Iodised salt has become uncool but many of us need to eat more iodine

When I was at uni, I had a biology lecturer who was obsessed with iodine, and whose life’s work had been tackling global dietary deficiencies. He urged us to always use iodised salt, telling us it had raised the IQ of whole nations and was one of the greatest public health inventions of all time. I still hear his voice in my head every time I’m in the salt section of the supermarket.

In recent years, however, I have found it increasingly difficult to even find iodised salt on the shelves. Over time, it has been crowded out by fancy-looking Cornish sea salt crystals, Himalayan pink rock salt, smoked salt flakes and Kosher salt. The few remaining containers of iodised salt come in drab packaging and look deeply uncool. This makes me wonder: are we about to undo all the benefits that have come from this unassuming food additive?

Iodine is an essential dietary mineral that the thyroid gland uses to make key hormones that regulate metabolism, growth, digestion, heart rate and body temperature.

Getting enough iodine is particularly important during pregnancy because thyroid hormones regulate fetal brain growth. Even mild to moderate deficiencies in utero have been estimated to reduce intelligence by 0.3 to 13 IQ points. Iodine is also important during childhood to support brain development and thyroid function. Case reports have described extremely picky eaters who are short for their age, struggle at school and are tired all the time because they are iodine-deficient. In children and adults, iodine deficiency can also lead to goitre – a swelling of the neck that occurs as the thyroid gland enlarges to try to capture more iodine.

Foods that are naturally rich in iodine include seaweed and seafood. Cow’s milk also contains iodine because it is often added to cattle feed, and iodine-based disinfectants are used to clean dairy cows’ teats and milking equipment. Fruit, vegetables and grains can absorb a small amount of iodine from the ground, but soil iodine levels vary greatly. Switzerland and Michigan, which was once part of North America’s “goitre belt”, both have very low soil iodine. Historically, they had high goitre rates, with up to 70 per cent of children in some Swiss towns affected.

In 1922, Switzerland was the first country to introduce iodised salt, which was made by adding a small amount of extra iodine to regular table salt. Within a short time, goitre had virtually disappeared, children became taller and they had an “injection of IQ”, as economist Dimitra Politi described it. That meant that more and more finished high school and went on to complete university degrees.

In 1924, Michigan also made iodised salt available, with other parts of the US and many other countries soon following. Its introduction has been credited as one factor driving the worldwide rise in IQ observed during the 20th century. Rarely has such a cheap invention had such extraordinary benefits. “For 5 cents per person per year, you can make the whole population smarter than before,” the late endocrinologist Gerald Burrow told The New York Times in 2006.

However, now that goitre is long forgotten, iodised salt is suffering a popularity crisis. For one, it cannot compete with the prettiness of pink Himalayan flakes. Some of the trendy non-iodised salts specifically advertise their lack of iodine additives, hinting that they may be somehow bad for you. I know parents who deliberately avoid giving iodised salt to their children because they are worried about chemical additives (even though iodine is a natural substance).

At the same time as people are using less iodised salt in home cooking, we are also eating more processed and takeaway foods, which are typically made with non-iodised salt to avoid unwanted reactions during processing. More people are going vegan or switching from cow’s milk to plant milk, reducing iodine intake further.

Because of these trends, a study published in November found that the proportion of Americans who aren’t getting enough iodine has doubled since 2001. Even more concerning, the study found that 46 per cent of pregnant women now have inadequate intakes.

The story is similar in the UK. The average iodine level measured in reproductive-aged women is “now considerably below the threshold for adequacy”, according to a study published in January. And in Australia, 62 per cent of pregnant and breastfeeding women have insufficient iodine levels. (Although it should be noted that some places, like parts of Japan, have the opposite problem of too much iodine consumption, which comes with its own thyroid problems).

This has led public health experts to urge people in the US, the UK and Australia to re-embrace iodised salt, to avoid harms to cognitive and thyroid health and the re-emergence of goitre.

Really, it is an odd time. The supplement industry is booming and people are loading up on zinc, selenium and ginkgo biloba pills to boost their brain health, even though there is slim evidence to support any benefits. In contrast, iodine supplements and salts are being overlooked despite many people having legitimate iodine deficiencies that carry real risks. I can’t wrap my head around it.

But fashionable or not, I’m going to keep rummaging around the supermarket shelves to find the iodised salt, still too scared of what my old lecturer would think if I went for the pretty pink flakes.