Iodine is an essential micronutrient for the synthesis of the thyroid hormones, Triiodothyronine (T3) and Thyroxine (T4). The amount of iodine required for the production of these hormones varies according to age and physiological state (1). Thus, the World Health Organization (WHO) recommends a daily iodine intake of 120 μg for schoolchildren and 150 μg for adolescents (over 12 years) and adults (2).

The recommended daily intake during pregnancy is higher (250 μg) due to body changes, fetal growth and development, and specific physiological changes such as thyroid hormone stimulation, hormone delivery to fetus, and increased glomerular filtration (urinary excretion of 30 to 50% of iodine) (2). This period is also marked by nutritional vulnerability given the energy needs for delivery and lactation, and fetal development (3).

Among pregnant women, a urinary iodine concentration lower than 150 μg is considered iodine deficiency. In schoolchildren, this concentration is <100 μg. Iodine deficiency in pregnant women causes hypothyroidism, goiter, and impaired fetal growth and poor brain development (2, 4). While in schoolchildren, it can culminate in significant reduction of intelligence quotient (IQ) and increased dropout rates (5). On the other hand, ingesting more than 300 μg of iodine is excessive for both groups, which can lead to thyroiditis, goiter, and Hashimoto's hypothyroidism (6).

In order to promote adequate iodine intake among the Brazilian population, salt iodization is mandatory by law in Brazil (Decree N° 39.814) since 1956. However, the implemented iodization range (20–60 ppm) was reduced in 2013 (15–45 ppm, Resolution N° 23) due to the high consumption of salt (12 mg/daily) among Brazilians (data from the Brazilian Institute of Geography and Statistics) (7, 8).

According to the National Survey on the Impact of Salt Iodization, 20.7% of schoolchildren presented urinary iodine levels between 200 and 299 μg/L (more than adequate) and 44.6% presented urinary iodine levels ≥300 μg/L (excessive). When stratified by sex, 49.7 and 40.8% of boys and girls, respectively, presented excessive iodine intake (9).

Studies have shown insufficient iodine intake in pregnant women, hence they are considered a risk group for iodine deficiency (3, 10, 11). From the above considerations, the study focused on two distinct groups, schoolchildren and pregnant women, the former with excessive iodine status and the latter with iodine deficiency. To plan effective interventions for these groups, iodine intake from all sources must be estimated. Therefore, the objective of this paper is to construct a theoretical model of iodine intake of schoolchildren and pregnant women in a city located in the Zona da Mata Mineira region, considering a healthy diet, salt consumption and water intake.

According to our theoretical model, the daily iodine intake of pregnant women and schoolchildren is higher than recommended (2). Brazil is currently considered a country which presents more than sufficient iodine intake (23). This classification was based on studies with schoolchildren, and does not reflect the real iodine status of pregnant women.

Based on our model and a healthy eating pattern, schoolchildren would have an iodine intake of 253.5 μg, which is higher than the 120 μg recommended by WHO (2). Corroborating our results, a cross-sectional study carried out in Vespasiano (Minas Gerais state) with 428 schoolchildren showed that 46.7% had more than adequate iodine levels and 20.1% had excess iodine (12). In Ribeirão Preto (São Paulo state), a cross-sectional study evaluated iodine intake among 8-and-10 year old children via urinary iodine, iodine concentration in table salt, and thyroid volume determined by ultrasonography. From the study, 59.5% of the children had iodine levels above 300 μg/L, classified as excessive (24).

For pregnant women following a healthy eating pattern, the calculated recommended intake is also exceeded. However, 279.5 μg is very close to the recommendation of 250 μg. In fact, cooking and food storage can result in possible iodine losses which coupled with a healthy eating pattern can meet the iodine requirement (25).

Pregnant women rarely present excessive iodine status. In a case-control study conducted in Ribeirão Preto (São Paulo state), 9.9% of pregnant women had urinary iodine >250 μg/L, above the requirement (23). In another study conducted in São Paulo, 4.4% of pregnant women had urinary iodine above 250 μg/L (25). A study in Rio de Janeiro reported excess iodine (≥500 μg/L) in 4.5% of pregnant women in Rio de Janeiro (3). As expected, the prevalence of excessive iodine is low due to the high nutritional needs of pregnant women.

Drinking water was not a protective factor against iodine deficiency, since very low iodine values were detected. The level of iodine in water reflects its concentration in the rocks and soils of the region, and varies according to geographic location, where mountainous regions and those far from the sea tend to have lower concentrations of iodine in the soil, water, and food (14).

The mean iodine concentration in the salt samples was within the recommended range and 69.2% of the samples were in accordance with the current legislation (8). However, 30.8% of the salt brands presented iodine concentration below or above the recommended value, which reinforces the importance of effective inspection to ensure compliance with the legislation.

Furthermore, if a schoolchild ingests 5 g of salt per day, he will be consuming 149.4 μg/day of iodine, which already exceeds the recommended daily intake. However, if we consider the current salt intake observed by the Family Budget Survey, of 12 g, even with a healthy pattern, the daily intake of iodine for pregnant women and schoolchildren can reach 488.1 and 462.1 μg, respectively. This worsens the scenario of excessive intake because iodine intake is almost three times (2.98) higher than the recommendation for schoolchildren and almost two times (1.43) higher than that of pregnant women.

If pregnant women consume only 5 g of salt, they will not meet the recommended daily intake of iodine, thus increasing the risk of deficiency. In a cross-sectional study carried out in São Paulo, 57% of pregnant women were diagnosed with iodine deficiency (23). A cross-sectional study carried out in Rio de Janeiro showed that 48.7% of pregnant women were iodine deficient (3). These results demonstrate the importance of further studies to develop effective public policies to control and prevent iodine deficiency.

The strength of this study is the theoretical model of daily iodine intake in pregnant women and schoolchildren, which can guide actions and public policies that targets all forms of iodine consumption. Furthermore, new studies evaluating the nutritional status of iodine will be able to formulate hypotheses about what may be causing changes in the pattern of iodine intake, which, depending on age, alter required daily intake. A limitation of the study is that iodine nutritional status was based on dietary estimates and not urinary iodine concentration, which is the gold standard for assessing a population's nutritional iodine status. Moreover, to assess the concentration of iodine in foods, a table with compiled information from the literature was used. Although the table has not yet been validated in Brazil, it is currently the only table in the country containing iodine content of foods.

The daily iodine intake of schoolchildren and pregnant women was estimated to be above the recommended level, considering a feasible healthy dietary pattern.

With the reduction of the iodization range of salt, the main source of iodine, if schoolchildren consume 5 g of salt per day based on a mean iodine range (30 ppm), they exceed the recommendation. For pregnant women, this does not occur. Therefore, the theoretical model of iodine intake shows the need for current policies to be reviewed with emphasis on the peculiarity of each population group. Moreover, different iodine concentrations were observed in salt brands, which reinforce the importance of a more effective inspection to ensure compliance with the current legislation.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

AC contributed significantly to the conception of the paper, its design, data collection, data interpretation, and analysis. SR contributed significantly to the interpretation of data and analysis, participated in the writing and critically revised the paper in a manner sufficient to establish ownership of the intellectual content. MM, EF, ED, ML, SP, MP, RM, and SF critically revised the article in a manner sufficient to establish ownership of the intellectual content. All authors contributed to the article and approved the submitted version.

We would like to thank the Coordination of Improvement of Higher Education Personnel - Brazil (CAPES) - Financing Code 001. National Council for Scientific and Technological Development (CNPq), case 408295/2017-1. Foundation of Support and Research of the State of Minas Gerais (FAPEMIG) case APQ-03336-18.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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