Iodine deficiency has historically affected mankind extensively, especially populations living inland or at high altitudes. Iodine was first associated with thyroid function in the late nineteenth century, and a few decades later there was evidence that its deficiency caused a series of health problems generally labelled “iodine deficiency disorders” (IDDs) that affect foetal and child development (cretinism, physical and cognitive development, increased rate of miscarriage and childhood mortality, and congenital anomalies) as well as the adult population (hypothyroidism and goitre).1

From the late twentieth century, the World Health Organisation (WHO), the United Nations Children's Fund and the International Council for Control of Iodine Deficiency Disorders have promoted numerous initiatives that seek to raise awareness in citizens and national and regional governments regarding the importance of eradicating these disorders by increasing dietary iodine intake. Since it is very cost-effective, iodised salt has been the measure adopted by most countries in the world.2 In 1983, Spain regulated the iodisation of salt for human consumption, establishing an amount of 60mg of iodine per kilogram of salt with a tolerance margin of ±15% (51–69mg/kg).3

In 1982, goitre was endemic in Asturias.4 For that reason, in 1983 the Asturias Council of Public Health started launching campaigns to promote the consumption of iodised salt in homes and mandated its use in school canteens. Recommended iodine intakes were achieved in 1992, so the campaigns were considered a success.4,5 A study conducted in 2010 found that iodised salt was used in 66% of the homes and 100% of school canteens, with median urinary iodine levels of 180μg/L in children aged 6–14 years and 176μg/L in women of child-bearing age that corresponded to optimal dietary intakes of iodine.5

On the other hand, iodine supplementation of animal feeds to prevent IDDs in poultry and cattle (which was regulated by the European Economic Community in 19706) resulted in increases in the iodine contents of foods of animal origin (which were highest in egg yolks and milk). Thus, in the past three decades, the high frequency of consumption of milk and dairy products and the amounts consumed have made these foods major sources of iodine in several countries, including Spain.7–10 Since dairy consumption is high in the young population, our study focused on the population aged 2–5 years to assess the impact of dairy in iodine intake.

We conducted our study in the south west of Asturias, a rural area that at the time of the study had a population of 27,847 inhabitants holders of a personal public health care card (Tarjeta Sanitaria Individual [TSI]), and where cattle farming and mining are the main economic sectors. In 1986, this region had the highest prevalence of goitre in the autonomous community of Asturias, and while the prevalence declined significantly by 1994, it was still of 16.4%.11

We conducted a cross-sectional descriptive study between October 2013 and February 2014 that was approved by the Clinical Research Ethics of Asturias and the Fiscalía de Menores (Child Welfare Office).

Nutritional information was obtained from 198 preschool-aged children and urinary iodine levels from 193 (97.5%). Of these, 20.2% (n=40) were replacements, 22 due to refusal to take part in the study and the rest because of change of address or lack of response to telephone calls. The distribution of the sample by socio-demographic variables (Table 1) mirrors the population, with the exception of 3-year olds, which are over-represented (27.3% versus 23.6%). Boys accounted for 55.1% and the majority came from the municipalities of Cangas del Narcea (57.1%) and Tineo (31.8%). Most children (77.8%) had lunch at home while the remaining 22.2% had lunch at the school canteen from Monday to Friday.

We found an adequate iodine intake in the sample under study, with a median urinary iodine level that was higher than the one observed in the 6-to-14-year old population of Asturias in 20105,15 (184μg/L versus 180μg/L) and in other regions of Spain, such as La Axarquía in 1997–1998 (120μg/L)8 and the Basque Country in 2012 (147μg/L).7

The mean number of dairy servings consumed per day was also higher than the one observed in the age 6-to-14 years population in 20105,15 (3.8 versus 3.3). The current recommendation for children is two to three servings a day,16 but 58.5% of the sample under study consumed more than three servings a day. There was a clear difference in consumption between municipalities (4.3 versus 3.5 servings/day) that was accompanied by a difference in the corresponding urinary iodine levels, with participants from Tineo-Pola de Allande-Ibias consuming amounts of iodine that exceeded recommendations and participants from Cangas del Narcea-Degaña of preschool age having a higher risk of insufficient iodine intake. The difference in dairy consumption patterns may be related to the livestock that predominates in the towns with the highest populations, such as beef cattle in Cangas del Narcea and dairy cattle in Tineo (in 2013 Tineo produced 18.8% of the milk in Asturias).17

Most children consumed at least two glasses of milk a day and one serving a day (equivalent to two units) of dairy products, mainly yoghourt and Petit Suisse®. Although cheese was consumed frequently, it was in lesser amounts.

We found an association between urinary iodine levels and milk intake measured in glasses consumed. Children that drank four or more glasses of milk a day had an iodine intake that could be a mild health hazard due to exceeding daily requirements (urinary iodine level, 282.5μg/L), something that was also observed in consumers of low fat milk (urinary iodine level, 233μg/L).

Considering that both deficiencies and excesses in intake need to be prevented,13,18 a salient finding in our study was that 26.1% of participants had extreme urinary iodine levels of less than 50 or of 300 or more micrograms per litre. Extreme intake can lead to problems associated with hyperthyroidism in individuals with a history of goitre that reside in previously iodine-deficient areas (Jod Basedow effect), autoimmune thyroiditis13,14,18 and subclinical hypothyroidism.19 In 2013, ten countries in the world were categorised as having excessive iodine intake,18 and in 2005 the European Union, concerned that peak levels of iodine in milk and eggs could be exceeding the levels allowed in adolescents and adults,20 mandated the reduction of the iodine content in cattle and poultry feeds from 10 to 4ppm.21

Milk was already described as a significant source of iodine in 1997–1998 in La Axarquía8 and in subsequent studies in Spain.9,10 This is also consistent with the fact that in the United Kingdom—the first country in Europe that implemented iodine fortification of livestock feed, before 1940—, while sales of iodised salt in the continent amounted to only 5% of the total salt, adequate iodine levels were achieved when consumption of milk increased in schoolchildren due to its promotion by successive governments.18,22 The decrease in milk consumption observed in recent years has resulted in the resurgence of iodine deficiency in this country.18,23

The lack of a statistically significant association between increased urinary iodine levels and increased overall consumption of dairy products in our study could be due to the lower content of iodine in dairy products and the variability of iodine intake depending on the product consumed. The dairy products consumed most frequently were whole milk yoghourt and Petit Suisse®, estimated to contribute 6 and 8μg/100g of iodine, respectively.12

The contribution of eggs and fish to the total iodine intake is modest, as these foods are not consumed daily.13 Most of the sample under study consumed an adequate amount of eggs (2–4/week) and while consumption of fish was frequent, only 16% consumed the recommended four servings per week.

The daily recommended allowance of iodine in children aged 1 to 8 years is 90μg/day,9 and in the sample under study, this requirement would be met by consumption of milk alone, as it contributed 98.4μg/day (a value obtained from adjusting the estimate of 89.9μg/day, as in the analysis of iodine content in milk in the form of iodide, the amount recovered is 87%10). This estimate is similar to the one calculated by Arrizabalaga et al.10 of 50μg of iodine for one serving of 200–250mL of milk based on the weighted annual mean of monthly analyses of the iodine content of milk samples, with the highest iodine concentrations occurring between January and May and the lowest between July and November, and intermediate concentrations from June to December. Our study was conducted between October and February, and thus encompasses a full range.

The consumption of iodised salt was higher than it was in Asturias in 20105,15 (85.9% versus 66%). Since neither study verified iodine intake by checking the packaging label of the salt consumed, both may have overestimated intake. A study conducted in the Basque Country in 2012 that assessed intake based on the packaging label reported consumption of iodised salt in 53% of households.7 We also ought to note that iodised salt brands do not always reach the established concentrations, as in 1995 only 15% of salt packages distributed in Madrid and Cuenca had adequate iodine contents,24 and in 2014 in Asturias, 12 out of the 16 commercial brands did not meet regulatory standards (contents were either insufficient or excessive), although in April 2015, this issue had been resolved in 10 out of the 12 brands after appropriate measures were taken by the General Directorate of Public Health.25 Furthermore, iodine contents may decrease in households due to humidity. For all the above reasons, it is difficult to guarantee adequate iodine intake through salt consumption, even if the latter is widespread.

• 1.

  Dairy consumption must be monitored to adjust it to current recommendations, because while we observed an overall adequate intake of iodine in preschool-aged children, iodine intake may be insufficient or excessive in a considerable percentage.

• 2.

  In order to maintain an optimal intake in the population, the health authorities should periodically monitor iodine concentrations in milk, as there may be changes in the production process or in dietary habits.7

This study was funded by the General Directorate of Public Health of the Health Department of the Autonomous Community of Asturias.

The authors of this article have no conflicts of interest to declare.