Original Paper Ann Nutr Metab 1992;36:129-134

Departments of a Human Nutrition and b Experimental Biology, TNO Toxicology and Nutrition Institute, Zeist, The Netherlands; c General Practitioner, Brielle, The Netherlands

Key Words Iodine Surveillance Nutritional status Dietary factors

Indicators of Iodine Status among Adults Dutch Nutrition Surveillance System

Abstract To evaluate the iodine status of Dutch adults we used three different iodine status indicators, namely urinary 24-hour iodide excretion (1/24 h), iodide/creatinine ratio in 24-hour urine samples (I/Cr) and 24-hour iodide excretion per kilo­ gram body weight (I/kg). Additionally, the habitual daily iodine intake was calculated. Men had higher mean 1/24 h and mean iodine intake than women. No differences between men and women were found for mean I/Cr and mean I/kg. Rela­ tively high (> 18%) prevalences of low values for the different iodine status indicators were found, especially for 1/24 h. The intraindividual variance for all three indicators was high. Risk assessment regarding iodine supply, based on urinary iodide excretion, depends on the indicator used, and therefore cau­ tion is called for when drawing conclusions. Our results do not argue against the usage of 1/24 h as iodine status indicator. More research is needed to solve the question whether iodine requirement depends on such factors as body size, body com­ position and physical activity.

Introduction Sufficient intake of iodine, an essential trace element required for synthesis of the thyroid hormones, prevents goiter in most cases [1], Urinary iodide excretion is often used to assess iodine status [2-5], Subjects with an iodide excretion below a certain cutoff point

This study was financially supported by the Ministry o f Welfare, Public Health and Cultural Affairs of The Netherlands.

Received: May 2.1991 Accepted: December 18. 1991

are considered to be at risk of a marginal iodine supply and therefore of goiter and other iodine deficiency disorders [2, 5-7], It is unknown whether similar cutoff points are valid for different population groups. Factors like age. gender and physical state, as determi­ nants of body size and body composition, might affect iodine requirement [7, 8]. If so, iodide excretion relative to body weight or

M.R.H. Lowik Department of Human Nutrition TNO Toxicology and Nutrition Institute PO Box 360 ’ 3700 AJ Ze st (The Netherlands)

© 1992 S. Karger AG. Basel 0250-6807/92/ 0363-0129S2.75/0

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Johannes Bruga Michiel R.H. Low ika JaapJ. van Binsbergenc Jacobus Odinkb Rob J. Eggera Michel Wedela

Subjects In the autumn of 1981, a survey was conducted to study the association between sodium exposure and blood pressure [9], The study design consisted of a gen­ der-stratified random sample, taken from the Regis­ trar's Office, that accounted for 18% of the adult ( > 18 years) men and women of Brielle (a small town in the southwestern part of the Netherlands). The response rate was 62%. About 11 % of the respondents were excluded from the study because data were insufficient or unreliable due to use of specific drugs or abnormal nutritional habits. The study population available for analysis consisted of 824 subjects (representing almost

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11% of the adult population of Brielle). A complete data set was obtained for 812 subjects. Sampling design, investigation procedures and results concern­ ing blood pressure have been described in detail pre­ viously [9].

Data Collection and Methods Two self-administering, structured questionnaires, with an additional letter with information on the study, were sent out by the coordinating general practi­ tioner. The completed forms regarding socioeconomic characteristics and food consumption habits were checked during a visit at the physician’s practice. Dur­ ing this visit information on the health status of the respondents was collected, and anthropometric mea­ surements were taken. Urine containers were supplied with oral and written instructions on how to collect one 24-hour urine sample. From a subsample of the subjects in Brielle (n = 81) a second 24-hour urine sam­ ple was obtained and analyzed in order to assess within-person variation in urinary iodide excretion. The classification into socioeconomic categories was based on information on education, occupation and position of the respondent and the respondent's partner [11], Usage of food products known as potential dietaryiodine sources was assessed by means of a structured semiquantitative food frequency questionnaire which comprised 20 food groups (9, 12]. The subjects were asked to indicate the frequency in which the listed food items were usually consumed and to specify the con­ sumed portions in household measures (e.g. slices of bread, glasses of milk). Options for frequency were dai­ ly, 4-6 days a week. 1-3 days a week, once per 2 weeks, once per month and rarely/never. The consumption was calculated and expressed in grams per day. Con­ sumed amounts of a food item known to be a potential dietary iodine source [13-18] were multiplied by the mean iodine content of that food item obtained from food composition tables [ 19-21 ]. Addition of the re­ sults obtained for all iodine-containing food items resulted in an assessment of individual total iodine intake per day. The 24-hour urines were analyzed for iodide con­ centration by ion-pair reversed-phase high-perfor­ mance liquid chromatography with electrochemical detection [22]. Urinary creatinine concentration was determined with a Technicon Autoanalyser [23]. In this study the World Health Organization crite­ ria for low and marginal iodide excretions were used.

Brug/Löwik/Binsbergen/Odink/Egger/ Wedel

Urinary Iodide Excretion Parameters

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fat-free body mass might be better indicators for iodine status. To obtain more insight into the conse­ quences of using various iodine status indica­ tors with different cutoff points for risk as­ sessment, we used the data of an adult popula­ tion group surveyed in 1981 in Brielle, the Netherlands [9], Three iodide excretion pa­ rameters were used as indicators of iodine sta­ tus: 24-hour urinary iodide excretion (1/24 h), urinary iodide/creatinine ratio in 24-hour urine samples (I/Cr) and 24-hour urinary io­ dide excretion per kilogram of body weight (I/kg). Since iodine-containing hormones stimulate oxygen consumption in the cells and thus have a calorigenic effect [7, 8], iodine requirement might be related to body size, especially lean body mass. Urinary 24hour creatinine excretion is a crude indicator of the amount of muscle mass which makes up most of the fat-free mass [10], Factors known or expected to influence the different iodine status indicators were studied as well. The existence of particular associations be­ tween such factors and the iodide excretion parameters can provide additional evidence for the most appropriate iodine status indica­ tor. Furthermore, the intraindividual vari­ ance in 1/24 h was calculated, and its conse­ quences for risk assessment are discussed.

Table 1. Selected variables (means ± SD) according to gender Women (n = 440)

Men (n = 376)

Variable

44.5 ±15.9 77.3 ±9.8 1.02 ±0.56 0.059 ±0.032 0.013 ±0.007 167 ±66 79 ±24 1.10 ± 0.40 159 ±62

Age, years Body weight, kg Urinary iodide excretion, gmol/24 h Urinary iodide/creatinine, gmol/mmol Urinary' iodide/body weight, gmol/kg Urinary' sodium excretion, mmol/24 h Urinary potassium excretion, mmol/24 h Calculated total iodine intake, gmol/24 h Bread consumption, g/day

43.8 ±15.8 67.1 ±9.9* 0.77 ±0.42* 0.060 ±0.033 0.012 ±0.006 137 ±56* 66 ± 19* 0.89 0.38* 112 ±41 *

* p < 0.05: significant difference between men and women.

Statistical Methods Analysis of variance was used to study the impact of gender on the different iodide excretion parameters. Simple regression analysis was used to study the asso­ ciations of age, body weight, bread consumption (most bakeries use iodized bread salt in the Netherlands) and total iodine intake with the different iodide excretion parameters. The intraindividual variance in 1/24 h. based on 2 separate days, was estimated for a subsample (n = 81) of the Bridle study population using analysis of vari­ ance, A paired t test was carried out, testing fora signif­ icant difference between the first and the second 1/24 h obtained in this subsamplc. The univariate distributions of the iodine status indicators were skewed to the right. Therefore, the val­ ues of the iodine status indicators were transformed logarithmically prior to analyses. Data analysis was performed with the BMDP statistical computer pack­ age [24). Effects with a p value of less than 0.05 were considered to be significant.

Table 2. Prevalence (%) of marginal and low uri­ nary 1/24 h and of urinary I/Cr ratios based on 24-hour excretion and of urinary 1/kg according to gender Indicator

Men (n = 379)

Women (n ■=440)

1/24 h < 0 .3 9 gmol 1/24 h < 0 .7 8 gmol 1/Cr < 0.0 2 2 gmol/mmol I/Cr < 0.045 gmol/mmol I/kg < 0.004 gmol/kg I/kg

Indicators of iodine status among adults. Dutch Nutrition Surveillance System.

To evaluate the iodine status of Dutch adults we used three different iodine status indicators, namely urinary 24-hour iodide excretion (I/24 h), iodi...
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