A CENTURY AGO, much of northern America was known as the goitre belt. In some regions, anywhere from 26% to 70% of school-aged children had the characteristic neck swellings, or goitres, caused by an enlargement of the thyroid gland. After trials showed that supplements of iodine could prevent goitre, iodized salt was rolled out commercially in 1924, starting in grocery stores in Michigan. By the 1940s, the goitre belt had vanished.

The beneficial effects of iodine are not purely cosmetic. Without enough iodine, the thyroid is unable to produce hormones which regulate the body’s metabolism and are crucial for brain development. This can result in metabolic disorders, delayed development and cognitive impairment. For growing children and pregnant women especially, iodine deficiency can have lasting consequences. In today’s America changing diets, a lack of public health education, and complacency mean that iodine deficiency may once again be on the rise.

These problems are long-simmering ones. In the 1970s the median concentration of iodine found in Americans’ urine exceeded 300 micrograms per litre (µg/L), well above the minimum recommended threshold of 100 µg/L. “Nobody worried about it again for another 20 years,” says Elizabeth Pearce, an endocrinologist at Boston University. By the 1990s, however, those numbers had dropped by more than half. A study published in 2022 suggests that concentrations have fallen further still in the intervening decades. Between 2017 and 2020, say its authors, the figure in adults was just 116 µg/L.

Why the decline? In part, says Dr Pearce, because salt iodization was never federally mandated. This has had several knock-on effects. For one, only about half of American table salt (which makes up 11% of the salt Americans consume today) is actually iodized. Faddish alternatives, like sea salt or pink Himalayan, tend not to be. More importantly, the salt used in processed foods—which accounts for a dominant and ever-increasing share of American salt consumption—is also iodine-free.

Changing salt consumption is not the only dietary trend at play. Decreasing demand for meat and fish, both good natural sources of iodine, is also having an effect. According to a study published in JDS Communications in February, one cup of cow’s milk, which is often supplemented with iodine, provides about half the daily intake needed for adult women. Increasingly popular alternatives to dairy, such as oat milk and soy milk, by contrast, typically offer no such benefits: a study published in 2022 in the British Journal of Nutrition found that less than a third of dairy alternatives are fortified with iodine.

Even diehard milk drinkers may not consistently feel the benefits. The iodine in milk is not added for human health, says Michael Zimmermann, a professor of nutrition at Oxford University, but for cow’s health. This means that the amount of iodine in milk products is not regulated, and can vary widely based on the season, the cow’s diet and even the sanitation practices of any particular farm.

Certain cohorts are especially at risk. Pregnant women, for example, require 50% more iodine than other women to provide enough iodine for their babies. Iodine deficiency during pregnancy can result in miscarriage, stillbirth and stunted cognitive and physical development of the fetus. It remains the leading cause worldwide of preventable intellectual disability. It is troubling, therefore, that a pair of studies on American women published since 2021 found that one-quarter of pregnant women—and about half of non-pregnant women of reproductive age—do not meet minimum iodine thresholds. Whereas current deficiencies are likely to be too small to be detectable, a continued decline could pose a risk to public health.

This problem is not exclusive to America. In a report taking stock of iodine intake in Europe, published in June, the World Health Organisation (WHO) sounded the alarm about deficiencies resulting from changing diets. According to Dr Pearce, these are particularly widespread in pregnant women. As in America, she says, dairy has become a key source of iodine: only 40% of European countries have mandatory salt-iodization policies. Deficiencies have risen as Europeans consume fewer dairy products.

Overall, the WHO estimates that iodine deficiency—one of the world’s most common nutritional gaps—affects around 2bn people worldwide. Most live in environments with low natural iodine levels (such as in parts of sub-Saharan Africa and South-East Asia), or in countries without universal access to iodised salt.

The problem is fixable at a modest cost. Mandating, rather than encouraging, salt iodization, adding iodine to prenatal vitamins or fortifying dairy alternatives could all help reduce deficiencies. America’s goitre belt has been thrown off before similar approaches could stop it from returning. ■