Iodine is an essential nutrient for the synthesis of thyroid hormones, which are involved in regulating the body’s metabolism and they are required for normal growth and the development of the brain and central nervous system [1
]. Iodine deficiency in humans results in a variety of disorders depending on the severity of the deficiency and at what age the deficiency occurs [1
]. Sufficient iodine intakes are of pivotal importance during pregnancy and early childhood. Later in life, inadequate levels of iodine may result in goiter and other adverse health outcomes [1
]. On the other hand, excessive iodine intake has been associated with increased rates of thyroid dysfunction [1
]. Since both iodine deficiency and excess iodine exposure are associated with adverse outcomes, assessing population iodine status is an imperative public health initiative. To assess iodine status in a population, measurement of urinary iodine concentrations (UIC) is recommended, while estimations of intake may be done using dietary assessment methods and food composition data [5
Historically, studies conducted in Norway between 1914 and 1938 showed high prevalence of goiter, especially among people living in inland areas where intakes of fish were low. In some inland areas, almost 80% of schoolchildren had goiter [7
]. Preventive measures were implemented using voluntary salt iodization (5 µg/g). Further, iodine enriched cattle fodder was introduced, which resulted in dairy products with elevated iodine concentrations. Follow-up studies performed in the 1970s and 1980s showed a marked reduction in goiter [7
]. Up until now, no studies estimating iodine intake in both genders have been conducted in Norway. Recent studies in pregnant women suggest that the iodine intake in the general population has changed and may now be too low [7
]. The worldwide recommended strategy to secure sufficient iodine intake in a population is implementation of Universal Salt Iodization Programs [2
]. In Norway, neither industrial nor household salt have mandatory iodization.
People obtain iodine from the diet, their drinking water, and dietary supplements [14
], therefore, all dietary iodine sources are of importance when assessing total intake. Iodine is among the micronutrients with the largest variation in concentrations in foods; between different food groups, as well as between otherwise similar food items; i.e. between and within different species of seawater fish [14
]. This variation is due to several factors; as most of the earth’s iodine is in the seas, fish and seafood generally have high content of iodine and so have plant foods that are grown near the coast, as compared to plant foods grown in inland areas [14
]. In addition, changes in the environment in which fish live and in the processing of fish, changes in soil, use of fertilizers, changes in the composition and fortification of cattle and chicken fodder, and use of iodized salt in food processing, all influence the total iodine content in our diets [14
]. Furthermore, changes in food habits may influence the dietary intake of iodine and individuals whose diets exclude or restrict iodine-rich food sources, for health, religious, or other reasons are at a special risk of deficiency [17
Both the international research community [15
] and the Nordic Nutrition Recommendations of 2012 [19
] underline the need for better surveillance and more data on the level of iodine in foods. Studies in nutrition epidemiology are dependent on high quality food composition databases to make good estimations of intake. The reliability of estimates of iodine intake relies on the quality of the iodine food composition data [20
] and reliable and representative iodine food composition data is therefore an important prerequisite [15
]. Comprehensive and high quality data on food iodine content have been sparse; values have been lacking or have been obsolete [3
]. However, initiatives have recently been taken to fill the missing data gap and analytical food iodine content data are now emerging [17
]. Analysis of iodine in food and beverages has historically not been included in the Norwegian analytical food composition projects and has until recently only been done in a limited number of foods. Thus, there has been limited iodine food composition data available for Norwegian foods. This is, however, changing and recent analytical food composition projects now include determination of iodine.
In the present project, our first aim was to compile and update the iodine food composition databases at the University of Oslo and the Norwegian food composition table (matvaretabellen.no). The second aim was to use these data together with the latest national dietary survey among adults, in order to estimate the populations’ dietary iodine intake in both men and women and in different age groups.
We have compiled a comprehensive database on the iodine content of Norwegian foods, from analytical projects, other food composition databases and scientific food composition literature. Men were in better compliance with the iodine recommendations than women, yet nearly one in five males had high probability of inadequate iodine intake. In women, almost a third had high probability of inadequate iodine intake, and especially among the younger women the iodine intake was low. The present study shows that other food groups in addition to lean fish, milk, and dairy products contribute considerably to the total iodine intake. The present study adds important new knowledge to an area with sparse data, regarding the total iodine intake and the contributions of the different dietary iodine sources of Norwegians.
The low intake of iodine in women in the present study is in agreement with earlier studies showing a low iodine intake among women in Norway [7
]. The young women are at particular risk of inadequate iodine intake; only 41% had an iodine intake above recommended intake and among women age 18 to 29 years, 46% and 24% had high and very high probability of inadequate intake, respectively. Low iodine intakes in women in childbearing age have also been reported from other Nordic and European countries [8
The iodine intake in both men and women showed great inter-individual variation, different iodine food sources, and varying intakes across age groups, which indicate that the Norwegian population depends on a variety of dietary iodine sources to fulfill their iodine needs. Young men and women in the present study had not only lower intakes of iodine than older participants did, but also other food sources. In young adults, the most important iodine sources were milk, meat, and white cheese, while in the older age groups, the foods contributing mostly to iodine intakes were fish and especially lean fish and fish products, in addition to whey cheese, milk, and coffee. This difference in iodine sources is partly explained by the differences in intake of fish and milk products seen between young and older age groups in the Norkost 3 survey [31
] and by changes in the diet of the Norwegian population; while the intake of white cheese has increased, there has been a decrease in intakes of other milk products and fish [35
]. Dietary changes will influence the iodine intake of the population and the present findings are in agreement with the study of Katagiri et al. [36
], showing changing dietary patterns among younger men and women, resulting in lower intake of iodine as compared to the traditional Japanese diet.
Coffee is, in the present study, identified as a source of iodine in the Norwegian population. This is a consequence of the traditionally high consumption of coffee in Norway [31
]. Again, it is interesting to see a difference across age groups; the contribution of iodine from coffee is higher among the older participants of the population than among the young.
The iodine intake increased with age for both men and women, but still 12% and 27% of men and women, respectively, in the age group 60 to 70 years, had a relatively high probability of inadequate intake. In the other end of the intake distribution, 5% of the men in this study had iodine intake above UL 600 μg/d [19
], and this high intake was mostly due to iodine from fish. This upper intake level is however low when compared to the upper intake level defined for Americans at 1100 μg/day [37
] and 3000 μg/day set by Japanese dietary guidelines [21
There are several limitations with the dietary data used in the present study. The iodine recommendation refers to the average intake over a longer period of time [19
]. However, the present iodine intake data was based on two 24 h recalls, which provides dietary data on group level, but it is somewhat inadequate for estimations of individual habitual diet [38
]. Moreover, with two recalls, the intake of food items eaten infrequent or seldom most probably will be underreported. This may have implications for the estimations of iodine intake from fish in the present study, which may be underestimated.
To assess iodine status in a population, the WHO/UNICEF/ICCIDD recommend measuring UIC [2
]. The present study did however not collect urinary samples and may therefore only estimate iodine intakes without the evaluation of UIC [5
In Norway, table salt and salt used in food manufacturing are not fortified with iodine, and only a few commercially available table salts have iodine content (5 μg/g). The contribution of iodine from iodized salt is therefore very uncertain, but may contribute to the overall iodine intake in the population.
In the present compilation project the iodine value in milk was based on a large number of individual samples that were analyzed by the TINE SA dairy company and the IMR. Based on scientific literature on the production of milk and its content of iodine, which showed no significant differences in iodine content based on milk fat content, we used the average value of iodine in all milk types, irrespective of fat content [39
]. Furthermore, the development of new cattle fodder including soy-based ingredients has influenced the content of iodine in Norwegian milk recent years [42
], and thus only analytical values for the two most recent years were included in our iodine food composition database. Not surprisingly, whey cheese had a much higher content of iodine than white cheese. Iodine is mostly found in the water fraction of milk (the whey), and thus it is concentrated in whey cheese.
The iodine content of industrially produced bakery products may be higher than the values that we have compiled in this project. The reason is that between 2004 and 2016 at least 168 bread and other bakery products on the Norwegian food market, mostly imported products, were produced while using iodized salt in different concentrations, according to the NFSA list of fortification permits for bakery products that are imported and sold in Norway [44
]. We do not know how well the analytical samples, which are the basis of the present compilation, represent the total market of bread and bakery products, including the imported products. The average iodine content of bread and bakery products in our food composition database may therefore be too low, and therefore the estimations of iodine from bread may also be too low. Bread is a staple food in Norway, therefore new analyses of iodine in bread, both produced in Norway and imported products are warranted.
Up until recently, analytical food composition iodine values have been scarce. Fortunately, new initiatives are being taken and we look forward to the development of more and higher quality iodine food composition data from both Norwegian and international initiatives. Comprehensive coverage of iodine values will strengthen the intake estimations. We will continue to compile and update our iodine values in the future when new analytical values emerge from future analytical projects.