In 1916, a pathologist named Dr David Marine walked into a school board meeting in Ohio, and asked for permission to run an experiment on four and a half thousand schoolgirls.
But the school refused him.
Dr Marine wanted to give them iodine.
At the time, iodine was thought of as a poison. Obviously, you don't want to poison children.
But Marine thought that the poison could be a cure for goitre – a swollen thyroid at the front of the neck. He'd seen goitre in the dogs that he passed on his walk to the hospital.
And back in 1909, a trout hatchery had called him in about fish supposedly dying of thyroid cancer. It wasn't cancer. It was goitre. He put iodine in the water, and it vanished.
Now Marine wanted to test his theory on humans.
Little did he know that his experiment would trigger the largest rise in human intelligence anyone has ever engineered. It would take ninety years before anyone realised. And right now, quietly, we're running the same experiment in reverse, and the problem is getting worse.
Table of Contents
- The Experiment on 4,500 Schoolgirls
- Five Cents Per Person Per Year
- How America Quietly Lost Its Iodine
- Iodine and IQ
- Who Feels It First
- What I Do in the Clinic
- References
The Experiment on 4,500 Schoolgirls
Marine went back to the Ohio school board and managed to convince them to run his experiment, for two reasons.
The first was a local doctor called O.P. Kimball, who had sat in Marine's lectures on goitre and came away convinced.
The second was that he asked the school board if he could examine the necks of the schoolgirls first, to establish how bad goitre actually was in their town — and only then propose doing something about it.
When Marine and Kimball published their results, it convinced the school board to proceed with the iodine experiment [1].
The trial ran from 1917 to 1922. Marine and his colleague O.P. Kimball only included girls in their experiment, because goitre was twice as likely compared to the boys.
Every pupil from fifth grade up whose parents returned the permission slip got sodium iodide dissolved in water, a short course repeated every six months. The girls whose parents never replied became the comparison group.
Among the girls who started with normal thyroids and got nothing, more than a quarter developed a goitre.
But among the girls who got iodine: 0.2% [2].
Wild.
Five Cents Per Person Per Year
Now the challenge is to deliver iodine to everyone to help prevent goitre. But how do you do that?
The answer came out of Michigan, from a paediatrician named David Murray Cowie.
Michigan sat in the goitre belt — the band across the northern United States where ice-age glaciers had stripped the soil away, and with it the iodine. When the First World War draft boards examined young men from those counties, they often had swollen necks from goitre, caused by iodine deficiency.
Dr Cowie's move was to stop thinking like a doctor and start thinking like a supply chain.
Patients often forget to take pills. Particularly children. So instead of taking iodine pills, what if you mix it with salt?
Everybody buys it. It never spoils. Rich and poor eat about the same amount of salt.
Put the iodine in the salt, and the entire population doses itself, forever, for nothing.
So from 1924, American table salt carried potassium iodide [2].
Goitre collapsed. Switzerland and Italy ran their own school programmes and watched the same thing happen. Today, the worldwide cost of iodising salt runs at two to five cents per person per year. It is very possibly the best public health money our species has ever spent.
Especially when we consider the unintended benefit on human intelligence, which I'll explain later in this article.
But now, there's a huge problem that's only getting worse.
How America Quietly Lost Its Iodine
Salt iodisation in the United States has been voluntary from 1924 to this morning. It's never been made into a law. And it worked, because everyone at the time happened to keep doing it.
But by the second half of the century, Americans were still eating just as much salt, but the added iodine began to decline. Luckily, this wasn't a problem, because of dairy.
And this part is wild.
Cows get iodine in their feed, for their own thyroids.
But iodine is also an antiseptic. And dairy farmers used iodine-based sanitisers to clean the cows and the milking equipment. Some of it ends up in the milk [3].
So even though iodine from salt was declining, at a population level we were still ok because we were accidentally getting iodine from dairy.
And, also accidentally, from bread.
For decades, industrial bakers used iodate as a dough conditioner to make the dough easier to machine.
But these two accidental pillars of iodine intake slowly gave way.
Dairy farmers moved to other sanitisers.
Around 1980, bakers swapped iodate for bromate.
And between the early seventies and the early nineties, iodine levels in Americans fell by more than half. From about 320 down to about 145 [3].
And when the Australian dairy industry in the 1990s phased out its iodine sanitisers, within 5 years iodine deficiency was back [4].
Iodine and IQ
But it wasn't just goitre from iodine deficiency that was concerning.
In 2009, a randomized controlled trial of 184 schoolchildren in Dunedin, New Zealand was done to look at the effects of iodine on cognition. The schoolchildren in the iodine group's overall cognitive score came out 0.19 standard deviations higher, with clear improvements in perceptual reasoning [5].
And that raises the bigger question. Could iodine not only reduce the risk of goitre, but also reliably improve human intelligence?
So in 2013, a group of economists went digging through American military draft records.
The timing was perfect.
Remember, salt was iodised from 1924.
Eighteen years later, the young men born around that moment were lining up for the Second World War — and they did cognitive tests.
Those test results could be compared to the earlier generation of young men who fought in the First World War, who didn't have supplemental iodine in their salt. Who were likely iodine deficient.
Get this: the men born after iodised salt scored about one full standard deviation higher — roughly fifteen IQ points higher [6].
Now of course this isn't a perfect study, it's not a randomized controlled trial, but it fits everything else we now know.
Modern reviews link moderate-to-severe childhood iodine deficiency to around 12 to 13.5 lower IQ points.
And right now, we're becoming at risk of deficiency.
The World Health Organization is sounding the alarm: iodine deficiency is the most common preventable cause of intellectual disability on Earth [3].
Some countries like Australia and New Zealand are trying to deal with this, and in 2009 both countries made iodised salt mandatory in bread.
But in the USA, between 2001 and 2020, median iodine in adults continued to fall from 153 to 116. Remember, in the 1970s it was 320. In children, from 243 to 166 [7].
To be clear about where that leaves things: 116 is still above the sufficiency line of 100. America is not a deficient country. What it's lost is its buffer, and if this trend continues, then it will become deficient.
Who Feels It First
This is happening because America has lost its three pillars of iodine: iodised salt, and accidental iodine from dairy and bread.
Fancy salt that people buy: sea salt, kosher salt, pink Himalayan — the upgrades we've all seen the adverts for — carry essentially no iodine [3].
And processed foods that carry most of the salt that Americans actually eat: manufacturers almost always use the non-iodised kind.
If you're thinking "I eat fish and eggs, I'm covered" — less than you'd hope. An egg helps. Canned tuna carries about seven micrograms against the 150 you need in a day, and most fruit, veg and grains sit at one to three.
Who feels this first? Pregnancy. Requirements jump by half when a brain is being built, and the American pregnancy numbers have sat below the adequacy line in every national survey since the mid-2000s. Pregnant women who'd had no dairy in the previous day: a median of 100, against an adequacy range that starts at 150.
So how do you know if you're deficient? Urine spot tests don't work.
A spot urine test only describes populations; your own level swings with every meal. Even TSH, the standard thyroid blood test, can sit dead normal while intake is low [3].
What I Do in the Clinic
For pregnant women, in line with the clinical guidelines I suggest an iodine supplement of 150mcg, alongside folate supplements.
For adults and children, the fix is not to eat more salt that has iodine in it, unless a patient has medical issues such as POTS (postural orthostatic tachycardia syndrome).
Most people overconsume salt, and we know from a meta-analysis that combined 85 trials that when we reduce salt intake, we reduce blood pressure [8].
This was further confirmed by the salt-substitute studies, where a salt substitute reduced stroke risk by 14% during a 4.74-year follow-up period [9].
Yes, eating enough potassium is critical, but for most people we also want to reduce salt intake.
Most of us should also probably reduce bread intake, which again removes a source of iodine, as often breads have iodine added to them.
To me, this is where supplementation has a real part to play for human health. I've talked a lot on this channel about the importance of reaching the recommended daily intakes of vitamins and minerals without megadosing, and iodine is the same.
The recommended daily allowance for iodine is 150mcg, and a third-party tested, low-dose multivitamin from labdoor.com can be a reasonable place to start.
References
1. https://pubmed.ncbi.nlm.nih.gov/2405081/
2. https://jn.nutrition.org/article/S0022-3166(22)10117-3/fulltext
3. https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/
5. https://pubmed.ncbi.nlm.nih.gov/19726593/
6. https://academic.oup.com/jeea/article-abstract/15/2/355/2691480
7. https://www.liebertpub.com/doi/10.1089/thy.2022.0103
8. https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.120.050371
