Copyright! Reproduction and dissemination – also partial – applicable to all media only Peer Review | Selenium and Iodine with written permission of Umschau Zeitschriftenverlag GmbH, Wiesbaden. Selenium and iodine: essential trace elements for the thyroid Kristina Lossow, Tanja Schwerdtle, Anna Kipp Introduction Abstract Selenium and iodine are among the essential Selenium and iodine are essential trace elements that work together to en- trace elements. Ensuring a comprehensive, sure that the thyroid functions optimally. A deficiency in one or both of these adequate supply of these elements remains elements leads to fluctuations in thyroid hormone production, which have a challenge across the globe to this day, de- far-reaching consequences in terms of metabolic processes, neurological de- spite extensive efforts (such as the iodination velopment, and disease. Iodine and selenium supply also play an important of table salt). Factors that play a decisive role role in autoimmune diseases of the thyroid. Both the total selenium concen- tration in the serum and the concentration of selenoprotein P are suitable in the supply of these nutrients include not biomarkers for determining selenium status. Iodine concentration in the only soil conditions and traditional foods, but urine is the most commonly used method of determining iodine status. In also the increasing spread of dietary trends, order to improve assessment of supply status for these two essential trace particularly in Europe. elements plus an additional four, the TraceAge research group is identifying A recently published study recorded the nutri- age- and sex-specific trace element profiles as well as new functional biomar- ent intakes of 32,423 study participants (EP- kers for the individual trace elements. In addition, the research group will IC-Oxford cohort) using diet questionnaires investigate interactions with other trace elements in more detail. adapted to the participants’ eating habits [1]. Keywords: selenium, iodine, thyroid gland, autoimmune diseases of the thyroid, Both, this study and similar studies conducted selenoproteins, TraceAge in Germany showed that vegetarians had a lower selenium intake than people who con- sume meat and fish. Vegans are particularly at risk of inadequate iodine intake (exception: those who consume seaweed) [2]. The same conclusion was drawn by studies that used biomarkers to determine the status of both of these trace elements [3, 4]. In addition to diet, another cause of inadequate supply of trace elements are diseases such as inflammatory bowel disease or renal insuffi- ciency, which can cause changes in the uptake or excretion of trace elements, respectively. Selenium The selenium content of plant-derived foods varies depending on the soil in which they grow. As a consequence, European plants are Citation less rich in selenium than identical products Lossow K, Schwerdtle T, Kipp A (2019) Selenium and iodine – es- sential trace elements for the thyroid. Ernahrungs Umschau 66(9): from North America. Plant-derived foods that 175–180 are relatively rich in selenium include Brazil This article is available online: nuts, legumes, cruciferous vegetables, and DOI: 10.4455/eu.2019.032 bulb vegetables. In Europe, adequate selenium intake is primarily achieved through the con- Peer-reviewed sumption of animal-derived foods such as Manuscript (overview) received: November 09, 2018 meat, eggs, or fish. The selenium content of Revision accepted: March 14, 2019 Ernaehrungs Umschau international | 9/2019 175 Copyright! Reproduction and dissemination – also partial – applicable to all media only Peer Review | Selenium and Iodine with written permission of Umschau Zeitschriftenverlag GmbH, Wiesbaden. these products is ensured in EU member states through the wide- Selenium Iodine spread practice of feeding animals (especially pigs and poultry) [μg] [μg] with selenium-rich mineral mixtures. The recommended daily Infants 10–15 40–80 selenium intake for adults is 60–70 μg ( Table 1). Ingested selenium compounds are metabolized in the body, re- Children 15–30 100–140 sulting in the incorporation of selenocysteine into selenoproteins (1–10 years) which mediate most selenium effects. The selenoproteins include Adolescents 45–70 180–200 enzymes such as glutathione peroxidases (GPx), which catalyze Adults 60–70 200 the reduction of peroxides, thioredoxin reductases (TR), which Senior citizens 60–70 180 maintain the body’s oxidation-reduction system, and deiodinases Pregnant women 60 230 (DIO), which activate or deactivate the thyroid hormones. Breastfeeding 75 260 Looking at the world as a whole, Germany has intermediate lev- women els of selenium supply. Severe selenium deficiency-related diseases Upper limit (EFSA) 300 600 such as Keshan disease and Kashin-Beck disease ( Box), which Tab. 1: Estimated values for daily intake of have been described in China and in African countries, do not selenium and recommended intakes of occur here. iodine from the German Nutrition Soci- ety (Deutsche Gesellschaft für Ernährung [DGE]) [5] and upper limits from the European Food Safety Authority (EFSA) [6, 7] for adults (including breastfee- Iodine ding women and pregnant women, non-age-specific) Iodine is an elemental component of the thyroid hormones tri- iodothyronine (T3) and tetraiodothyronine (T4). These hormones human body. Both plant and animal products regulate growth and development, body temperature, and the generally contain only low amounts of iodine. metabolic processes for carbohydrates, proteins, and fats in the The iodine content of these products varies de- pending on the soil and water in the region and depending on production processes (such Keshan disease as the use of plant fertilizer containing io- Keshan disease is a disease of the heart muscle (cardiomyopa- dine, iodine-enriched animal feed, or table salt thy) that mainly affects growing children and pregnant women. enriched with iodine). By contrast, seaweed, It is thought to be due to an interaction between selenium de- saltwater fish, and seafood naturally contain ficiency and Coxsackie virus infection. Selenium supplementa- high levels of iodine. tion significantly reduces the occurrence of the disease. According to the estimation of the World Health Organization (WHO), Germany has Kashin-Beck disease had an optimal intake of iodine since the in- troduction of iodized salt. However, this eval- Kashin-Beck disease is characterized by degenerative joint uation is in large part based on an erroneous changes. It mainly occurs in children and manifests as defor- iodine analysis by one of the three German mation of hand and foot joints, inflammation of the nerves, in- laboratories where study-based iodine analy- ternal bleeding, and stunting. Possible triggers of the disease ses were regularly performed on urine sam- that have been suggested include contamination of food with ples. The analytical error, which resulted in mycotoxins, selenium deficiency, and a high level of fulvic acid an iodine concentration that was 36% higher in drinking water. than the actual value, has now been identified Studies focussing on genetic aspects also came to the con- [11] and corrected. This also explains why the clusion that in affected persons, the genes associated with two representative studies by the Robert Koch apoptosis and selenium were expressed differently in the Institute – DEGS (German Health Interview articular cartilage [8]. There are discussions about whether and Examination Survey for Adults, 6,978 iodine deficiency may also be a cause alongside selenium participants, 2008–2011) and KiGGS (German deficiency. However, there is no direct evidence that iodine Health Interview and Examination for Chil- deficiency alone is a risk factor for Kashin-Beck disease. Sele- dren and Adolescents, > 14,000 participants, nium supplementation does indeed have a positive effect on 2003–2006) – recorded lower iodine excretion the occurrence of the disease; however, it cannot prevent it values, resulting in the suggestion that about completely [9, 10]. 30% of the subjects examined had an inade- quate iodine supply [12, 13]. 176 Ernaehrungs Umschau international | 9/2019 Copyright! Reproduction and dissemination – also partial – applicable to all media only with written permission of Umschau Zeitschriftenverlag GmbH, Wiesbaden. The daily intake recommended by the German Nutrition Soci- vulnerable to oxidative cell damage. Along ety is 180–200 μg of iodine ( Table 1). During pregnancy and with other antioxidant enzymes, the seleno- breastfeeding, intake should be around 10–20% higher. In the protein families GPx and TR contribute to the early stages of pregnancy in particular, a sufficient supply of io- breakdown of reactive compounds. dine is essential for the neural development and maturation of As selenium deficiency progresses, the activity the fetus. Therefore, the administration of iodized oil, particularly of selenium-dependent DIOs is reduced, which in the first and second trimester, could reduce the prevalence of in turn reduces both activation and break- neurological damage and improve child development [14]. Opti- down of thyroid hormones. When an iodine mal iodine intake is also necessary for the critical phases of brain deficiency and a selenium deficiency occur at development later on. For instance, a randomized, double-blind, the same time, this effect is getting stronger. placebo-controlled study in school pupils in New Zealand aged Due to decreasing T4 and T3 concentrations 10–13 years showed