nutrients Review Is There an Ideal Diet to Protect against Iodine Deficiency? Iwona Krela-Ka´zmierczak 1,† , Agata Czarnywojtek 2,3,†, Kinga Skoracka 1,* , Anna Maria Rychter 1 , Alicja Ewa Ratajczak 1 , Aleksandra Szymczak-Tomczak 1, Marek Ruchała 2 and Agnieszka Dobrowolska 1 1 Department of Gastroenterology, Dietetics and Internal Diseases, Poznan University of Medical Sciences, Heliodor Swiecicki Hospital, 60-355 Poznan, Poland; [email protected] (I.K.-K.); [email protected] (A.M.R.); [email protected] (A.E.R.); [email protected] (A.S.-T.); [email protected] (A.D.) 2 Department of Endocrinology, Metabolism and Internal Medicine, Poznan University of Medical Sciences, 60-355 Poznan, Poland; [email protected] (A.C.); [email protected] (M.R.) 3 Department of Pharmacology, Poznan University of Medical Sciences, 60-806 Poznan, Poland * Correspondence: [email protected]; Tel.: +48-665-557-356 or +48-8691-343; Fax: +48-8691-686 † These authors contributed equally to this work. Abstract: Iodine deficiency is a global issue and affects around 2 billion people worldwide, with preg- nant women as a high-risk group. Iodine-deficiency prevention began in the 20th century and started with global salt iodination programmes, which aimed to improve the iodine intake status globally. Although it resulted in the effective eradication of the endemic goitre, it seems that salt iodination did not resolve all the issues. Currently, it is recommended to limit the consumption of salt, which is the main source of iodine, as a preventive measure of non-communicable diseases, such as hypertension or cancer the prevalence of which is increasing. In spite of the fact that there are other sources of iodine, such as fish, seafood, dairy products, water, and vegetables, the high consumption of processed food with a high content of unionised salt, alternative diets or limited salt intake can still Citation: Krela-Ka´zmierczak, I.; lead to iodine deficiency. Thus, iodine deficiency remains a relevant issue, with new, preventive Czarnywojtek, A.; Skoracka, K.; solutions necessary. However, it appears that there is no diet which would fully cover the iodine Rychter, A.M.; Ratajczak, A.E.; requirements, and iodine food supplementation is still required. Szymczak-Tomczak, A.; Ruchała, M.; Dobrowolska, A. Is There an Ideal Keywords: iodine deficiency; diet; iodine; vegetarian diet; alternative diet Diet to Protect against Iodine Deficiency? Nutrients 2021, 13, 513. https://doi.org/doi:10.3390/ nu13020513 1. Introduction Iodine is absorbed in the stomach and the small intestine where it is transported via Academic Editor: Lutz Schomburg sodium/iodide symporter (NIS) and pendrin to the thyroid gland, and then stored in the Received: 13 December 2020 follicular cells. Subsequently, in the presence of hydrogen peroxide (H2O2), iodine ions Accepted: 1 February 2021 − Published: 4 February 2021 (I ) are oxidised by thyroid peroxidase. Then, the tyrosine residues are iodinated—first, at position 3, which forms monoiodotyrosine (MIT), and then at position 5 to form diiodoty- Publisher’s Note: MDPI stays neutral rosine (DIT). MIT and DIT in a coupling reaction form thyroxine (T4) and triiodothyronine with regard to jurisdictional claims in (T3) hormones [1,2]. As a trace element, iodine is essential in human nutrition, mostly published maps and institutional affil- due to its role in the thyroid hormone synthesis. Moreover, both T4 and T3 are involved iations. in the regulation of metabolic processes in the human body and are responsible for the optimal growth of the central nervous system and brain [3,4]. Iodine also serves as an antioxidant and exhibits the protective effects of inflammatory states and cancer [5]. Io- dine deficiency constitutes a global issue, and is reflected by urinary iodine concentration (UIC) < 100 µg/day [3]. It is prevalent in the eastern Mediterranean, Asia, Eastern Europe, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. and Africa. Additionally, mild iodine deficiency is observed in Australia, Great Britain, This article is an open access article and New Zealand, as well as in specific groups, such as vegans or vegetarians [6]. In fact, distributed under the terms and proper iodine intake is crucial among pregnant women, as iodine deficiency is the leading conditions of the Creative Commons cause of mental retardation in children [7]. Since 1993, World Health Organization (WHO) Attribution (CC BY) license (https:// and United Nations Children’s Fund (UNICEF) recommend a universal salt iodisation. creativecommons.org/licenses/by/ Currently, around 70% of households in over 120 countries have access to iodised salt—in 4.0/). 1990, only less than 10% had this advantage [8]. Iodine is absorbed in 90% by stomach and Nutrients 2021, 13, 513. https://doi.org/10.3390/nu13020513 https://www.mdpi.com/journal/nutrients Nutrients 2021, 13, x FOR PEER REVIEW 2 of 16 have access to iodised salt—in 1990, only less than 10% had this advantage [8]. Iodine is absorbed in 90% by stomach and duodenum and is mostly delivered from the fortified Nutrients 2021, 13, 513 salt, and other sources such as fish, seafood, dairy, water, eggs, broccoli, peas,2 or of 15 spinach [4,9]. However, it is vital to notice that different concentrations of iodine in food products have been noticed, which is presumably associated with different growth environments [10].duodenum Both the and deficiency is mostly delivered and excess from of the iodi fortifiedne can salt, lead and to other an sourcesimpaired such thyroid as fish, gland functionseafood, and, dairy, subsequently, water, eggs, broccoli, to an peas,impaired or spinach functioning [4,9]. However, of the itentire is vital organism to notice that(Figure 1) [9].different A diet concentrations deficient in iodine of iodine can in lead food productsto mental have retardation, been noticed, hypothyroidism, which is presumably congenital anomalies,associated withgoitre different or low growth IQ, whereas environments the iodine [10]. Bothexcess the can deficiency result andin iodine-induced excess of iodine can lead to an impaired thyroid gland function and, subsequently, to an impaired hyperthyroidism [6]. Therefore, an adequate iodine intake—150 mg/day for adults, 120 functioning of the entire organism (Figure1)[ 9]. A diet deficient in iodine can lead to mg/daymental retardation,for children, hypothyroidism, and 250 mg/day congenital for pregnant anomalies, women—is goitre or low essential IQ, whereas for the the proper thyroidiodine excessgland canfunction result [3]. in iodine-induced The aim of the hyperthyroidism study is to evaluate [6]. Therefore, and review an adequate diets in search ofiodine the best intake—150 diet to protect mg/day against for adults, iodine 120 defic mg/dayiency. forMoreover, children, in and this 250 paper mg/day we wanted for to assesspregnant which women—is populations essential are at for risk the properof iodine thyroid deficiency gland functionor excess. [3]. In The order aim to of collect the the literaturestudy is to evaluaterelated andto review the diets inpresented search of thetopic, best diet tothe protect PubMed against iodinedatabase (www.pubmed.ncbi.nlm.nih.gov,deficiency. Moreover, in this paper weaccessed wanted toon assess 20 January which populations 2021) was are atexplored risk of with referenceiodine deficiency to the terms or excess. “iodine”, In order “diet”, to collect “deficiency”. the literature Iodine related Global to the Network presented materials topic, and the PubMed database (www.pubmed.ncbi.nlm.nih.gov, accessed on 20 January 2021) was reports were also analyzed. We have focused on popular diets—for example; paleolithic, explored with reference to the terms “iodine”, “diet”, “deficiency”. Iodine Global Network glutenmaterials free and or vegan reports diets—and were also analyzed. investigated We have if individuals focused on popular following diets—for them example;may be at risk ofpaleolithic, iodine deficiency. gluten free The or veganrisk of diets—and iodine deficiency investigated may if individualsbe increased following due to themthe type of productsmay be at that risk are of iodineeliminated, deficiency. for example The risk of dair iodiney. However, deficiency we may also be increaseddiscussed due if there to are anythe food type ofsubstitutes, products that which are eliminated, could decrease for example this risk. dairy. Further, However, salt-restrictive we also discussed and plant- basedif there diets are anymay food be associated substitutes, with which iodine could def decreaseiciency this due risk. to Further,reduction salt-restrictive or elimination of saltand and plant-based animal dietsderived may befoods, associated respectively. with iodine We deficiency also mentioned due to reduction low-salicylate or elimi- diet, gluten-freenation of salt diet and and animal parenteral derived foods,nutrition respectively. that are We associated also mentioned with low-salicylate many nutritional diet, gluten-free diet and parenteral nutrition that are associated with many nutritional deficiencies. deficiencies. FigureFigure 1. 1. IodineIodine can can bebe ingestedingested through through the the diet diet and an dietaryd dietary supplements. supplements. The primary The primary sources sources of of iodineiodine are are salt, salt, seafood, fish, fish, algae, algae, milk, milk, and and dairy. dair Iodiney. Iodine is taken is taken up by theup thyroidby the thyroid gland and gland used and usedfor synthesisfor synthesis MIT (monoiodotyrosine)