore predictable enough to carry a measured dose of iodine.

Otto Bayard tested the idea before anyone had built a national programme. Just before the winter of 1918, he travelled to a remote Swiss village, used a snow shovel to mix potassium iodide into salt, and gave it to five families. The train stopped running for the winter. When Bayard returned in spring, nobody had been harmed and the families’ goiters had disappeared. A larger trial with more than 1,000 people produced the same result over six months. In 1922, Hunziker, Bayard, and fourteen Swiss health leaders formed a commission that taught the country about iodine and introduced iodized salt. Within a year, it was sold in more than half of Switzerland.

Michigan and the first mass test

Michigan encountered the same problem through military recruitment. In 1917, more than 20,000 men in northern Michigan were rejected from service because of goiters. School surveys found goiter rates between 70% and 100% in some places, and the northern band across the United States became known as the Goiter Belt.

Dr David Cowie, chair of the Michigan Medical Society, heard about the Swiss salt experiment after the First World War. He spent two years organising almost 200 lectures for 26,000 people and persuaded Michigan’s salt producers that iodisation could change the state’s health without changing the price or taste of salt. Five producers put iodized salt on sale in May 1924.

Morton Salt held out. The company could not separate salt intended for Michigan from the rest of its production, although its size also meant that its decision mattered nationally. Iodized salt reached 90% of Michigan sales within months. Morton then offered it across the United States and announced the first nationwide iodized salt.

The results arrived before the mechanism was fully understood. Goiters disappeared from Switzerland within a few years. The national rate of deaf-mute births fell from one in 600 to one in 3,000, and Rabbit Hole reports zero documented cases of a Swiss child born with cretinism after 1930. In Michigan, iodized salt settled at 90% to 95% of sales without a legal mandate. Within ten years, fewer than 2% of children in the Detroit area had goiters, a fall of more than 90% from the peak.

The invisible gain in cognitive scores

The video presents the largest effect as one that nobody could see on a neck. A timeline accident created a useful comparison. Researchers studied two million men born before and after salt iodisation because the men were examined for the Second World War draft at about eighteen years old. In the 25% of the United States with the most iodine deficiency, recruits born after iodisation scored one standard deviation higher on military cognitive exams. Rabbit Hole translates that difference as 15 IQ points in the most deficient areas and estimates an average gain of 3.5 points for every American born during the period.

Multiplying that average across births from the start of US salt iodisation through the Second World War produces the video’s estimate of 180 million added IQ points. The estimate covers a defined twenty-year period and describes a population-level calculation. It cannot count points inside individual people. The video also names studies from China, Albania, and New Zealand that find a similar relationship between moderate iodine deficiency and weaker cognition. It presents iodine deficiency as the most preventable cause of intellectual disability and iodine sufficiency as one of the simplest ways to improve general intelligence.

The historical evidence supports a serious claim about population health. The 2013 study remains unnamed in the captions. Draft data produces an estimate rather than a direct count of IQ points, and the cognitive comparison comes from a period and population with a particular military testing system. Those limits belong beside the striking number.

A few nozzles at the end of the salt plant

Iodisation turns out to be a small processing step. Salt producers evaporate seawater in open-air ponds or vacuum pans, clean the salt until it is roughly 97% sodium chloride, crush it, and send it for packaging. Near the end, nozzles spray water and potassium iodide onto the grains at a concentration of about 0.01%. The iodine compound sits on the salt. The video says there is no chemical reaction despite claims that iodized salt behaves as though it were a different substance.

Colder countries tend to use potassium iodide, or KI. Warmer countries often use potassium iodate, KIO3, because it stays more stable in humidity. Rabbit Hole says KIO3 becomes KI during cooking or in the body, and that the United States only allows KI without a rigorous reason being clear to the presenter. Dextrose can stabilise KI. Calcium silicate and similar substances keep salt from forming a solid block. Morton’s Rain Girl and “when it rains, it pours” slogan come from that particular problem with humid weather.

The intervention spread because salt is cheap, ordinary, and difficult to replace during a crisis. The video reports that nearly 90% of the world’s population has access to iodized salt, that 128 of 196 countries legally require it, and that iodisation has prevented an estimated 720 million iodine-deficiency disorders in the past thirty years. The United States still has no general legal requirement. Its main trace of the old campaign is the declaration required by the FDA on salt containers.

The programme can still recede

The success has made the original problem easy to forget. Rabbit Hole reports that around one billion people still lack access to iodized salt, more than 200 million people still have goiters, and an estimated five million babies are born with iodine-deficiency disorders each year. Elizabeth Pearce, whom the video identifies as a leading iodine researcher, says funding and attention have fallen away in places where iodine deficiency is returning.

The source also corrects one of its own possible simplifications. A commonly repeated figure of two billion people with iodine deficiency is out of date. Maria Andersson, whom the presenter identifies as the lead author of the study behind the number, and Pearce both explain that iodine levels fluctuate too much within an individual for a single check-up to measure deficiency reliably. Researchers can use the median iodine level across a large group, such as pregnant women, because the group result smooths out individual fluctuations. That tells them whether the group is deficient. The group median leaves the distribution of values within the group unknown.

The video’s numbers therefore describe medians or visible deficiency symptoms. They cover a population pattern rather than a precise count of every person whose iodine intake falls below an ideal level. Mild deficiency alone cannot predict a goiter or cretinism, and the majority of people today have sufficient iodine. The practical conclusion is dietary rather than diagnostic: the video advises against trying to discover a personal iodine number through a routine test. It advises people to make sure their diet supplies enough iodine.

Where the iodine in food comes from

The argument that natural food makes iodized salt obsolete sounds plausible until the video follows iodine back through the environment. Rain carries small amounts of iodine into soil, and plants take it up. Inland regions, flooded ground, and places stripped by retreating glaciers can have very little. Switzerland and the American Midwest both lost deep soil layers during the ice age.

Food also concentrates iodine unevenly. Rabbit Hole gives a daily recommendation of at least 150 micrograms and compares foods per 100 grams. Fruits, vegetables, and grains usually contain one to three micrograms. Most meat contains less than five. Some white fish and shellfish contain much more, whilst fish as a category remains unreliable. Seaweed and iodized salt stand out at roughly 2,100 and 5,200 micrograms per 100 grams. A teaspoon of salt provides the intended dose, and a person would normally reach the point of eating too much salt before reaching an excessive iodine dose.

In the United States, dairy supplies almost half of dietary iodine. Some iodine enters cow feed, and iodine-based cleaners are used on cows and milking equipment. A little of that cleaner reaches the milk. Australia removed iodine-containing cleansers from its dairy industry after public concern about contaminants, and the video links the change to an iodine deficiency that appeared almost overnight. That route into the diet is useful and fragile at the same time.

US iodine levels have fallen by about 50% over the last fifty years as plant-based milks have become more common. The group with the lowest median iodine levels is Asian women aged 15 to 44. Pregnant and breastfeeding women need more iodine than other groups, and the video gives a US median for pregnant women that falls from 327 micrograms per litre in 1971 to 144 in 2014, which is deficient by the stated measure. People can avoid those risks through other foods, yet the alternatives depend on a diet that supplies the element consistently.

The taste allegation

Chefs often tell people that iodized salt tastes metallic or bitter. Rabbit Hole asks whether the taste difference survives a proper test. Gary Beauchamp, whom the video calls a leading salt and taste scientist, explains the value of a double-blind test. The presenter then asks America’s Test Kitchen to run a double triangle test, in which one round contains one iodized sample and two non-iodized samples, whilst the second reverses the arrangement.

Ten tasters take part. Three identify the odd sample in the first round and four identify it in the second. The combined result is about 35%, close to random chance. The test is small, and the participants sometimes noticed intensity differences without identifying which salt they had tasted. The presenter also reviews 34 studies of iodized salt in food. Most report no change. One mortadella study finds a small colour shift, one lemon flavouring becomes less sweet, and potassium iodate produces minor effects in some cases.

That result leaves room for iodine to affect taste in particular foods or at particular concentrations. It makes the stronger claim about universal metallic flavour hard to defend. The practical variables may be the shape and density of the crystals. America’s Test Kitchen shows that a teaspoon of table salt is equivalent to two teaspoons of Diamond Crystal kosher salt and one and a half teaspoons of Morton’s kosher salt. The chemical identity stays the same whilst the texture changes how much salt reaches the food and how evenly it spreads. For meat, the presenter recommends seasoning from above onto a dry surface. The method distributes the crystals more evenly and keeps them from sticking to wet fingers.

Keeping the ordinary measure

The video ends with a modest instruction. People who add salt at the table or in the kitchen should use iodized salt. Anyone planning a pregnancy, pregnant, or breastfeeding should take a daily supplement containing 150 micrograms of iodine, following the medical guidance stated in the video. For everyone else, the salt can carry the dose, and the video advises against testing an individual iodine level.

The arrangement works because it asks for almost no attention. That also makes it vulnerable to fashion, taste claims, processed-food manufacturers that avoid iodized salt, and the memory loss that follows a successful public health programme. The video treats iodized salt as a century-old intervention that still works, with current access and dietary changes making its continuation a live question.

Further reading / references

• Iodized Salt video notes, linked in the video’s description.
• America’s Test Kitchen, whose team runs the taste test in the video.