Could a mother's lack of iodine harm her child's IQ?

Behind the Headlines

Wednesday May 22 2013

Iodine plays a role in the brain's development in the womb

“Mothers’ diets may harm IQs of two babies in three,” warns The Independent. The newspaper reports on its front page that iodine deficiency is widespread among pregnant women.

Iodine is recognised to play a role in the healthy development of the baby’s brain and nervous system while in the womb and the World Health Organization recommends that pregnant women eat iodine-rich foods.

Severe lack of iodine is one of the leading causes of brain damage in the developing world. But a new study, reported in most of the media today, suggests that even mild-to-moderate iodine deficiency during pregnancy may be associated with poorer cognitive function in the child.

In this large study, the iodine levels of pregnant women were measured, and their child’s IQ at age eight and reading ability at age nine were tested.

The researchers found that children of women who didn’t get enough iodine were more likely to be in the lowest quartile for verbal IQ, reading accuracy and reading comprehension. However, there was no significant difference in overall IQ.

A study of this kind has limitations, for example the fact that it relies on measurements being taken at a single point in time. Also, although the researchers adjusted for many factors that may have influenced the relationship (for example, parental lifestyle and socioeconomic factors), the study cannot prove a direct cause and effect relationship between a mother’s iodine consumption during pregnancy and her child’s cognitive ability. It is also not clear whether the differences seen in the children’s verbal and reading skills would translate into ‘real-world’ problems for these children.

Nevertheless, the study does highlight the need for pregnant women to get enough iodine during pregnancy.

Iodine – the facts

Iodine is a mineral that the body needs to produce thyroid hormones that help control the body’s growth and metabolism.

 

The World Health Organization recommends that pregnant and breastfeeding women consume 250 micrograms of iodine a day. Good dietary sources of iodine include oily fish and dairy products – particularly cows’ milk. For most pregnant women, eating two portions of fish a week and three portions of dairy products a day should be enough.

 

Where did the story come from?

The study was carried out by researchers from the University of Surrey and the University of Bristol. No specific funding was reported for the current study, but researchers were supported by the Waterloo Foundation, the Commission of the European Communities, the US National Oceanographic and Atmospheric Administration and Wassen International. The latter is a company that makes and sells iodine supplements. However, none of these organisations had any role in how the study was conducted or how the collected data was interpreted.

This study used information taken from a much larger ongoing cohort study known as the Avon Longitudinal Study of Parents and Children (ALSPAC), which is looking at the health outcomes of children born during the 1990s. The ALSPAC study is supported by the Medical Research Council, the Wellcome Trust and the University of Bristol.

The study was published in the peer-reviewed medical journal The Lancet.

The media reporting is generally representative of the study, although the Mail Online headline writers got into a serious muddle. When they first published the story they used the headline “Drinking organic milk in pregnancy is 'vital for the baby's future brain power'". This was then changed later in the day – "Drinking organic milk in pregnancy could be harming baby’s IQ".

Neither claim is supported by this study. The study did not assess women’s dietary iodine intake from different sources. So it is not possible to say how many women drank organic milk and whether those who did were more likely to be in the iodine deficient group.

 

What kind of research was this?

The researchers say that the World Health Organization considers iodine deficiency to be “the single most important preventable cause of brain damage” worldwide. Iodine has a role in regulating the thyroid gland, and thyroid hormones have a role in brain and nervous system development. 

The researchers say that changes to dairy farming after the 1930s increased the amount of iodine in milk in the UK. After this and due to the reduction in cases of goitre associated with thyroid problems in the UK it was considered that iodine intake in the UK was sufficient.

However, some more recent UK studies have suggested that mild iodine deficiency may be quite common among adolescent schoolgirls and pregnant women.

The current study used data collected from participants in the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort study to see whether there was an association between pregnancy iodine levels and child cognitive performance. The researchers speculated that women with lower iodine levels during pregnancy would have children with poorer cognitive outcomes.

 

What did the research involve?

The ALSPAC cohort was eligible to all pregnant women in southwest England with a due date between April 1991 and December 1992.

A total of 14,541 pregnant women were enrolled and 13,988 of their children survived for at least 12 months.

The researchers selected 1,040 women for whom they could measure iodine in the first trimester of pregnancy (up to 12 weeks) and their child’s IQ when they were eight years old.

Iodine was measured in a single urine sample. Urinary iodine levels are said to be a good indicator of iodine levels in the body as 90% of ingested iodine is excreted into the urine. However, the results would have been more accurate if the researchers had been able to measure iodine based on 24-hour urine collection.

To try to reduce the impact of this issue, the researchers looked at the iodine-to-creatinine ratio, which is said to be a good way to get a more accurate iodine measurement. The researchers defined adequate iodine as an iodine-creatinine ratio of 150 micrograms or more per litre. Iodine deficiency was sub-categorised as mild-to-moderate (50 to 150) or severe (less than 50).

Child IQ at the age of eight was assessed using a validated scale (the Wechsler Intelligence Scale for Children). At the age of nine psychologists also assessed children’s reading speed, accuracy and comprehension. 

The researchers looked at the association between pregnancy iodine status and IQ at the age of eight and reading at the age of nine. They adjusted analyses for a wide range of confounders including:

  • mother’s age
  • mother’s ‘parenting score’ (assessed by looking at cognitive stimulation of the baby, parental education and socioeconomic status)
  • home environment, including baby’s emotional and cognitive environment
  • family adversity
  • stressful events during pregnancy
  • infant birth weight and prematureness
  • breastfeeding history
  • maternal smoking and alcohol intake
  • other dietary factors during pregnancy, including intakes of omega-3 fatty acids and iron

 

What were the basic results?

The researchers found that, overall, the women in the study had an average (median) urinary iodine concentration of 91 micrograms per litre, and average iodine-to-creatinine ratio of 110 micrograms per litre. About two-thirds of women in the study (67%) were iodine deficient in pregnancy. None of the women was using an iodine supplement.

Compared with mothers with adequate pregnancy iodine, those with iodine deficiency were significantly younger and less educated, but had less exposure to stressful life events in pregnancy.

Compared with children of women with adequate pregnancy iodine levels and after adjustment for confounders, children of women with iodine deficiency were at significantly higher risk of:

  • having a verbal IQ score in the lowest quartile (odds ratio 1.58, 95% confidence interval (CI) 1.09 to 2.30)
  • having a reading accuracy score in the lowest quartile (odds ratio 1.69, 95% CI 1.15 to 2.49)
  • having a reading comprehension in the lowest quartile (odds ratio 1.54, 95% CI 1.06 to 2.23)

However, there was no significant association between pregnancy iodine deficiency and performance IQ or overall IQ score – only verbal IQ. There was also no significant association between iodine deficiency and reading score or number of words read per minute – only reading accuracy and comprehension.

 

How did the researchers interpret the results?

The researchers say that their results demonstrate the importance of having adequate iodine intake during early pregnancy. They say that the results “emphasise the risk that iodine deficiency can pose to the developing infant, even in a country classified as only mildly iodine deficient”. The researchers consider iodine deficiency during pregnancy to be an important public health issue that needs attention.

 

Conclusion

This is a valuable study that demonstrates that in this subsample of a large cohort of pregnant women in the UK, the majority had inadequate iodine levels during pregnancy.

They also found that this deficiency was associated with poorer verbal IQ in their children at the age of eight, and reading accuracy and comprehension at the age of nine.

The study benefits from its relatively large sample size, from the fact that it followed participants up over time and from the fact that it adjusted for extensive confounding factors.

However, there are some limitations to this study:

  • As the researchers say, several 24-hour urine collections would have been the ideal way to measure iodine levels, rather than a single measure, but this would be impractical in a large-scale study.
  • It would also be useful to continue to reassess the children’s IQ and reading performance at different time points, particularly as the associations were only found for certain measures of IQ and reading ability. Related to this, it is also unclear what impact these differences in verbal IQ and reading accuracy and comprehension would have had on the children’s learning and school performance. Children’s IQs are not thought to be fixed for life but can change over time.
  • Studies in other population samples from other countries would be valuable.

The researchers note that a randomised controlled trial assessing the effect of iodine supplementation in pregnant women on child cognitive ability in areas with mild-to-moderate iodine deficiency would be valuable. They say that they hope to run such a trial in the UK, as current evidence from trials in this area is weak. 

Overall, the study highlights the need for pregnant women to obtain sufficient iodine during pregnancy. The World Health Organization recommends that pregnant and breastfeeding women consume 250 micrograms of iodine a day.

Dietary sources of iodine include dairy products and fish. Pregnant or breastfeeding women who are unable or unwilling to eat these types of iodine-rich dietary sources may need supplements.

If you are pregnant or breastfeeding and are concerned about your iodine levels, speak to your GP or midwife before taking supplements. Supplements will not be suitable for every woman.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.

Analysis by Bazian

Edited by NHS Choices

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