Iodine nutrition: Disorders, monitoring and policies

Iodine is an essential mineral nutrient and an integral component of thyroid hormones. Iodine deficiency is typically associated with goiter, but can have more serious health implications. Adequate iodine status is important for normal brain development. Iodine deficiency in utero or in early life can cause severe neurological and cognitive impairment. Over the last three decades, global efforts have reduced the prevalence of iodine deficiency disorders (IDD) in many areas of the world with implementation of nutrition policies and programs such as "salt" iodization. However, in a number of areas iodine deficiency is still widespread. Iodine deficiency in remote regions with high poverty will be more difficult to eradicate. Efforts to eliminate IDD in affected areas and sustaining successful iodine programs will be a priority given the substantial public health and economic benefits. A key component will be periodic monitoring of population iodine status to ensure sufficient intakes and the absence of excessive intakes. Median urinary iodine concentration (UIC), a validated biomarker for assessing population iodine status, will facilitate monitoring. Research validating "usual" UIC for use in combination with the Estimated Average Requirement cut-point method will expand its utility and allow accurate determination of the prevalence of inadequate intakes in populations. Further research on the development of biomarkers for assessment of individual iodine status for routine patient care will be important.

Assessing Cassava Flour Consumption Impact on Iodine Nutritional Status in Schoolchildren from Public Schools in Brazil

Cassava (Manihot esculenta Crantz) is an important food source in many developing countries. This root contains cyanoglucosides, which can aggravate iodine deficiency disorders (IDDs). To analyze the impact of cassava flour consumption (CFC) on urinary iodine concentration (UIC) among schoolchildren from public schools in Bahia, Brazil. Cross-sectional study was conducted on 1231 schoolchildren (ages 6-14 years old). Anthropometric parameters, household food insecurity, UIC, and CFC were evaluated. CFC prevalence was 90.8%. The mean UIC indicated adequate iodine nutrition in group A (CFC positive) and group B (CFC negative) (203.29 ± 81.08 μg/L versus 225.98 ± 76.59 μg/L, respectively). We found that daily cassava flour intake did not significantly raise the risk of iodine deficiency (ID) (odds ratio [OR] = 1.43 [confidence interval (CI) 0.72-2.82]; p = 0.29), nor did it significantly protect against excessive iodine intake (EII) (OR = 0.70[CI 0.39-1.26]; p = 0.24). The prevalence of iodine deficiency (ID) (12.6% versus 8.9%) and EII (9.6% versus 15%) did not significantly differ between both groups. The results of this study suggest that the cyanides present in cassava flour have very little influence on iodine metabolism, which is probably justified by cassava processing methods.

The role of micronutrients in thyroid dysfunction

Thyroid hormones are essential for normal growth and development in children. Nutritional factors are closely related to thyroid dysfunction due to deviation from normal physiology of the gland. Iodine, a main constituent of thyroid hormones (T3 and T4), deficiency is one of the commonest causes of hypothyroidism in children and adults, worldwide. Other micronutrients, such as Cruciferous vegetables, Pearl Millet, Soy products and Cassava, were also attributed to cause thyroid dysfunction. Environmental factors, namely, contamination of water with goitrogens could also contribute to the aetiology of goitre in some endemic areas. Dietary advice and avoidance of excessive use of goitrogens in diet are part of guidance on nutritional safety that needs to be established, especially in the areas of endemicity.

Interaction between iodine and glucosinolates in rutabaga sprouts and selected biomarkers of thyroid function in male rats

Rutabaga sprouts belong to the Brassicaceae family and may exert a negative influence on thyroid function, because they are a rich in glucosinolates. These sprouts are also valuable source of iodine (6.5 ± 0.6 μg/100 g of fresh weight). Sprouts were tested in a long-term experiment with young male rats as an element of their diet, combined with two models of hypothyroidism, the first - deficit of iodine and the second - sulfadimethoxine ingestion as a pharmacological agent caused inhibition of thyroid peroxidase. Evaluations were performed for the serum TSH and thyroid hormones together with analyzes of thyroid histopathology, cytosolic glutathione peroxidase (GPX1), thioredoxin reductase in the thyroid, plasma GPX3 and CAT, erythrocyte GPX1. Rutabaga sprouts' intake by healthy rats did not cause any harmful effect on their health, including thyroid function. For animals with hypothyroidism, rutabaga sprouts enhanced the adverse effect of iodine deficiency or ingestion of sulfadimethoxine on the organism. According to the results obtained for young male rats thyroid function, the interpretation of data for human exposure to rutabaga sprouts has to be avoided. Furthermore, unless new scientific data confirms a lack of the negative effect of brassica sprouts on thyroid function in human, they should not be excluded from the group of goitrogenic products.

Assessing the potential impact on the thyroid axis of environmentally relevant food constituents/contaminants in humans

Occurrence and mode of action of potentially relevant goitrogens in human nutrition and their mode of action (MOA) are reviewed, with special focus on the anionic iodine uptake inhibitors perchlorate (PER), thiocyanate (SCN) and nitrate (NO3). Epidemiological studies suggest persistent halogenated organic contaminants and phthalates as well as certain antimicrobials to deserve increased attention. This also applies to natural goitrogens, including polyphenols and glucosinolates, food constituents with limited data density concerning human exposure. Glucosinolates present in animal feed are presumed to contribute to SCN transfer into milk and milk products. PER, SCN and NO3 are well-investigated environmental goitrogens in terms of MOA and relative potency. There is compelling evidence from biomarker monitoring that the exposure to the goitrogens SCN and NO3 via human nutrition exceeds that of PER by orders of magnitude. The day-to-day variation in dietary intake of these substances (and of iodide) is concluded to entail corresponding variations in thyroidal iodide uptake, not considered as adverse to health or toxicologically relevant. Such normal variability of nutritional goitrogen uptake provides an obvious explanation for the variability in radioactive iodine uptake (RAIU) measurements observed in healthy individuals. Based on available data, a 20 % change in the thyroidal uptake of iodide is derived as threshold value for a biologically meaningful change induced by perchlorate and other goitrogens with the same MOA. We propose this value to be used as the critical effect size or benchmark response in benchmark dose analysis of human RAIU data. The resulting BMDL20 is 0.0165 mg/kg bw/day or 16.5 μg/kg bw/day. Applying a factor of 4, to allow for inter-human differences in toxicokinetics, leads to a TDI for perchlorate of 4 μg/kg bw/day.