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Monday, July 29, 2013

Childhood IQ and maternal iodine status

About a year ago, I blogged on the subject of iodine and its increasing importance in public health nutrition in developed countries[1]. Of course, iodine deficiency is one of the three elements of global hidden hunger. According to a letter to the Lancet from the WHO “urinary iodine has been collected for 92% of the world's population and globally, more than 1·9 billion individuals have inadequate iodine nutrition (defined as urinary iodine excretion <100 (μg/L), of whom 285 million are school-aged children.”[2] In developing countries, iodine deficiency primarily affects energy metabolism and reduces the capacity for physical work. More recently, the spotlight has also been shone on the developed world where the role of iodine in brain development is the main concern. Iodine is a component of the thyroid hormones, which play a central role in the brain development of the fetal infant. In a recent Lancet paper[3], the offspring of 1040 women who had spot urine analysed for iodine during her first trimester were studied. Only those children who had an IQ test at 8 years and a reading ability test at age 9 years were included. The study drew on a longitudinal health study of mothers and their offspring, the Avon Longitudinal Study of Parents and Children.[4] The iodine to creatinine ratio, a measure recommended by the WHO was used to determine maternal iodine status and these levels were classified above or below a cut off point recommended by the WHO (150 μg/g). The average urinary value was 91 μg/g, which indicated that this population had a mild-to-moderate level of iodine deficiency (Two thirds of the population were below the WHO cut-off). The problem with cut off values is that they are often, if not always, set by scientists working in the relevant field who tend (in my view) to always go for the highest value so that their pet problem is seen to be a really important issue. This isn’t skeptism on my part. It is cynicism.

The research output of this paper would make me retract my cynicism because what the authors found was a direct association between cognitive performance and maternal iodine status. Looking firstly at the mothers themselves, those below the 150 μg/g cut off tended to be younger and to have had less education than mothers above the cut off.  For the children of these mothers below the cut-off, followed up at age 8 years, the IQ values were significantly lower for total function and for verbal and performance function. At 9 years of age, their reading ability was also lower: words per minute, accuracy, comprehension and reading score.  Clearly, a child’s cognitive function could be influenced by a wide range of confounding factors and the authors accounted for a total of 21 possible confounding factors such as maternal age, life events, breastfeeding, alcohol and tobacco intake, the use of fish oil supplements in pregnancy, birth weight, maternal depression and so on. The inclusion of all these variables did not alter the conclusions.

Using a simple cut off can sometimes be a bit too simple because it wont show if a trend exists so the authors re-analysed the data into a continuous regression analysis rather than the dichotomous cut off approach. In general they found a linear positive relationship between maternal iodine status and subsequent childhood cognitive function, again after allowing for all manner of confounding variables.

These findings are really very important. Basically, soil levels determine dietary iodine levels and there are many regions of Europe, which were known in the past to be “Goitrogenic regions” by virtue of low soil iodine levels. All that vanished when dairy farmers used iodophors to clean milking byres. That has now vanished and we are back to reliance mainly on soil levels. As I wrote in my previous blog: “Ironically, the many pregnant women who shift to organic foods in the belief that this will help ensure as healthy a baby as possible, will see a very significant fall in iodine intake.  Organic animal production greatly restricts the use of mineral and vitamin supplements in animal feeds.  Recent survey of the iodine content of milk from organic and conventional farms shows that the organic milk is 42% lower in iodine than conventional milk, and milk accounts for almost half the UK iodine intake. In fact, pregnant women should be counselled to avoid organic milk”.
Now here is an interesting question. Which would you rather have if you had the utterly unthinkable choice between an overweight 8 year old or a cognitively impaired 8 year old. Childhood obesity is very important. But there are other equally important if not more important issues for childhood nutritional wellbeing. Methinks iodine is top of the future list.

[1] “Iodine, now a problem in developed countries July 23rd , 2012 -
[2]De Benoist et al (2003) Lancet, 362, (9398), 1859-1860
[3]  Bath SC et al (2013) , Lancet. 2013 May 21. E pub ahead of print

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