The story comes from a survey of 800 UK schoolgirls aged 14 to 15 years old, in which urine samples were taken to measure their levels of iodine. It found that just over half the group (51%) had iodine levels indicating a mild deficiency, 16% had moderate deficiency and 1% had severe deficiency. The researchers suggest the finding is of major importance to public health, especially since iodine deficiency in pregnant women could lead to harmful effects on the development of an unborn baby’s nervous system.
While the findings of this study are concerning, the results are based on a single urine sample from each participant. Since levels of minerals and vitamins in urine can fluctuate, a clearer picture of iodine levels would be provided by a larger study of different populations, which measured iodine levels in several urine samples over a period of time.
Iodine is a trace mineral that is important to health, in particular for healthy thyroid function. It is found in milk, yogurt, eggs and fish.
Where did the story come from?
The study was carried out by researchers from the Royal Free Hampstead NHS Trust in London, Cardiff University, University College Dublin, the National University of Ireland in Galway, Aarhus University Hospital in Denmark and the University of Birmingham. The research was funded by the Clinical Endocrinology Trust.
The study was published in the peer-reviewed medical journalThe Lancet.
The media reported the study accurately, although some of the language used – such as the “dangerously low” levels of iodine in the Mail’s headline – was alarmist. The study was not large or long enough to support The Independent’s claim that “a generation of schoolgirls” is growing up deficient in iodine.
What kind of research was this?
This cross sectional survey assessed levels of iodine in schoolgirls aged 14 to 15 years old, who were attending nine UK secondary schools. This kind of study involves surveying people at one point in time, and is often used to assess the prevalence of a condition (in this case, iodine levels in UK schoolgirls).
The researchers say that iodine deficiency has substantial effects on growth and development and is the most common cause of preventable mental impairment worldwide. Iodine levels are measured in the urine and deficiency is defined by the World Health Organization (WHO) as an average of less than 100 micrograms of iodine per litre of urine (100μg/l), with mild deficiency classed as 50-99μg/l, moderate deficiency as 20-49μg/l and severe deficiency as less than 20μg/l. They say that mild iodine deficiency in pregnant women impairs mental function in their children. Iodine is a trace mineral that is important to health, in particular for healthy thyroid function. It can be found in milk, yogurt, eggs and fish. Deficiency in adults can lead to a swelling of the thyroid gland and in children to developmental delays and other health problems.
While 45% of people in continental Europe have evidence of iodine deficiency, no current data are available for the UK, although concern has been expressed about current iodine intake. The researchers aimed to assess the iodine status of the UK population and focussed on 14- to 15-year-old schoolgirls, since this group and their future offspring are most susceptible to the adverse effects of iodine deficiency.
What did the research involve?
The researchers recruited participants from nine schools across the UK. They took a sample of early-morning urine from each participant in June-July 2009 and November-December 2009 and measured the levels of iodine in the urine using standard laboratory techniques.
The researchers also took a 5ml sample of tap water at each sampling site at the same time to measure iodine levels.
Each participant was asked about their diet (through a validated diet questionnaire), ethnic origin, date of birth and postcode. The researchers then analysed the results using validated statistical methods.
The researchers explained that household iodised salt is rarely available to buy in the UK and that few, if any, manufacturers use iodised salt in the preparation and manufacture of foods.
What were the basic results?
Of the 810 schoolgirls who participated, 737 provided urine samples. Data for dietary habits and iodine status were available for 664 participants.
- The average urinary iodine level was 80.1μg/l, which indicates mild iodine deficiency and 75% of samples fell between 56.9μg/l and 109.0μg/l (the inter-quartile range).
- 51% of the sample had urinary iodine levels indicating mild iodine deficiency.
- 16% had urinary iodine levels indicating moderate deficiency.
- 1% had levels indicating severe deficiency.
- The prevalence of iodine deficiency was highest in Belfast, where 85% of the participants were deficient.
- Tap water iodine concentrations were low or undetectable (except in London) and were not associated with urinary iodine levels.
- Low iodine levels were more likely in summer and also associated with diet, including low intake of milk and high intake of eggs. Iodine levels also varied according to where the girls went to school.
The researchers compared dietary habits in Belfast with those of other areas of the UK because Belfast had the highest number of participants with iodine deficiency. They found that dietary habits, in particular consumption of milk and eggs, were not significantly different between Belfast and other areas of the UK.
How did the researchers interpret the results?
According to the researchers, the findings suggest that the UK is now iodine deficient. They say that the findings are of major public health importance, since developing foetuses are most susceptible to iodine deficiency and even mild disturbances of maternal and foetal thyroid function can have an effect on the development of the foetal nervous system. They call for an urgent comprehensive investigation of UK iodine status.
Unlike some other European countries, the UK has no salt iodisation programme. In the past, children’s milk consumption helped meet their iodine needs. However, UK milk consumption has fallen recently, and the researchers suggest that this could be responsible for low iodine status.
The findings of this carefully conducted survey are concerning because they seem to indicate that iodine levels could be lower than recommended in some people in the UK. Although the survey only assessed iodine levels in 14-to 15-year-old schoolgirls, the researchers used methods recommended by WHO guidelines for recruiting their participants.
However, as the researchers point out, further investigations are needed to give a clearer picture of the UK population’s iodine status.
The study’s main limitation is that the results were based on a single urine sample from each participant. Since levels of minerals and vitamins in urine can fluctuate, a more accurate assessment of iodine levels would be provided by a larger study that measured iodine levels in urine samples over a period of time.
Whether the iodine status of this population is similar to that of people of other ages and sexes is unclear. Iodine is a trace mineral that is important to health, in particular for healthy thyroid function. It is found in milk, yogurt, eggs and fish.
Links to the headlines
Girls' lack of iodine 'could harm babies' (and it's due to a lack of milk). Daily Mail, June 2 2011
Babies at risk as girls fail to get enough iodine. The Independent, June 20 2011
Links to the science
Vanderpump MPJ, Lazarus JH, Smyth PP et al. Iodine status of UK schoolgirls: a cross-sectional survey. The Lancet, June 2 2011 (early online publication)
Angermayr L, Clar C. Iodine supplementation for preventing iodine deficiency disorders in children. Cochrane Database of Systematic Reviews 2004, Issue 2
Mahomed K, Gülmezoglu AM. Maternal iodine supplements in areas of deficiency. Cochrane Database of Systematic Reviews 1997, Issue 4
Wu T, Liu GJ, Li P, Clar C. Iodised salt for preventing iodine deficiency disorders. Cochrane Database of Systematic Reviews 2002, Issue 3