Every mother knows how important nutrition is in pregnancy; specifically taking folic acid and vitamin-D supplements to ensure their growing baby gets the best start. Yet few have given much thought to iodine; but they should – it boosts brain development in the womb, and iodine deficiency can cause learning disabilities in the child.

Now evidence is building that suggests many women in the UK are iodine deficient and that low levels during pregnancy may put the unborn child at risk. In response, iodine experts are calling for government recommendations that pregnant and breastfeeding women take iodine supplements to make up for the deficiency. Here’s what they say about iodine deficiency and what needs to be done:

What is iodine and why is it important?

Iodine is essential for the production of thyroid hormones, which are important for healthy metabolism, growth, and development, according to the UK Iodine Group that promotes the importance of the mineral and aims to ensure that iodine deficiency is eradicated in the UK. Iodine is particularly important during pre-conception and the first 16 weeks of pregnancy to ensure the healthy development of the baby’s brain.

What can happen if someone is iodine deficient?

Goitre, known for causing a large swelling of the neck where the thyroid gland is located, is typical of severe iodine deficiency. Prolonged deficiency can cause thyroid hormone production to dip or dry up altogether. Mild iodine deficiency can also cause tiredness, weight gain, achy muscles, brittle nails and dry skin.

But the real problem is in pregnancy, so why is iodine so important at this time? Iodine is essential for building a healthy brain, skeleton and metabolism. During the first 14-16 weeks of pregnancy, a foetus is entirely dependent on the mother for its supply of thyroid hormone. If she is deficient in iodine and thyroid hormone both during pregnancy and during pre-conception, the baby risks mental impairment. Severe iodine deficiency can lead to the extreme disability known as cretinism.

Are women in the UK iodine deficient?

Yes – mild to moderately so, but this is enough to put the foetus at serious risk of brain abnormalities and low IQ. Professor John Lazarus, Chair of the UK Iodine Group and a specialist in hormone disorders, points out that there have been at least five recent surveys of pregnant women living across the UK showing a substantial amount of iodine deficiency in the first trimester of pregnancy. In 2011, another study also showed that two-thirds of 15-year-old girls in the UK were iodine deficient. The concern is that many of these girls will go on to become mothers.

What evidence is there that iodine deficiency in pregnancy is linked to low IQ?

A landmark study of 1,000 families in south-west England, published in The Lancet in 2013, showed that pregnant mothers who were deficient in iodine were more likely to have children with learning difficulties. Not only was iodine deficiency found to be more common than thought – found in 62% of the mothers enlisted in the survey – but their eight-year-old children were more likely to have IQ scores in the bottom 25%, according to Dr Sarah Bath, lead researcher from the University of Surrey. “This is an important public health issue,” she says. Other studies support these findings with impairment in the child worsening as a mother’s iodine levels drop.

Where can iodine be found in our diets?

Our bodies cannot make iodine so we need to source it from our diets, mainly from milk and dairy products, but also white fish which provides the richest source in the average British diet.

The UK Iodine Group argues, though, that it is not that simple. Ironically, organic milk, often considered by consumers to be “healthier” than non-organic, contains very little iodine because organic-reared cows feed on grass that is iodine-poor.

Also, say experts, winter milk has a higher iodine content than summer milk, due to cattle eating fortified feed, whereas in summer they just feed on grass. Some countries add iodine to table salt, and indeed the World Health Organization (WHO) recommends this. This is widely seen as the most effective way of raising a population’s iodine level, but the UK does not do this.

So has the UK population always been iodine deficient?

No. Around a century ago, goitre was so widespread in parts of the UK that in Derbyshire, it was nicknamed the Derbyshire neck. Then in the 1940s, dairy farmers started to use iodine-rich disinfectant and iodine-fortified cattle feed, making milk the primary dietary source of iodine.

So should pregnant women take iodine supplements?

Experts argue that in countries with iodine deficiency, such as in the UK, taking an iodine supplement is the most convenient way of consuming sufficient levels, especially during pregnancy. Sourcing it from the diet alone “would require a woman to eat unrealistically large amounts of fish”, says Professor Lazarus. The WHO recommends that if iodised salt is scarce, “vulnerable groups, pregnant and lactating women, should be supplemented with iodine”. The recommended levels are 250µg per day during pregnancy and breast-feeding, otherwise 120µg per day. Currently, there is no UK government recommendation.

Can we consume too much iodine?

Yes. The British Dietetic Association says excessive iodine intake can cause thyroid problems. Eating kelp or brown seaweed, which contains concentrated iodine, is discouraged.