Iodine in table salt: How a public health victory is becoming a victim of its own success
Iodine deficiency can cause severe developmental delays
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Whether it's kosher, Himalayan pink or sea salt, Canadians have a wide range of choices when it comes to salts.
But what many of these don't have — or don't have much of — is iodine. The mineral is essential for the body to make thyroid hormones, but can be rare in many parts of the world. And researchers say it's time for Canadians to pay attention to how much iodine they're getting.
Table salt is one major source of iodine in Canada. In 1949, the government made it a mandatory additive. While Canadians can also get their iodine in by eating seafood (seawater is an abundant source of iodine) and dairy (farmers often use iodine-based antiseptics on udders), much of the soil in Canada is iodine-deficient — which means the crops and fruits that grow from our soils often are, too.
Not having enough iodine could lead to a variety of health conditions, like goitres — a large, usually benign swelling of the thyroid gland — which used to be quite common in parts of Canada. It's still commonly seen in inland countries in Asia and Africa that don't add iodine to the food.
Another, more serious, irreversible condition commonly caused by iodine deficiency is congenital iodine deficiency syndrome, also known as cretinism. It happens when a fetus doesn't get enough iodine while in the womb, and can cause stunted physical and mental growth.
It's become very rare in North America and the developed world. But Dr. Anna Banerji, a pediatrician at Unity Health Toronto, and chair of the North American Refugee Health Conference, says she recently saw a case of a 17-year-old who had moved to Canada from an iodine-poor country.
"She was about this tall," said Banerji, gesturing around three feet, "had the classical face of cretinism and was intellectually delayed — severe, severe intellectual deficits."
She says it's easy for Canadians to forget the importance of iodine because serious deficiencies like this are so rare, thanks to public health measures — like having iodine added to table salt. It's still in our salt, but Canadian dietary patterns have changed, she says, raising concerns of a comeback.
"We eliminated iodine deficiency, cretinism, goitre, and all that in general in many parts of the world, and we don't see it anymore, it's off the radar," she said.
"I think it's a [victim] of its own success," Banerji said. In Canada, she says, the problem could be coming back, as dietary habits have changed
No iodine in most processed foods
Iodine deficiency appears to be on the rise across North America, say researchers.
A 2022 study out of McMaster University in Hamilton found 11.9 per cent of the Canadians they monitored had a moderate to severe deficiency.
The authors, who looked at iodine levels collected from 800 participants in four clinical sites across Canada, say recent public health policies that call for people to reduce their sodium and dairy intake may also have inadvertently reduced iodine intake.
Canadians are also eating less seafood than you might think — almost two kilograms fewer than Americans per capita in 2021, according to data from the United Nations.
"We now have a resurgence, at least of mild to moderate deficiency," says Philip Britz-McKibbin, a chemistry professor at McMaster University and one of the authors of the study.
Mild to moderate iodine deficiency might not cause visible symptoms — like a goitre — but it still has an impact on health, says Britz-McKibbin.
"There's historical evidence of how iodine deficiency can have consequences in terms of cognition and development," he said.
Those development delays are most crucial for young children, as well as expecting mothers, said Britz-McKibbin.
Some analysis suggests an IQ increase of about 15 points in the general population, as reflected in standard military aptitude tests in parts of the U.S. that were most profoundly iodine-deficient, years after the element started being added to table salt, says Dr. Elizabeth Pearce, an endocrinologist at Boston Medical Center.
That's quite a substantial difference, says Pearce. For comparison, one study estimates a mean loss of 14 IQ points after a minor traumatic brain injury in a car accident.
"What we're ... concerned about, really, would be sort of a subtle decrease in IQ across the population."
Pearce says while most North Americans ingest more salt than would be good for their heart, they're not getting the iodine they need.
"Most of that salt is in these commercially processed foods and prepared foods, [and] that salt is typically not iodized," she said. In Canada, about 75 per cent of the food supply is packaged or processed.
"So it's really only the salt that could be added, you know, at the table or added in the kitchen while cooking that is a potential source of iodine."
Concern for women of reproductive age
Iodine deficiency among pregnant women is particularly concerning, because of how important the mineral is during fetal development. But some expecting mothers in Canada aren't getting enough of it, say Quebec researchers.
A new study, which looked at 500 pregnant women in that province, found the problem was particularly concerning in early pregnancy, when the median intake level of iodine fell below the recommended level in the first trimester: 136 micrograms per litre. The recommended range for pregnant women is between 150 to 249 micrograms per litre.
Many prenatal vitamins contain iodine, and most of the women in the study took them. But two-thirds of them had started taking them before getting pregnant, while one-third of the women only started them after. That appears to have made a difference. Iodine intake levels became normal in the second and third trimesters, researchers found in the study.
Pearce says in Canada, women of reproductive age should generally be paying special attention, even if they don't plan on having a baby anytime soon, because that's the population most likely not to be getting enough iodine.
"Women of reproductive age is the population group that's least likely to add salt to their food at the table, so it may not be the best way today of reaching that population," she said.
"We are seeing maybe a re-emergence of mild iodine deficiency across the board in women of reproductive age."
It's already happened in the U.S., says Pearce, among pregnant women.
"That group in the U.S., in the past 15 years has slid into what [the World Health Organization] would regard as mild iodine deficiency," she said.
And it remains important to keep an eye on intake levels at a population level, she says.
Vegans and vegetarians
Other groups that need to watch their iodine: Those with certain dietary restrictions
"Say, vegetarians, perhaps they might be more concerned to ensure they have adequate iodine in their diet, especially if they refrain from milk or seafood, which are natural sources of iodine," said Britz-McKibbin.
It's also important to keep in mind factors that reduce the amount of iodine your body can process and use. Smoking cigarettes, for example, reduces iodine uptake, he said.
Ultimately, this is a problem with an easy fix, say researchers. People just need to be reminded that the issue exists in the first place.
"This will persist if you live in a part of the world where there isn't enough iodine in soil, as long as people aren't paying attention to the problem."
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