Iodine nutritional status in Brazil : a meta-analysis of all studies performed in the country pinpoints to an insufficient evaluation and heterogeneity

Objectives: Iodine deficiency disorder (IDD) is the result of an inadequate dietary intake of iodine, which physiological consequences are endemic goiter and thyroid dysfunction. The objective of this study was to a analyze studies that assessed the status of Brazil’s population iodine nutrition and IDD prevalence. Materials and methods: Systematic review using PRISMA statement. Electronic database: PubMed, Medline, SciELO and Lilacs. Quality of studies: Newcastle-Ottawa Scale. Metaanalysis was carried out with R Core Team Statistical Software, version 3.1.0 (2014). The summary measure (WMD) and its confidence interval (CI) of 95% were calculated. The “Funnel plot” graph assessed publication bias and heterogeneity. Results: Seventeen papers were eligible: pregnant women (2), school children (9), adults/elderly (4) and preschool children/infants (2). Geographic distribution: North (1), Northeast (1), Midwest (2), Southeast (13), South (3). Twenty-three thousand two hundred seventy-two subjects were evaluated between 1997 and 2013 and all have use urinary iodine (UI) measurement. However, only 7 studies could be included in meta-analysis, all from Southeast region. The overall prevalence of IDD in school children in southeast region was 15.3% (95% CI, 13-35%), however this data had an important heterogeneity, expressed by the I2 Statistic of 99.5%. Conclusion: Only few studies have been performed and enrolled populations from south/southeast region of Brazil. The actual IDD prevalence analysis is complex because it was detected bias due influence of individual studies and very high heterogeneity. IDD might still be high in some areas but this remained unknown even after this meta-analysis evaluation. The generation of a national program for analysis of iodine status in all regions is urgently required. Arch Endocrinol Metab. 2015;59(1):13-22


INTRODUCTION
I odine is an essential micronutrient for the synthesis of thyroid hormones (TH), which are important for homeostasis and neurodevelopment (1)(2)(3)(4)(5).The World Health Organization (WHO) recomends daily iodine intake of 50 µg for newborns, 90 µg for children between 13 months and 6 years, 120 µg for children (7-12 years), 150 µg for adults (after 12 years) and 250 µg for pregnant and lactating women (6)(7).Iodine deficiency disorder (IDD) is the result of an inadequate dietary intake of iodine, whose physiological consequence is an abnormal function of the thyroid gland, hypothy-roidism and endemic goiter (8).The harmful effects of IDD are even more severe in pregnant women, fetuses and children, being the worldwide most common cause of preventable mental retardation (6,7,(9)(10)(11).
The Brazilian Government have been controlling salt iodization in the country, according to Federal Law 6,150, in partnership with the National Agency for Sanitary Surveillance (Anvisa) and the salt productive sector (12)(13)(14).In accordance with Resolution RDC nº 130 (2003), it was deemed fit, for human consumption, salt content that corresponded to 20-60 ppm of iodine concentration and this recommendation have Arch Endocrinol Metab.2015;59/1 been prevailed for ten years (14).However, in the mean time, data from the ThyroMobil Project in Latin America, identified Brazil (with 17 sentinel sites and a total of 1,563 school children evaluated) as a country of excessive iodine consumption, with a mean urinary iodine excretion (UIE) concentration of 360 µg/L (15,16).Consequently, the levels of salt iodization were recently reduced to 15 to 45 milligrams of iodine per kilogram of product (http://www.in.gov.br)(17).Notwithstanding, the Brazilian Endocrine Society (SBEM) have strongly diverged on this reduction (http://www.tireoide.org.br/reducao-de-iodo-no-sal/).
The fact is that there is not recent national survey study about iodine content in table salt in households from different regions of Brazil neither a continuous monitoring of the overall population iodine status.Therefore, given the above, this review aimed to systematize and analyze all studies which assessed the prevalence of IDD in Brazil through UIE analysis in order to describe the current available information about iodine nutrition status.

Research questions
(1) What is the overall prevalence of IDD in Brazil?; (2) What is the prevalence of IDD in different areas and population groups?; (3) Are there enough studies to profile the population iodine nutrition status?; (4) Are there differences related to IDD in individuals of distinct ages, from one region to another, or in separate areas of the same region?;(5) Are the published data prevailing and have comprised all regions of Brazil?.

Search strategy and data collection
This systematic review is reported in accordance with the PRISMA (preferred reporting items for systematic reviews and meta-analyses) statement (18,19).Analysis of all studies conducted in Brazil in order to assess the status of iodine nutrition in populations -retrieved from the electronic database PubMed, Medline, SciELO and Lilacs.We have not limited the period of the study because we wanted to reach all studies conducted in the country.The search strategy used controlled vocabulary supplemented with keywords describing the following concepts, not only limited to English language publications, in the form of: "Iodine" and "Brazil" and "Iodine deficiency" and "Brazil" and "Thyroid function tests" and "Brazil".Unpublished studies were also investigated in the largest thesis and doctoral dissertations database of a Brazilian Agency (Capes).The searching by hand has been conducted in the references of the review papers and in a few non-indexed Medline Brazilian journals.Duplicate publication was checked and, if necessary, the corresponding author was contacted.After this, reviews of iodine status methods were collected in full text, the reference lists were checked and the included study list was updated accordingly.

Quality assessment
Two reviewers working independently (R.O.C. and H.E.R.) assessed the methodological quality of included observational studies using the Newcastle-Ottawa Scale and adapted Newcastle-Ottawa Scale to evaluate cross sectional studies (20).This instrument assesses the protection against bias due subject selection methodology, evaluation and data analysis.

Study selection
Inclusion criteria were defined as follow: a) the language in which the article was published was English or Portuguese; b) the main purpose of the article: assessment of iodine nutritional status of individuals; c) primary studies conducted in Brazil; and d) description of percentage of IDD, sufficiency and excess iodine, according to the criteria established by WHO.Experimental animal studies, review articles, case reports, studies investigating iodine nutrition in individuals with thyroid disorders or chronic diseases, studies on the iodine nutritional and duplicate articles in the databases were excluded.

Diagnosis of iodine deficiency disorders
The WHO recommendations regarding IDD assessment have been followed by the selected studies.UIE was measured using different methods and in a number of different units that could not always be interconverted to allow comparison between studies (Table 1).The studies selected for the meta-analysis had UIE generally, UIE was determined by the colorimetric ceric arsenite method based on the Sandell-Kolthoff (S-K) reaction, previously considered as the gold standard (21).Normal reference range was considered 100-299 μg/l for general population and 150-499 µg/l for pregnant woman, according to WHO (8,9).

Statistical analysis
Statistical analyzes were performed using the R Core Team Statistical Software, version 3.1.0(2014).The summary measure -weighted mean difference (WMD) -and its confidence interval (CI) of 95% were calculated.
The heterogeneity was assessed initially through a hypothesis test for homogeneity, using the Cochran Q test (at a significance level of 5%), and subsequent application of the I 2 Statistic and visual inspection of each "Forest plot".We have determined fixed and random model effects, but was systematically employed in cases where the I 2 Statistic found significant heterogeneity.As the available published data have been predominantly described in school children group, most of our data were achieved using this group.As we noted that only few studies could be selected for this investigation and the heterogeneity measured by the graph "Funnel plot" pointed to a possible publication bias, we decided to establish a cutoff of 15.3% (prevalence of overall meta-analysis) in WMD to independently calculate the IDD prevalence.

Studies characteristics
In our investigation, a total of 1,252 records were identified (Figure 1).We have found only 24 studies published between 1997 and 2013 eligible for this systematic review (16, (Table 1).A total of 26,148 subjects were subscribed for UIE assessment between 1997 and 2013.The most widely carried out type of study was cross-section (n = 20) (86,9%).Two followup (cohort) and one case control study were also enclosed.The school environment was the place where the largest amount of data collection was performed, representing 13 studies (56,5%).Seventeen/twenty-three studies received adequate qualification, by Prisma analysis, for quantitative synthesis (Tables 2 and 3): pregnant women (n = 2), school children (n = 9), adults/elderly (n = 4) and preschool children and/or infants (n = 2).The country distribution was very variable, with a clear shifting toward Iodine deficiency in Brazil southeast region: North (n = 1), Northeast (n = 1), Midwest (n = 2), Southeast (n = 13) and South (n = 3) (Tables 2 and 3).Considering only the selected studies, 23,272 individuals had UIE being investigated.The main reasons for study exclusion were: incomplete or repeated results (n = 5), gold standard methodology for UIE measurement (S-K) (n = 2) and data that could not be compared because absence of similar evaluation coming from others studies (10).

IDD prevalence in Southeast Brazil
The studies were subgrouped according to the region and population group (school and/or preschool children, adults, elderly and pregnant women).The overall meta-analysis performed could include only 7 studies from the Southeast region (Figure 2) the others subgroups could be not be analyzed by meta-analysis methodology.However, a high heterogeneity was immediately identified among the studies used for this meta-analysis (p < 0.0001, I 2 = 99.5%)(Figure 2).After applying the I 2 statistic and plot Forest visual inspection, it was noted that the studies should be analyzed separately because the large heterogeneity between then.The criteria for this separation was achieved by  No: number of subjects evaluated; µg/L: micrograms per liter.
Iodine deficiency in Brazil

DISCUSSION
This unprecedented systematic review showed the prevalence of IDD measured in populational studies conducted in Brazil and aimed to answer our main question: What is the overall prevalence of IDD in Brazil?.However, the vast majority of surveys (94, 1%) were conducted in the South (4/17) and Southeast (13/17) regions.Consequently, a comprehensive conclusion about the real IDD prevalence in the country can not be acchieved.While the Southeast region has been recognized as an iodine sufficient area, differences were also detected between states from the same region (Minas Gerais vs. São Paulo).In São Paulo, there is a rising concern regarding the excessive iodine exposure and predisposition to thyroid autoimmune diseases (Table 2) (29,33,36,40).In contrary, in the state of Minas Gerais, few studies have reported increased IDD prevalence, especially in children and adolescents from low-income populations (Table 2) (37,38).Macedo evaluated 540 children from schools in the municipality of Novo Cruzeiro (Minas Gerais) and observed IDD prevalence of 40% (38).This might be related to low salt intake, decay of iodine in salt due the storage form and/or expiration date and reduced education level Iodine deficiency in Brazil (38).Limitations of the number of studies conducted in all regions of Brazil and the possibility of "publication bias" identified in our investigation (I 2 = 99.5%;p < 0.0001), made impossible to conduct a meta-analysis for the entire country or particular region (Figure 2).Our quantitative analysis rejected 17 studies because lack of urinary iodine (UI) standardization and similar surveys conducted in the same area.
In our study, only a supplementary central questions (What is the prevalence of IDD in different areas and population groups?) could be partially addressed.The IDD prevalence rate predictied in school children from Southeast region diverged between 24% (95% CI, 13-35%) and 32% (95% CI, 25-39%), using different analysis methodology (Figure 3).Therefore, we have diagnosed a tremendous heterogeneity available of data.We have demonstrated that the higher rate of 32% of IDD prevalence, was clearly influenced by two individual studies performed by Esteves (24) and Macedo (38) (Figure 4).Thus, we assumed that the calculated IDD prevalence of 15.3% (95% CI, 6-23%) after biased studies exclusion is the one that could better represents the studies developed in this area, pinpointing to a possible compelling iodine deficiency.
The very few studies (N = 2) executed in North and Northeast regions, did not fit the inclusion criteria for the meta-analysis (Table 1) (24,35).Pontes and Adan (35) assessed the iodine nutritional status and cassava consumption of 180 school children in the city of Cabaceiras/Paraiba (northeast region).The high rate of cassava flour utilization (31.6%), associated with in elevated rate (33.3%) of IDD prevalence in school children.Nonetheless, in this study, the gold standard method for UI evaluation was not used (35).In another survey conducted in north/northeast region, Esteves identified in Bahia, the city of Cocos had low UI level, with median of 44 μg/L, (24).Almas, Arraias and Paraná (Tocantins) had median UI of 33 μg/L, 34 μg/L and 26 μg/L, respectively (24).Therefore, a limitation of the data collected in our review is that the greater number of studies was based only on school survey data, without any enough available information about other population groups.In summary, targeting other essential questions, our review shows that there   are not enough studies to profile the population iodine nutrition status in Brazil; and perhaps we might find huge differences related to IDD in individuals of distinct ages, from one region to another, or even in separate areas of the same region.In the manner that the available published data have not considered all country regions and was mostly concentrated in southeast.
The guidelines of the Brazilian Control Program for Iodine Deficiency Disorders (Pro-Iodo), recommended observation in schoolchildren between 6-14 years-old, as the child population is high vulnerable (5,8).Only few studies have analyzed pregnant women (25,30,43).Interesting, Ferreira evaluated 191 pregnant women in Ribeirao Preto, Sao Paulo, and found a very high IDD prevalence (57%) and an average UI of 144.4 μg/L (43).It is important to remark that all surveys were conducted when the level of salt iodi nation were still of 20-40 ppm (ref).Therefore, it is not known if the impact of salt iodization reduction on the health of pregnant and lactating women in Brazil, since this group has a greater need for iodine and previous studies pointed to higher susceptibility for IDD (45).Considering the IDD neurological potential damage during childhood development, the establishment of preventive evaluation for pregnant women and children might be essential (4).
Macedo and cols.(37) when assessing infants and preschoolers in Minas Gerais found a IDD prevalence of 34.4% (37).Therefore, this group of individuals (children until the fifth year of life) might be also extremely vulnerable for IDD (37).In contrast, Lima and cols.(40) have recently used Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to measure iodine content in breast milk and urine from children during the first six months of life in the state of São Paulo and the data revealed a high average concentration of iodine in breast milk (206 μg/L) and urine (293 μg/L) (40).
Therefore, it seems that only considering the risk of excessive iodine intake, based on studies from southeast region, Brazil has changed the contents of this micronutrient in salt traded domestically to 15-45 ppm.However, after adoption of this measure, it would be necessary to monitor the novel iodine nutritional status of the population.Indeed, we hypothesized that the recommended iodination could be modified based on the data about salt intake and UI concentration found for individual area coustolering Brazil each region, especially in a country of continental dimensions with vast differences in socio-demographic, geographic and climate aspects.
The main challenge is to adequate salt iodization and to promote educational and nutritional programs in order to strengthen the IDD or excessive control related to excessive consumption of iodine.Ideally, pe riodic monitoring of iodine nutritional status of the population in different states would be essential, especially less studied is essential to establishment of the proper individual range of salt iodization.The actual IDD prevalence analysis is complex, because pu blication bias high heterogeneity between studied.IDD might still be high in some areas but this remained unknown even after this meta-analysis evaluation.
In conclusion, nutritional status of iodine in Brazil has improved over the past few years, in general, in order to control the supply of iodine and reducing the rate of endemic goiter.However, concern about iodine optimal nutrition persists in all regions of the country, especially, after the reduction in the levels of salt iodization (15-45 ppm).
We hypothesized that changes in diet, differences in goitrogens consumption (including cassava), geographical and social demographic characteristics, road construction/commercial negotiation and processed foods availability may explain the coexistence of IDD and excessive intake iodine in different areas of the country.Most studies have been conducted many years ago and the generation of a national program for the analysis of the actual situation iodine in all regions is an urgently needed crucial for establishing the specific salt iodization needed for each region.

Figure 3 .
Figure 3. Meta-analysis of studies from the Southeast with IDD prevalence of less than 15.3.
Iodine deficiency in Brazil CI: confidence interval; W: weighted.

Figure 4 .
Figure 4. Meta-analysis of studies from the Southeast with IDD prevalence of greater than 15.3.

Table 2 .
Urinary iodine excretion in Brazilian population

Table 3 .
Urinary Iodine Excretion in Brazilian Pregnant Women CI: confidence interval; W: weighted.Figure 2. Overall meta-analysis of studies from the Southeast.CI: confidence interval; W: weighted.