The iodine intake level influences thyroid function in a population and both high and low iodine intake levels are associated with increased risk of diseaseReference Delange1, Reference Laurberg, Bulow Pedersen, Knudsen, Ovesen and Andersen2.
Dietary iodine content is decisive for iodine intakeReference Andersen, Petersen and Laurberg3, Reference Rasmussen, Ovesen, Bülow, Jørgensen, Knudsen, Laurberg and Perrild4. Water is a ubiquitous component of the diet and ground water is an important drinking water resource in many countriesReference Andersen5.
Tap water iodine content exhibits regional differencesReference Andersen5–Reference Pedersen, Laurberg, Nøhr, Jørgensen and Andersen7, which are associated with differences in population iodine intake levelsReference Rasmussen, Ovesen, Bülow, Jørgensen, Knudsen, Laurberg and Perrild4, Reference Pedersen, Laurberg, Nøhr, Jørgensen and Andersen7, Reference Munkner8. Also, the iodine content of drinking water has been shown to influence the occurrence of thyroid disease in populationsReference Felgentäger, Gerth and Fanghänel6, Reference Knudsen, Bülow, Jørgensen, Laurberg, Ovesen and Perrild9, Reference Bülow Pedersen, Knudsen, Jorgensen, Perrild, Ovesen and Laurberg10. However, thyroid disease may persist despite iodine-replete dietsReference Jahreis, Hausmann, Kiessling, Franke and Leiterer11, Reference Saikat, Carter, Mehra, Smith and Stewart12. This raises the question of iodine bioavailability.
The bioavailability of elemental iodine was demonstrated decades agoReference Keating and Albert13 and confirmed recently in a study that also demonstrated a reduced bioavailability of iodine in seaweedReference Aquaron, Delange, Marchal, Lognone and Ninane14, as found by othersReference Katamine, Mamiya, Sekimoto, Hoshino, Totsuka and Suzuke15.
Chemical analysis of natural waters has demonstrated variable amounts of humic substancesReference Andersen, Petersen and Laurberg3, Reference Thurman, Aiken, Mcknight and Wershaw16–Reference Nissinen, Miettinen, Martikainen and Vartiainen18, which bind iodineReference Andersen, Petersen and Laurberg3, Reference Christensen and Carlsen19 to form iodo derivativesReference Moulin, Reiller, Amekraz and Moulin20. This may modify the biogeochemical behaviour of iodineReference Santschi and Schwehr21 and a reduced uptake has been found in animalsReference Middlesworth22, Reference Harrington, Shertzer and Bercz23 and suggested in manReference Hurrell24, Reference Fordyce, Johnson, Navaratna, Appleton and Dissanayake25. Previous studies of tap water iodine content have looked into the association with thyroid disordersReference Felgentäger, Gerth and Fanghänel6, Reference Fordyce, Johnson, Navaratna, Appleton and Dissanayake25–Reference Amelsvoort28 and subsurface geologyReference Fordyce, Johnson, Navaratna, Appleton and Dissanayake25, Reference Amelsvoort28, while data on whether it is bioavailable are lacking.
We previously found that natural waters in Skagen had a high content of iodineReference Pedersen, Laurberg, Nøhr, Jørgensen and Andersen7, but that this was bound in humic substancesReference Andersen, Petersen and Laurberg3. In addition, we have identified a town with a low content of iodine in tap water, likely due to a different subsurface geochemistry.
We aimed to obtain data on the content of humic substances in tap water low in iodine compared with iodine-rich waters to evaluate the influence of iodine in humic substances in drinking water on the urinary iodine excretion levels on a population level and to assess if iodine bound in humic substances in natural waters is bioavailable.
Methods
Subsurface geology
The investigations were performed in two towns, Randers and Skagen, both situated on the peninsula of Jutland in Denmark. Skagen is situated in the northernmost part of Jutland, which still rises after deglaciation approximately 15 000 yeas ago. In combination with the apposition of sand by the North Sea to the northern tip of Jutland, this has built the Isthmus of Skagen. Consequently, the tap water from the waterworks of Skagen is derived from an aquifer source rock (buried sea floor) that contains marine depositsReference Andersen, Petersen and Laurberg3. In contrast, Randers is situated on the phenoscandinavian brim with no uplift of landReference Andersen5. Thus, the waterworks of the two towns, although being situated relatively close, differ by aquifer source rock, which was the background for selection of these two areas for investigation.
Water samples
Water samples were collected on 3 separate days from both locations. Furthermore, Skagen tap water was collected at 2-month intervals for 6 months and once every year for 4 consecutive years from 1997 to 20003. The iodine content was unaltered with timeReference Andersen, Petersen and Laurberg3.
Skagen had a high iodine content of tap water (three to four times sea water level)Reference Andersen, Petersen and Laurberg3 and Randers had a low iodine content of tap water.
Procedures and solutions
Tap water samples were collected in iodine-free polyethylene containers from the final tap before leaving the waterworks. Samples were kept dark at 4°C until freeze drying. Freeze-dried samples were stored in an oxygen-free environment until they were re-dissolved 1:10 in ultrapure water from a Mili-Q water purification system (Millipore Corporation, Billerica, MA, USA) for further analysis. Iodine content was unaffected by freeze drying when re-dissolved 1:1 (recovery 90–103 %; average 98 %).
HPLC size exclusion was performed on ÄKTApurifier™ (Amersham Pharmacia Biotech, Freiburg, Germany) using a Superose 12 HR 10/30 column (Amersham Pharmacia Biotech) as described in detail previouslyReference Andersen, Petersen and Laurberg3. This is an agarose gel with exclusion limits from 1000 to 300 000 D (limits stated by supplier). Raw or resuspended tap water (500 μl) was added to the column after filtering through a 0·20 μm membrane (Minisart®; Sartorius, Göttingen, Germany) to eliminate particulate matter. Identical iodine concentrations were seen before and after filtering. Tris buffer 10 mm, pH 7·0 was eluent. Elution speed was 1 ml/min and pressure was 1·45–1·52 MPa. Absorbance at 280 nm was registered and effluent was collected in fractions of 1·5 ml. Experiments were carried out at 21°C and performed in triplicate.
Populations
Participants were men and women living in the towns of Randers or Skagen in Jutland, Denmark. Names and addresses were obtained from the national civil registration system, in which all individuals living in Denmark are recorded. In Randers, all men and women born in 1920 were invited to take part (n 483). Skagen had a smaller population and all individuals born between 1918 and 1923 were invited to participate (n 432).
The investigation took place at the local hospital (90 %) or, at request, as home visits (10 %) in 1997 and 1998, i.e. before the recent iodine fortification of salt in DenmarkReference Laurberg, Jørgensen, Perrild, Ovesen, Knudsen, Pedersen, Rasmussen, Carlé and Vejbjerg29. Information about medication and the use of iodine containing vitamin and mineral preparations was collected by a questionnaire. Among these long-time Skagen dwellers, 95 % had lived in the town for more than 10 years and 91 % for more than 30 years. Also, this population had retired and thus had the time to participate. In addition, the intake of water and other liquids was knownReference Andersen, Fagt, Groth, Hartkopp, Møller, Ovesen and Warming30.
The study was approved by the regional ethics committee for Nordjylland and Viborg County.
Urine
At the interview, a non-fasting spot urine sample was collected from all participants. Urine samples were frozen and stored at − 20°C in iodine-free polyethylene containers for subsequent measurements of iodine and creatinine.
Iodine and creatinine determination
Iodine was determined by the Sandell-Kolthoff reaction modified after Wilson and van ZylReference Wilson and van Zyl31 as described previouslyReference Laurberg32, Reference Andersen, Hvingel, Kleinschmidt, Jørgensen and Laurberg33. The principle is evaporation and alkaline ashing of the sample followed by resuspension and measurement of iodine by the spectrophotometric detection of the catalytic role of iodine in the reduction of ceric ammonium sulfate in the presence of arsenious acid. For determination of iodine content, a 1·5 ml sample was used giving an analytical sensitivity of 1·0 μg/l. The intra-assay CVs were 9·2 % (interval 2–4 μg/l, n 8); 8·7 % (interval 5–9 μg/l, n 4); 4·2 % (interval 10–15 μg/l, n 4); 1·5 % (interval 15–50 μg/l, n 5). Urinary creatinine was determined by a kinetic Jaffé methodReference Bartels, Bohmer and Heierli34.
Urinary iodine excretion was expressed in μg/l and as an estimate of the 24 h urinary iodine excretion by adjusting the iodine:creatinine ratio for the average 24 h urinary creatinine excretion in an age and gender matched group of Caucasians (men 0·95 g/24 h; women 0·7 g/24 h)Reference Kampmann, Siersbæk-Nielsen, Kristensen and Mølholm Hansen35, Reference Kesteloot and Joossens36.
Statistical analysis
The χ2 test was used to compare frequency among populations. Mann Whitney U-test was used to compare the median iodine content of samples. Bartlett test for homogeneity of variance was used for comparing variances. Factors important to urinary iodine excretion were tested in a multivariate logistic regression model with urinary iodine excretion entered as dependent variable. Explanatory variables entered were gender, origin of tap water, use of iodine-containing supplements and the lifestyle factors smoking and alcohol use. A P-value of < 0·05 was considered significant.
Results
In Skagen, the mean iodine content of raw water was 152·7 ( ± 4·0) μg/l. It was 139·7 ( ± 5·2) μg/l after water treatment consisting of aeration, sedimentation and chemical coagulation before sedimentation, adsorption in granular activated carbon contractors and tandem sand filtration. Thus, the extensive water treatment reduced the iodine content by 8·5 %. In Randers, the average tap water iodine content was 2·0 μg/l.
Fig. 1(b) shows Skagen tap water with a high content of iodine bound in macromolecules (Fig. 1(a)) previously found to be humic substancesReference Andersen, Petersen and Laurberg3, Reference Grøn, Wassenaar and Krog17. Randers tap water had a low iodine content (Fig. 1(d)) not bound in humic substances (Fig. 1(c)).
Fig. 1 HPLC size exclusion performed on preconcentrated tap water from Randers (left) and Skagen (right). Elution of iodine measured (b,d) and absorbency at 280 nm was registered (a,c). For details of subjects and Procedures, see Methods.
Table 1 shows the characteristics of the 430 participants from Randers and Skagen. The participation rate was 43·9 % in Randers and 50·5 % in Skagen. An equal number of long-term Randers- and Skagen-dwelling men and women participated from the two towns (χ2; P = 0·98). More women (n 264) than men (n 166) participated in accordance with the demographic characteristics of the population aged 75 to 80 years. Iodine-containing supplements were taken more frequently in Randers than in Skagen (χ2; P = 0·003).
Table 1 Descriptions of the participants from the two towns, Randers and Skagen, in Jutland, Denmark§
* 12 no answer
† 5 no answer.
‡ 1 no answer.
§ For details of subjects and procedures, see Methods.
Urinary iodine excretion in participant groups is shown in Table 2. Urinary iodine excretion among participants not taking iodine-containing supplements was markedly lower in Randers than in Skagen (Mann–Whitney, P betweentowns < 0·001). Participants with a daily use of iodine-containing supplements had a 66 μg/24 h (Randers) and 54 μg/24 h (Skagen) higher urinary iodine excretion, with no difference between towns (Mann–Whitney, P = 0·88). The fraction of urine samples with iodine excretion below 100 μg/24 h, i.e. the level set to discriminate iodine deficiency when investigating groups37, was higher in Randers than in Skagen (Table 2, both in participants with no use of iodine-containing supplements and all participants: χ2, P < 0·001). Conversely, the fraction of samples indicating more than adequate iodine intake (>200 μg/24 h) was clearly higher in Skagen (χ2; P < 0·001), as was the fraction of samples indicating excessive iodine intake (>300 μg/24 h) (χ2, P < 0·001), as can be seen in Table 2. Expressing the results in μg/l did not alter the results of comparisons.
Table 2 Urinary iodine excretion among participants from the two towns Randers and Skagen in Jutland, Denmark. The two towns have a different aquifer source rock, that caused different iodine content of drinking water: 2 μg/l in Randers and 140 μg/l in Skagen¶
* 205 urine samples available.
† 218 urine samples available.
‡ Corrected for age and gender specific creatinine excretions (men 0·95 g/l; women 0·7 g/l).
§ Excluding also individuals treated with thyroxin (n 23).
‖ n Randers 108; n Skagen 131.
¶ For details of subjects and procedures, see Methods.
Fig. 2 shows the distribution of estimated 24 h urinary iodine excretion among all Randers and Skagen dwellers. The difference was marked, even when blurred by the use of iodine-containing supplements, and it was also significant for both median (Mann–Whitney; P < 0·001) and dispersion (Bartlett test; P < 0·01) among supplement users.
Fig. 2 Distribution of estimated 24 h urinary iodine excretion among old Randers and Skagen dwellers at intervals of 25 μg/24 h: Randers (
); Skagen (■). Dispersion differed between towns (Bartlett's test; P < 0.01) as did median (Mann–Whitney; P < 0.001). For details of subjects and procedures, see Methods.
The factors important for urinary iodine excretion (>100 μg/24 h) were origin of tap water (reference:Randers; OR 72; 95 % CI 33, 153) and use of iodine-containing vitamins (reference:no users; OR 6·4; 3·4, 12) when tested in a multivariate logistic regression model. This suggests that iodine in humic substances in natural waters was absorbed. Gender may be important for iodine intake (reference:women; OR 1·8; 1·0, 3·3) while the lifestyle factors smoking (reference:no smoking; OR 1·4; 0·7, 2·7) and alcohol intake (reference:no alcohol; OR 1·0; 0·4, 2·5) were not.
The average creatinine concentration in spot urine samples was 856 mg/l in men and 556 mg/l in women, with no difference between towns (Mann-Whitney; men P = 0·54; women P = 0·45). This creatinine excretion suggests a 24 h urine volume of 1·26 litres in women and 1·11 litres in men. Weighted according to 264 women and 166 men, this equals an average of 1·2 litres in this population, in keeping with previous findingsReference Andersen, Fagt, Groth, Hartkopp, Møller, Ovesen and Warming30.
Table 3 shows an estimate of the bioavailability of iodine from natural waters calculated from differences between Randers and Skagen. The difference in iodine content of drinking water was 138 μg/l. If all drinking water iodine was bioavailable, the difference in the contribution to iodine intake from tap water would be 166 μg/24 h (138 μg * 1·2 l/24 h), of which 90 % (149 μg) was excreted in the urineReference Keating and Albert13. The difference in estimated 24 h iodine excretion in urine between Randers and Skagen dwellers was 127 μg/24 h. Hence, the fraction of iodine available can be calculated as 127 μg / 149 μg = 0·85, suggesting a bioavailability of 85 % (93 % and 79 % with a tap water intake of 1·1 and 1·3 litres respectively).
Table 3 Estimated bioavailability of iodine bound in humic substances in natural waters, calculated iodine excretion among long-time Randers and Skagen dwellers not using iodine-containing supplements or thyroxine (n 239)**
* Measured.
† Calculated: Skagen ! Randers.
‡ Calculated from creatinine and references 7 and 30, and includes tap water, coffee, tea and flavoured water.
§ Calculated: difference in tap water iodine * tap water ingestion.
‖ Urinary iodine excretion is approximately 90 % total iodine excretion.
¶ Calculated from u-iodine-difference: measured/expected * 100.
** For details of subjects and procedures, see Methods.
Discussion
Iodine, element no. 127, is involved in the cycle of organic matter in most surface environments. It has a biophilic nature and is abundant in marine environments, where sediments are particularly rich in iodineReference Aquaron, Delange, Marchal, Lognone and Ninane14, Reference Shishkina and Pavlova38, Reference Fuge39.
Northern Europe experienced several ice ages over the Quaternary period. Ice depressed the Earth's crust by up to several hundred metres. When the ice melted, the land rose after a delay. During this delay, sea flooded the deglaciated terrain. This was followed by an uplift exposing large areas of sea floor and marine deposits have been found up to 60 m above sea levelReference Andersen5.
The northern part of the peninsula of Jutland in western Denmark still rises and the most northern part, the Isthmus of Skagen, is less than 15 000 years old. The Skagen waterworks uses shallow wells. Thus, the aquifer source rock is marine sediments and Skagen tap water was previously shown to contain humic substancesReference Grøn, Wassenaar and Krog17 with iodineReference Andersen, Petersen and Laurberg3. The binding of iodine in organic matterReference Moulin, Reiller, Amekraz and Moulin20 has been hypothesized to modify bioavailability of iodine in manReference Moulin, Reiller, Amekraz and Moulin20, Reference Santschi and Schwehr21 and thus the occurrence of thyroid disorders in a populationReference Laurberg, Bulow Pedersen, Knudsen, Ovesen and Andersen2.
It has been demonstrated that 90 % of elemental iodine disposed from the human body is excreted in the urineReference Keating and Albert13 and, when a steady state is present, iodine in urine is generally accepted as a measure used to estimate bioavailable iodine in the dietReference Jahreis, Hausmann, Kiessling, Franke and Leiterer11, Reference Aquaron, Delange, Marchal, Lognone and Ninane14, Reference Katamine, Mamiya, Sekimoto, Hoshino, Totsuka and Suzuke15, Reference Harrington, Shertzer and Bercz23, Reference Hurrell24, Reference Sivakumar, Brahamam, Madhavan, Ranganathan, Vishnuvardhan, Vijayaraghavan and Krishnaswamy40.
The availability of dietary iodine was found to be incomplete in miceReference Middlesworth22 and ratsReference Harrington, Shertzer and Bercz23. In man, complete absorption of elemental iodine has been demonstratedReference Keating and Albert13. Also, iodine added to salt showed a completeReference Nath, Moinier, Tuilier, Rongier and Desjeux41 or near-complete absorptionReference Aquaron, Delange, Marchal, Lognone and Ninane14. A review of the bioavailability in man of naturally occurring iodine concluded that data on organically bound iodine were lackingReference Hurrell24. The relevance of the question has since been emphasized by the finding of a missing relationship between iodine in the environment and endemicity of goitre, in the UKReference Saikat, Carter, Mehra, Smith and Stewart12, on the continentReference Felgentäger, Gerth and Fanghänel6, Reference Jahreis, Hausmann, Kiessling, Franke and Leiterer11 and in the USAReference Gaitan42.
About 10 % of disposable iodine is excreted with faecesReference Jahreis, Hausmann, Kiessling, Franke and Leiterer11, Reference Hurrell24. Humic acids are absorbed in the gastrointestinal tract, where some enter the enterohepatic circulationReference Visser43. Thus, iodine in the humic substances in drinking water may be retained in this circulation and excreted in faeces. We did not measure iodine in faeces, but this provides an explanation for a reduced bioavailability of iodine bound in humic substances.
In keeping with these considerations, we calculated that approximately 85 % of iodine bound in humic substances in Skagen tap water was bioavailable on a population level. Concentration of tap water during boiling or other food processing may lower this estimate. Hence, it is similar to the findings by Aquaron et al. Reference Aquaron, Delange, Marchal, Lognone and Ninane14 that between 60 and 85 % of the iodine in seaweed was bioavailable. The available fraction of iodine in solid foods varied markedlyReference Katamine, Mamiya, Sekimoto, Hoshino, Totsuka and Suzuke15, while our previous finding that iodine in drinking water correlated with urinary iodine excretion across townsReference Pedersen, Laurberg, Nøhr, Jørgensen and Andersen7 suggested that the bioavailable fraction varies within certain limits between tap waters. This is in keeping with the present findings in tap water from Skagen.
There are limitations to our estimate of the bioavailability of iodine bound in humic substances. First, the volume of tap water ingested was from food tables for DenmarkReference Andersen, Fagt, Groth, Hartkopp, Møller, Ovesen and Warming30. It was, however, in keeping with the intake estimated from population spot urine creatinine. Second, urinary iodine content varies considerably due to factors such as dilution and variations in diet. To overcome some of the variation caused by dilution, we used estimated 24 h urinary iodine excretion as the correction for age and gender specific creatinine excretion diminishes this variation and describes the actual iodine excretion more accuratelyReference Andersen, Hvingel, Kleinschmidt, Jørgensen and Laurberg33, Reference Jolin and Escobar del Rey44–Reference Andersen, Pedersen, Pedersen and Laurberg46. Third, to overcome some of the dietary variations, we used differences in the iodine content of tap water and urine between towns in our calculations, as no systematic differences are known to exist between the populations in these two towns located in the same part of Denmark. Fourth, the Randers and Skagen dwellers differed in the use of iodine-containing supplements. Thus, individuals using iodine-containing supplements were excluded from the calculations in addition to thyroxin users. Hence, the difference in aquifer source rock for drinking water supply provided a method for estimating the bioavailability of naturally occurring iodine bound in humic substances in tap water on a population level. Still, a balance study including individuals with a fixed intake of organically bound iodine, i.e. Skagen tap water, is needed to detail the bioavailability.
The study was carried out just prior to the initiation of the Danish iodine supplementation programmeReference Laurberg, Jørgensen, Perrild, Ovesen, Knudsen, Pedersen, Rasmussen, Carlé and Vejbjerg29 and the iodine excretion shows the iodine intake from the natural diet in Denmark. Living in Randers caused a low iodine intake level but is likely to have increased. Among Skagen dwellers, the iodine excretion was markedly higher and 36 % of samples were in the range of more than adequate iodine intake. Thus, even though living in Skagen provided a stable iodine intake within the recommended range for the majority of the population, more than one third of samples suggested an iodine excretion above adequate37, which may adversely affect thyroid functionReference Laurberg, Bulow Pedersen, Knudsen, Ovesen and Andersen2. An increase may be considered an adverse effect of iodine supplementation and a study of the occurrence of thyroid dysfunction in this population is warranted. Also, this should consider that humic substances may influence thyroid functionReference Gaitan47.
The finding of high iodine excretion among Skagen dwellers is unique in Denmark and illustrates that iodine monitoring programmes, despite thoroughly considered design and keen efforts to portray population iodine status, may miss subgroups.
The use of iodine-containing supplements influenced iodine excretion markedly in individuals. It was associated with an equal increase in urinary iodine between towns. Thus, neither the humic substances nor the differences in intake of naturally occurring iodine seems to influence the availability of iodine from supplements.
In conclusion, iodine in humic substances was present in tap water from the Skagen aquifer only. This naturally occurring organically bound iodine influenced population iodine intake, the bioavailability was assessed to be about 85 %. The increase in iodine excretion associated with intake of iodine-containing supplements was unaffected by the humic substances and it reduced the number of samples in the range set to define iodine deficiency. Also, it increased the fraction of samples that indicate more than adequate and excessive iodine intake in Skagen with a high content of iodine in drinking water. This may be of concern and a study of thyroid function in this population is warranted.
Acknowledgements
We appreciate the aid from the Skagen municipality and from Frederikshavn-Skagen Hospital.
