A Unique Child: Health - A guide to ... iodine deficiency

Severe iodine deficiency is a major cause of irreversible, yet preventable, brain damage, and a problem normally associated with the developing world. However, research is now finding that mild deficiency levels in pregnant women in the UK, and other developed countries, can impair their child's cognitive development.

Early years settings can help tackle the problem by promoting healthy eating habits in young children and their families and in alerting mothers to the importance of iodine in their diet before and during pregnancy.

What is iodine and why is it important in our diets?

Iodine is a naturally occurring mineral and an essential component of thyroid hormones. These are secreted into the blood and carried to every tissue in the body, so regulating the body's metabolism and keeping muscles, brain and other organs functioning normally. The body does not make iodine, so it is a vital part of our diet.

How prevalent is iodine deficiency in the UK?

Current data for the UK is limited, as the country, like many other developed nations, was believed to be replete in iodine. A 2011 study aimed to assess the position by analysing the iodine levels of 14- to 15-year-old girls in nine areas across the UK. Of the 737 girls tested, over two-thirds (68 per cent) were found to be deficient - half (51 per cent) mildly, 16 per cent moderately and 1 per cent severely.

In the study, published in The Lancet, the researchers conclude, 'Our findings suggest that the UK is iodine deficient ... (and) are of potential major public health importance.' 1

What are the effects of iodine deficiency?

Iodine deficiency during pregnancy can impair a baby's physical growth, cognitive development and motor function. Severe deficiency is associated with congenital abnormalities and irreversible mental retardation, as well as miscarriages, pre-term deliveries and stillbirths.

Longer term, iodine deficiency can lead to the development of thyroid disease, principally goitre (the enlargement of the thyroid) and hypothyroidism (when the thyroid becomes underactive).

The impact of low-level deficiency on the child and longer term is less well understood, although the body of research in this area is growing.

In May, The Lancet published a study analysing the iodine levels of 1,040 women from the Avon Longitudinal Study of Parents and Children (ALSPAC) during their first 12 weeks of pregnancy and their children's IQ levels at eight years old. Two-thirds (67 per cent) of the women were found to be iodine deficient, while their children were found to have a higher risk of having verbal IQ, reading accuracy and reading comprehension scores in the lowest quartile. 2

A Tasmanian study, also published in May, found similar links. Here, nine-year-olds whose mothers were mildly deficient in pregnancy were found to have reductions of 10 per cent in spelling, 7.6 per cent in grammar and 5.7 per cent in English literacy performance compared with children whose mothers had adequate levels of iodine when tested. 3

The links remained significant after taking account of biological factors, such as maternal age. The same was true of the ALSPAC study which took account of 19 biological, social and economic factors, such as stress during pregnancy and family adversity.

What are recommended daily intakes?

The NHS Health A-Z (www.nhs.uk/Conditions/vitamins-minerals/Pages/Iodine.aspx) states adults need 0.14mg (140mcg) of iodine a day.

The World Health Organization (WHO) recommends daily intakes of 90mcg of iodine for infants and children up to five years, 150mcg for adults and 250mcg for women during pregnancy and when breastfeeding.

What are the main sources of iodine?

Iodine is present naturally in soil, seawater and certain foods. The richest sources of iodine are fish, seafood, milk and other dairy products, although iodine is also found in meat, eggs, some grains and cereals, soy, sea salt and seaweed. Although not widely available in the UK, salt fortified with iodine (iodised salt) is stocked by some supermarkets.

There are no standard measurements of iodine in food as concentrations vary, depending on iodine levels in the soil in which plants are grown, the species of fish, the type of feed given to dairy cows and the season (dairy products have a higher concentration of iodine in winter). However, the British Dietetic Association Food Fact Sheet - Iodine does provide guidance (see table).

Why might the incidence of iodine deficiency be increasing in the UK?

Milk is the main source of iodine in the UK, so the main cause of rising deficiency levels is likely to be due to people drinking less milk. Also iodine levels in organic milk are 42 per cent lower than in regular milk and soya milk is rarely fortified with iodine so is not a substitute for cow's milk.

How should women of child-bearing age adjust their diet?

It is important to recognise that 50 per cent of pregnancies are unplanned and that the foetus is dependent on their mother's iodine levels at a time when most women don't even know they are pregnant - it is only after 12 weeks that the foetus produces its own thyroid hormones. Eating a balanced diet at all times is, therefore, essential for women of child-bearing age.

Women may wish to take a daily supplement of 150mcg three months before trying to conceive, once they are pregnant and when breastfeeding, with the remainder of their iodine requirement coming from their diet.

But equally important to recognise is that absorbing too much iodine can be as harmful as ingesting too little. Adults should never exceed 600mcg a day and pregnant women should avoid kelp or seaweed supplements, which are extremely high in iodine.

Should more be done to tackle the problem?

Researchers involved in the ALSPAC study conclude, 'Iodine deficiency in pregnant women in the UK should be treated as an important public health issue that needs attention.'

Dr Mark Vanderpump is consultant physician in Diabetes and Endocrinology at the Royal Free Hampstead NHS Trust in London and was involved in a study of deficiency levels in schoolgirls. He says, 'Both our study and the recent (ALSPAC) study by Dr Sarah Bath and colleagues suggest that the evidence of iodine deficiency in young UK women needs to be addressed.'

One response adopted by many countries over the past 80 years and viewed by WHO as a simple, cheap and universally effective solution is fortifying salt. For example, since adopting a policy of mandatory salt fortification in bread in 2009, New Zealand has seen the numbers of children estimated to have inadequate iodine intakes drop from 38 per cent to 4 per cent. 4 To date, the UK Government has been cautious about adopting this policy. Part of its reluctance may come from the public health agenda to cut people's salt intake.

A Public Health England spokesperson says, 'Most people should be able to get all the iodine they need by eating a varied and balanced diet. In the UK, good sources of iodine include cows milk and fish.

'Public Health England has no plans to promote the use of iodised salt, but this does need to be set in a wider context. Much of the effort in public health nutrition in the past decade has been put into reducing salt consumption by the population as a whole. There is good evidence that this will have a positive impact on health by reducing blood pressure, which is a risk factor in cardiovascular disease. The Scientific Advisory Committee on Nutrition is looking at the evidence on iodine at the moment, although no formal position statement has been published yet.'

However, Dr Vanderpump says, 'The two policies would not be incompatible as the concentration of iodine in salt can be adjusted according to the requirements and degree of iodine deficiency in a particular population. Ninety per cent of our salt intake is via processed foods and the evidence-base is that a mandatory salt iodisation programme proves extremely efficient and cost-effective. This is the current WHO recommendation.'

REFERENCES AND MORE INFORMATION

1 'Iodine status of UK schoolgirls: a cross-sectional survey' by Vanderpump MP, Lazarus JH, Smyth PP, Laurberg P, Holder RL, Boelaert K, Franklyn JA; British Thyroid Association UK Iodine Survey Group, June, 2011, www.thelancet.com

2 'Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC)' by Sarah C Bath PhD, Colin D Steer MSc, Prof Jean Golding FMedSci, Pauline Emmett PhD, Prof Margaret P Rayman DPhil of Surrey and Bristol Universities, May 2013, www.thelancet.com

3 'Mild iodine deficiency during pregnancy is associated with reduced educational outcomes in the offspring: nine-year follow-up of the gestational iodine cohort' by Hynes KL, Otahal P, Hay I, Burgess JR of the University of Tasmania, J Clin Endocrinol Metab. 2013 May;98(5):1954-62. doi: 10.1210/jc.2012-4249. Epub 2013 Apr 30, www.ncbi.nlm.nih.gov/pubmed?term=236 33204

4 'Iodine intake of New Zealand children following fortification of bread', www.foodsafety.govt.nz/elibrary/industry/ child-iodine-intake-bread.pdf

• British Dietetic Association, www.bda.uk.com/foodfacts/Iodine.pdf.

• www.british-thyroid-association.org/Guidelines/

MAIN SOURCES OF IODINE

Food Portion Average iodine/
portion (mcg)
(actual iodine
content will vary)

Cow's milk 200ml 50-80**
Organic cow's milk 200ml 30-65**
Yoghurt 150g 50-100**
Eggs 1 egg (50g) 20
Cheese 40g 15
White fish 100g 115
Oily fish 100g 50
Shellfish 100g 90
Meat 100g 10
Poultry 100g 10
Nuts 25g 5
Bread 1 slice (36g) 5
Fruit and veg 1 portion (80g) 3

**Depending on the season, higher value in winter
Source: www.bda.uk.com/foodfacts/Iodine.pdf