Background
Iodine is an essential trace element with vital functions for human growth, metabolism and mental development, which are mediated by the synthesis of thyroid hormones. Long-term iodine deficiency is associated with an increased frequency of thyroid disorders [
1,
2]. The consequences of especially severe intrauterine iodine deficiency have been known for a long time and include an increased risk of miscarriage and stillbirth, or even the most extreme form of iodine deficiency disease referred to as cretinism, although this is very rare nowadays [
1]. However, even a mild to moderate iodine deficiency in utero and in early life-stages may have a negative impact on the child´s development with long-term health consequences. A mild to moderate iodine deficiency in adults may lead to adverse effects secondary to hypothyroidism, including impaired mental function with decreased educability and reduced work productivity [
1,
2].
The European Food Safety Authority (EFSA) has set an adequate intake (AI) for iodine at 130 to 150 µg/day for adolescents from 15 years of age and adults and at 200 µg/day for pregnant and lactating women [
3]. The main factor responsible for not achieving iodine sufficiency is an inadequate dietary intake. This mainly occurs in populations living in areas with a low iodine soil content [
4], but it may also occur due to infrequent consumption of sea fish and a low use of iodised salt at the household level and, above all, by the food industry [
1].
In order to ensure an adequate intake and prevent iodine deficiency disorders (IDD), salt fortification with iodine has been implemented as a prophylactic measure in both Germany and Greece, following official recommendations and based on the universal salt iodisation (USI) strategy by the WHO and UNICEF. However, in many countries of the world—including Germany and Greece—the addition of iodine to salt occurs on a voluntary basis and is thus far from being universal at present [
1]. According to the latest national food consumption survey (Nationale Verzehrsstudie II; NVS II) that was conducted in Germany between 2005 and 2007, the main natural food sources contributing to iodine intake—in order of the most relevant sources—were milk and milk products, non-alcoholic beverages (water, coffee, tea, fruit juices, vegetable juices and lemonades because of the natural iodine content of the water) and sea fish. However, when being produced with iodised salt, processed foods such as meat, meat products and bread are the most relevant iodine sources [
5]. There are no recent data available about the main food sources of iodine from Greece [
6], but data from 1993 indicate that milk and milk products, meat and sea fish served as the most important contributors besides iodised salt [
7].
Despite improved socioeconomic conditions and the voluntary implementation of salt iodisation in Germany and Greece, which ameliorated the population´s iodine status over the past decades [
1,
8], recent data indicate a decreasing iodine status in German children and adults [
9‐
11] and a suboptimal iodine status in pregnant women in Greece [
12,
13].
A few international studies revealed a positive impact of good knowledge and awareness about iodine/iodised salt on the use of iodised salt at home and on iodine intake [
14‐
16], and a low level of knowledge has been suggested to be a risk factor for suboptimal iodine intake or iodine deficiency [
17‐
19]. We recently published results about knowledge, awareness and behaviour regarding salt and iodine among university students in European and Asian countries (Germany, Greece, Poland, Slovenia, Sri Lanka and Taiwan), with a major focus on salt use [
20]. The objective of the present study is to assess knowledge and awareness about the importance of iodine/iodised salt for health and its use at home, based on data from the German and Greek subpopulations of the original multicentre study. We also aimed at determining whether the (conscious) use of iodised salt was associated with age, gender, Body Mass Index (BMI) as well as with knowledge regarding iodine and related factors such as attendance of a nutrition course, interest in nutrition and the habit of reading food labels.
Discussion
The present data analyses were undertaken with the aim to evaluate the knowledge and awareness about iodine and (iodised) salt and the use of iodised salt among university students in Germany and Greece. Our study revealed a relatively low degree of iodised salt usage, with 41% in the German and 37% in the Greek study group. The use level in the German study group was thus only about half of that observed in previous studies (75%—82% in studies from 1996 to 2021) [
1,
23‐
26] and in the Greek sample, it was about twice as high as in 2007, i.e. 37% versus 18% [
1]. However, it has to be taken into account that there are limitations when directly comparing these data since one of the previous studies from Germany was conducted in a non-adult population [
24] and in some of the studies, data on the iodised salt market segment for household use were collected [
1,
25], which are not the same as consumption data. Nevertheless, the iodised salt use level in our study did not reach 90%, as recommended by the WHO and UNICEF [
1]. The low prevalence of reported iodised salt usage may partly be explained by the fact that many participants were not aware of the type of salt they were actually using. This was particularly evident in Greece, where 45% of the study group was not aware of the type of salt used at home. Yet, the results of this study also revealed an insufficient knowledge about iodine/iodised salt among the study population, illustrated for example and very surprisingly by the fact that participants in both countries did not often identify iodised salt as a good iodine source, although iodised salt is one of the most important iodine sources in human nutrition. This is in contrast to results of a study from Norway in bachelor students [
27], but in agreement with a study in South African adults [
28]. Moreover, our participants often correctly identified foods that do
not serve as good iodine sources, but—in line with results from the Norwegian study [
27]—seldom knew that milk makes a very important contribution to the iodine supply. Despite the overall quite poor knowledge about iodine in the Norwegian study, the students´ knowledge about milk being an important source of calcium, which is vital for bone health, was good. However, this did not lead to a frequent milk consumption, as the majority of those students reported to rarely consume milk or having omitted milk from their diet. From our results, it cannot be concluded whether a better knowledge about the relevance of milk/milk products for the iodine supply would actually have lead to an increased iodine intake via milk/milk products consumption.
The participants in our analysis were also more likely to correctly identify health conditions
not associated with a low iodine intake (Germany) or population groups
not at an increased risk of iodine deficiency (Greece) than to identify potential negative health consequences of iodine deficiency (poor cognitive development and thyroid disorders) or to indicate population groups that require additional iodine or are at an increased risk of iodine deficiency (pregnant or lactating women and vegans). This was particularly evident for the male students. Female students in both countries, by contrast, had a much better specific knowledge about iodine deficiency and associated health risks and about population groups at risk of iodine deficiency. With regard to foods that
do serve as good dietary iodine sources, there was only one difference observed between males and females, i.e. that German females knew more often that seaweed is an iodine source. In our previous study [
20], which included six countries in Asia and Europe, males—by contrast—had a significantly better knowledge about iodised salt being a good iodine source.
Despite the higher awareness of females about the impact of iodine on health outcomes and about the population groups at risk of iodine deficiency in our study, they did not use iodised salt more often than the males, which is in line with the results of others [
29] who also did not observe gender differences in the use of iodised salt. This discrepancy could be caused by the lack of ability to correctly identify good iodine sources and thus act appropriately when choosing foods. Yet, an adequate iodine intake is particularly important for women of childbearing age who may be at risk of an insufficient iodine supply, if they become pregnant.
The overall relatively poor knowledge observed in our study is in agreement with that seen in previous studies from Germany, Italy and the aforementioned study from Norway [
26,
27,
29], in which university and even medical students lacked knowledge about iodine-related issues. In the Norwegian study, some students even mentioned that they had never heard of iodine before [
27]. It may thus be assumed that the lack of knowledge observed in our study is a general problem in young European adults. Moreover, at least in Germany—based on data from 1996 [
26] —, inadequate iodine-related knowledge may either have persisted throughout the past decades or, after an improvement following the introduction of salt iodisation and the assumed greater awareness about iodine-related topics, may have decreased again in the past years. This is of concern since information, communication and education cannot only improve knowledge, but are also important to increase the use of iodised salt [
16,
30]. Interventions to increase awareness and knowledge are particularly important in countries with an endemic iodine deficiency, where the use of iodised salt and the consumption of foods rich in iodine is essential for contributing to an adequate iodine intake, especially for women who wish to become or are pregnant or lactating as their iodine status is crucial for fetal development [
1].
The inadequate knowledge observed in this study is also worrying in the context of a growing interest in plant-based diets, often among young and highly educated people [
31,
32], since strict adherence to a vegan diet could increase the risk of developing iodine deficiency [
33]. In our German study sample, 2% of the students indicated to follow a vegan diet, which is a higher proportion than the estimated percentage of vegans in the German population as a whole (between 0.1% and 1%, based on data/reports published in 2007 and 2015) [
34].
A food that has a substantial iodine content is seaweed, which is also becoming increasingly popular, especially among vegans, in Western countries [
35,
36]. As seaweed can vary greatly in its iodine content, there may be a risk of an excessively high iodine intake with negative health consequences when eating it [
37]. Vegans in our study identified seaweed more frequently than other participants (88% vs. 53% of vegetarians/flexitarians/pescetarians and 47% of the omnivores) as a relevant iodine source. However, this study did not allow to assess whether this better knowledge was associated with a more frequent use of seaweed by the vegans, but Eveleigh et al. [
37] recently concluded from a systematic review that very high iodine intakes in vegans may be a result of seaweed consumption. Thus, vegans—who do not consume animal products including milk or fish—may on the one hand be at an increased risk of iodine deficiency, but on the other hand also at risk of an iodine excess. Only 25% of our vegan participants knew that their type of diet may pose a risk of iodine deficiency, and this was comparable to the participants who followed another type of plant-based diet (23% of the vegetarians/flexitarians/pescetarians). Moreover, when considering that 75% of the vegans in our study identified iodised salt as a good iodine source, it is surprising that only 57% of them consciously used iodised salt in their households. Nevertheless, this use level was still higher than that observed in those who followed a vegetarian/flexitarian/pescetarian or omnivorous diet, i.e. 36% and 47%, respectively. It has to be acknowledged, however, that the number of vegan participants in our study was low (
n = 8).
It is important that individuals make food choices and related dietary decisions based on good information and knowledge in order to ensure an adequate nutrient supply.
Despite the rather poor knowledge about iodine and iodised salt both in the German and Greek samples included here, our results suggest that an increasing level of knowledge is associated with a more frequent conscious use of iodised salt, although, after adjustment for multiple factors (age, gender, BMI, attendance of a nutrition course, interest in nutrition, food label use and knowledge score), this was only significant in the Greek sample. The attendance of a nutrition course, however, was associated with a more frequent use of iodised salt in both countries, even after adjustment. Furthermore, in the German sample, the interaction of a higher knowledge (score) and the prior attendance of a nutrition course was linked to an even higher frequency of iodised salt use. This shows that nutrition courses may be important as sound and reliable sources of information and education about nutrition and health. Indeed, in a study from Italy in school children, it was reported that their second most important source of nutritional information about iodine—after media sources like the internet, magazines, radio and television—were presentations on iodine at school [
16].
Interestingly, most of our study participants were moderately or very interested in nutrition, but a higher interest in nutrition was not clearly associated with a more frequent use of iodised salt, which may partly be caused by ineffective and unsustainable ways of informing the public about iodine and iodised salt.
Furthermore, paying attention to nutrition information on food labels when buying food did not result in a higher use level of iodised salt in Germany and, unexpectedly, it was associated with an even lower use level in Greece. This could have been caused by reservations and misconceptions about additives and foods produced with iodised salt, since they may be considered less natural and less healthy than ordinary foods [
38]. This is also supported by results of a recent survey from Germany, in which the decreasing usage of iodised salt by the food industry was partly explained with a low acceptance of iodised salt by the consumers [
39]. However, we cannot deduce the consumption of foods produced with iodised salt from our study.
In some segments of the population, there is a trend towards the consumption of specialty salts like Himalayan salt, sea salt, fleur de sel or others, assuming that these salts are more natural or healthier. However, since they contain less iodine than iodised table salt, they contribute to a lesser extent to an adequate iodine status [
40,
41]. This information does not seem to reach consumers, since the participants in our study were not very likely to know that Himalayan salt has a lower iodine content than iodised salt. This is even more remarkable since less than half of the participants in both countries indicated to use iodised salt, and many of the students—especially in the Greek sample—used a rock type salt like Himalayan salt or other salts like sea salt or fleur de sel instead. This could also partly be rooted in a negative attitude towards food additives or foods produced with iodised salt [
38‐
40].
Strengths and limitations
This study provides preliminary insights into the use of iodised salt and iodine-specific knowledge of university students in Germany and Greece, which is important because there is a lack of data about the household use of iodised salt, especially among young adults. Apart from that, knowledge and the use level of iodised salt were assessed in relation to the nutritional behaviour, at least in the German sample of this study, which also should be examined in further studies. One strength of this study was the use of the same methodological approach including the procedure followed for the recruitment of the students and the same questionnaire in the participating countries, which allowed direct comparisons of the results between the two countries. Concerning the use of the Rasch modeling approach in our study, the advantage of this method was that differences in the participants´ knowledge between Germany and Greece could be easily assessed and visualised. In addition, when making assumptions about more general associations, the advantage of the Rasch model lies in the independence of the results from the sample selected, meaning for instance that the selection of persons from a defined population does not have to be random since the results can be transferred to all persons with a certain ability or knowledge [
42]. This is important for the validity of the results revealed from the Rasch model analysis, since the number of participants in our study was rather small. A limitation of our study was the fact that our data were based on self-reported information, thus, our participants´ actual behaviour may have differed from the reported one. Moreover, our results could have been influenced by methodological issues. For instance, since in our study in multiple-choice questions with more than one correct answer, not only the ticking of correct answers, but also the non-ticking of incorrect answers was defined as “correct”, it cannot be concluded with absolute certainty whether the participants gave the correct answers deliberately in every case or not. The problem of possibly not accounting for participants´ guessing in using the so-called number right scoring method—which is widely used in multiple-choice tests—is well-known, but this method has nevertheless been used for decades [
43].
A possible solution to minimise the effect of guessing on test reliability is to increase the number of items in the test, as well as the number of alternative choices in multiple choice questions [
43], which was done in our study. Yet, despite the uncertainties regarding the use of multiple choice questions, there is also uncertainty as to whether we made a valid classification of the participants in terms of their knowledge and have therefore measured their “true” level of knowledge. Furthermore, our study groups consisted only of university students in Berlin, Larisa and Thessaloniki and were thus neither representative of the student populations in Germany and Greece nor for the general populations in these countries. Therefore, in order to gain more representative and thus reliable results about the knowledge and awareness regarding iodine and iodised salt and the actual iodised salt usage at population level, it is desirable that representative data be collected in future studies.
Conclusions
The present study shows a relatively low level of conscious iodised salt use among university students in Germany and Greece. This may partly be explained with inadequate knowledge about the role of iodine/iodised salt in human nutrition and health. In addition, almost half of the participants in Greece were not able to recall the type of salt used at home, suggesting that a lack of awareness could have been another cause for the low use level observed.
Considering the overall rather unsatisfactory knowledge, it is emphasized that education on the importance of iodine should be offered consistently and continuously to obtain a sustainable effect. As the attendance of nutrition courses and a better knowledge about iodine/iodised salt had a positive impact on the use of iodised salt, it would be desirable that education about iodine be offered in nutrition courses in school curricula to ensure equal access to this information for all levels of education, preferably starting from an early age. There is also a need for widespread information campaigns on the importance of iodine in order to assure a sustainable awareness about iodine and related health matters in the public across all age groups, health care professionals, food producing companies and wide-reaching multipliers on social media platforms, as an information source particularly for younger people. In order to be effective, these information campaigns should be planned in a target-group specific manner.
A better knowledge about the importance of iodine and iodised salt for health could not only have a positive impact on the use of iodised salt in private households, but also improve consumer acceptance and demand for processed foods manufactured with iodised salt (such as bread, cheese or meat), thus contributing to an increased iodised salt usage in food manufacturing, which would eventually contribute to an improved iodine status of the population, including those at risk of iodine deficiency.
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