Iodine in Pregnancy and Children's Brains - Nutrition Matters

Historically, neurological iodine deficiency syndromes including ‘cretinism’, involving profound physical as well as mental disabilities, and ‘deaf-mutism’ were common in large parts of the UK, along with ‘goitre’, a marked swelling of the thyroid gland in response to lack of iodine. Inland regions like Derbyshire were among the worst affected, because fish and seafood are the richest natural sources of iodine. Goitre even became known as ‘Derbyshire neck’, as it was so common there. 

By the late 1960s, however, iodine deficiency in the UK had largely disappeared. Importantly, however, this was not because of any public health measures - such as iodising salt, or other basic foodstuffs, which almost all other developed countries had implemented. Instead, it was a purely accidental result of milk and dairy products having become good sources of iodine (owing to new dairy feed and hygiene practices) along with increases in milk consumption. Nonetheless, for several decades, it was simply assumed that iodine deficiency was now a thing of the past, and simply need not be considered by UK public health authorities.

Such complacency now looks misguided - because by early in the 20^th century, evidence was already emerging that iodine deficiency was common in at least some population subgroups, particularly in young women - and in pregnant women.

What is the impact of low iodine status?

What still wasn’t known, however, was whether even mild or moderate levels of iodine deficiency in mothers-to-be might impair their children’s brain development, until ten years ago, a pioneering study to investigate this issue showed that:

• Two-thirds of pregnant UK women had mild to moderate iodine deficiency during pregnancy; and
• This ‘sub-clinical’ level of iodine deficiency in pregnancy predicted significantly poorer verbal ability and reading in their children at 8-9 years of age

The researchers used data from over 1,000 women who took part in the ‘Children of the 90s’ birth cohort study; and they were able to control for numerous socioeconomic, psychological and biological factors known to affect children’s intelligence and cognitive development.

Although observational studies can never provide definitive evidence of causality, these findings therefore provide strong evidence that low maternal iodine status in pregnancy can impair childhood mental development. Since then, other studies have also reported similar findings. For example a similar study in Norway found

• children of mothers with low iodine intake in pregnancy had poor language development and fine motor skills, as well as more behavioural problems, when assessed at three years of age.

In 2015, independent researchers using conservative estimates calculated that it would be cost-efficient for all pregnant women to be given iodine supplements in countries like the UK with high levels of mild-moderate deficiency, because while for individuals, the impacts on intellectual ability and educational achievement may be subtle; at the population level, the overall economic and health consequences of early life iodine deficiency are likely to be significant and longlasting.

How much iodine is enough?

The W.H.O. recommended iodine intake for adults (>12 years) is 150 µg/day. During pregnancy and breastfeeding, they recommend 250 µg/day, to meet increased needs and ensure adequate thyroid hormone production for both the mother and the fetus. (EU and UK recommendations are the same as the W.H.O for adults, but lower for pregnancy and breastfeeding, at 200 µg/day).

The main dietary sources of iodine in the UK are milk and dairy, by far, because consumption of fish and seafood, although a very rich source, has fallen so low. And consumption of both these types of foods has been declining further in recent years as 'plant-based diets' have become more popular, particularly among young women.

Other food sources include eggs and meat, but most plant foods provide little or no useful contribution to iodine requirements, so vegans are at high risk of deficiency without supplementation. Seaweed is an exception here – especially brown seaweeds such as kelp, which are very high in iodine. However, excessive iodine intakes are harmful, so kelp and seaweed supplements are NOT recommended, as these are not regulated and can easily provide an excess.