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 (41% versus 75% − 82% in studies from 1996 to 2021) [1, 24, 25, 26, 30] and in the Greek sample, it was about twice as high as in data from 2007, i.e. 37% versus 18% . However, it has to be taken into account that there are limitations when directly comparing these data since one previous study was conducted in a non-adult population  and in some of the studies, data on the iodised salt market segment for household use were collected [1, 26]. Nevertheless, the iodised salt use level in our study did not reach 90%, as recommended by the WHO and UNICEF in order to ensure an adequate iodine intake as part of several different measures . The low prevalence of reported iodised salt usage may be partly explained by the fact that the participants were not always 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 of iodine/iodised salt among the study population: for example and very surprisingly, participants in both countries did not often identify iodised salt as a good iodine source (question item 14), as seen in the results of the Rasch model, although iodised salt is one of the most important iodine sources in human nutrition. This is in contrast to the results of a qualitative study from Norway in bachelor students who often identified iodised table salt as a source of iodine (as they had seen the word on the table salt) , but in agreement with a study in South African adults .
Our results showed, in addition, that while the participants often correctly identified foods that do not serve as good iodine sources, they also seldom knew that milk makes a very important contribution to the iodine supply. This is in line with the aforementioned Norwegian study , in which only a few participants could name good iodine sources and knew that milk contains iodine. Despite the quite poor knowledge about iodine in that 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. The attitudes and behaviour towards milk and milk products consumption was not assessed in our study, it therefore cannot be concluded from our results whether the knowledge about the high iodine content of milk/ milk products leads to an increased iodine intake via an increased 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 who knew more frequently than the female students that elderly people are not at an increased risk of iodine deficiency (Germany) and that nuts and soy sauce are not considered good iodine sources (Greece). Female students, in contrast, in both countries had a much better specific knowledge about iodine deficiency and associated health risks and about population groups at risk of iodine deficiency, but there were no differences between female and male students in their knowledge about the foods that serve as good dietary iodine sources, except that German females knew more often than males that seaweed is an iodine source. In our previous study , which included six countries in Asia and Europe, however, males had a significantly better knowledge about iodised salt being a good iodine source than females.
Despite the better 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  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 [27, 29, 30], 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 about iodine before. It may be assumed that this lack of knowledge is possibly a general problem in young European adults. Moreover, at least in Germany - based on data from 1996  -, the problem of 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, has been recently decreasing 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 [17, 31]. Interventions to increase awareness and knowledge are especially important in countries with an endemic iodine deficiency, where the use of iodised salt is essential for contributing to an adequate intake, including for women who wish to become or are pregnant or lactating as their iodine status is crucial for fetal development .
The inadequate knowledge observed in this study is also worrying in the context of a growing interest in plant-based diets, especially among young and highly educated people [32, 33], since strict adherence to - for instance - a vegan diet could increase the risk of developing iodine deficiency . In our German study sample, 2% of the students indicated to follow a vegan diet, which is a proportion twice as high as the estimated percentage of vegans in the German population as a whole (1%) .
A food that has a substantial iodine content is seaweed, which is becoming increasingly popular, especially among vegans, in western countries [36, 37]. 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 . 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 may on the other hand also be at risk of an iodine excess. Only 25% of the vegans in our German study sample knew that their type of diet may pose a risk of iodine deficiency, and this was comparable to the participants who followed a vegetarian or other types of plant-based diets (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 the vegan participants consciously used iodised salt in their households. Nevertheless, this use level was still higher than that observed in those who followed a vegetarian or omnivorous diet, i.e. 36% of the vegetarians/flexitarians/pescetarians and 47% of the omnivores reported the use of iodised salt. Likewise, vegans 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 a better knowledge was associated with a more frequent use of seaweed by the vegans, but Eveleigh et al.  recently concluded from a systematic review that very high iodine intakes in vegans may be a result of seaweed consumption. It is particularly important that young adults, as all 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 was 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, the children reported that their second most important source of nutritional information about iodine were presentations of iodine at school, followed by media like the internet, magazines, radio and television .
Interestingly, most of the study participants were moderately or very interested in nutrition, but a higher interest in nutrition was not associated with the 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 fortified foods such as iodised salt, since some consumers consider fortified foods less natural and less healthy than ordinary unfortified foods [39, 40].
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 [41, 42]. 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 fortified foods (such as iodised salt) , which has been 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 . However, we cannot deduce the use of processed foods with iodised salt from our study.
This study provides preliminary insights into the use of iodised salt and iodine-specific knowledge of university students in Germany and Greece. This 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 use level of iodised salt were assessed in relation to the nutritional behaviour, at least in the German part of this study, which 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 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 participants´ knowledge between Germany and Greece could be easily assessed and visualized. Moreover, 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 . 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. The small number of participants, and especially the small number of vegans, was one of the limitations of this study. Another limitation was the fact that our data were based on self-reported information, thus, our particpants´ actual behaviour may have differed from the reported one. Moreover, our results could also have been influenced by methodological issues. For instance, since 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 whether the answer was given by guessing or coincidence in the case of not ticking an incorrect answer. 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 .
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 , which was done in our study. Yet, despite the uncertainties regarding the methodology of multiple choice questions for answering our questions, there is also uncertainty as to whether we have 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.