Swiss pilot study of individual seasonal fluctuations of urinary iodine concentration over two years: is age-dependency linked to the major source of dietary iodine?

Abstract

Objective: Because of known significant seasonal fluctuations of iodine concentration in cow's milk (ICM) in Switzerland (winter=summer (w=s) ICM ratio averages 5.6), we looked for seasonal variations and familial aspects in urinary iodine and sodium concentrations (UIC, UNaC). Design: Prospective sequential cohort investigation. Setting: University hospital. Subjects: Thirteen children (six girls, seven boys) aged 3 -10 y and their parents (n ¼ 9) aged 30 -47 y. Interventions: The volunteers collected 1729 spot urine samples (5.1996 -5.1998). UIC data from winter (UIC w : DecemberFebruary) and summer seasons (UIC s : July -September) were compared with UNaC and average milk consumption. Iodine intakes from milk and salt were modelized. Results: Highest seasonal fluctuations of UIC were found in six pre-school children (w=s UIC group average ¼ 1.56, significant), followed by seven school children (w=s UIC group average ¼ 1.24, N.S.); none existed in adults. UIC=UNaC showed corresponding seasonal fluctuations in children, but not in adults. Winter milk was an important iodine source for children, as proven indirectly by similar seasonal fluctuations of ICM, UIC, UIC=UNaC and an important part within UIC due to milk. Conclusions: Contribution to UIC from milk intake during winter was high in children (40 -50%) and lower in adults (about 20%). Compared with children, dietary habits of adults are more complex and their iodine supply depends mainly on iodized salt and not on milk, so the effect of seasonal ICM variations on UIC is less marked. Because of significant seasonal UIC fluctuations in consumers of fresh milk products in Switzerland, results of future UIC studies conducted during summer and winter seasons should be compared cautiously, especially in young children. Furthermore, consumption of milk is to be promoted, since, besides calcium and vitamins, it is an essential source of iodine. UNaC determination should be included in epidemiologic studies if the dietary source of iodine is questioned. Sponsorship: This work was supported by grants from the University Hospital in Bern, the Swiss Federal Office of Public Health, the 'Swiss National Foundation for Scientific Research' (32-49424.96), the 'Fondation Genevoise de Bienfaisance V Rossi di Montelera', the 'Schweizerische Lebensversicherungs-und Rentenanstalt' and the 'Schüpbach Foundation of the University of Bern'. European Journal of Clinical Nutrition (2003) 57, 636 -646. doi:10.1038=sj.ejcn.1601590 Keywords: iodine; season; milk; child; diet; salt *Correspondence: C Als, Division of Clinical Chemistry, Inselspital, University of Bern, CH-3010 Bern, Switzerland. E-mail: claudine.als@insel.ch Guarantors: C Als, B Zimmerli and M Haldimann. Contributors: CA had the initial idea, wrote and coordinated the project over several years. CA and BZ wrote the manuscript. MH and BZ did the calculations and the statistical evaluations. MH did the analyses of urinary iodine in his laboratory. All contributors helped in planning the study, evaluating the results a...

Quantification of urinary iodine: a need for revised thresholds

“…The unstable iodine supply over time has been explained by changes in alimentary habits, depending progressively more on foreign industrial food preparations with uniodized salt (Als et al, 1995). If the criteria recommended by WHO are considered, then the present data are the sixth confirmation of a resurgence of mild ID since the beginning of the 1990s (Als et al, 1995;Solca & Gerber, 1996;Als et al, 1997Als et al, , 2003Brander, 1997). Fortunately, and despite this situation, no increase of thyroid volume in school children in several Swiss regions has been registered in 1995-1996(Truong et al, 1997Bürgi et al, 1999), as well as in 1996-1998 (Als et al, 2003).…”

“…Following recently published knowledge and in conditions of a temperate climate, urinary iodine concentration is higher in winter than in summer months in milk-drinking subjects (Als et al, 2003). Owing to iodine enrichment of industrial cattle-food, iodine content of milk is higher in winter, when cattle is kept in stables, than in summer, when cattle weeds outside (Als et al, 2003). Despite our sampling in winter, iodine supply was nevertheless mildly to intermediately insufficient in Bern and Brussels, if the recommended daily intake of 1.18 mmol/day (150 mg I/ day) was considered as an optimum threshold (Delange, 1993).…”

“…Following recently published knowledge and in conditions of a temperate climate, urinary iodine concentration is higher in winter than in summer months in milk-drinking subjects (Als et al, 2003). Owing to iodine enrichment of industrial cattle-food, iodine content of milk is higher in winter, when cattle is kept in stables, than in summer, when cattle weeds outside (Als et al, 2003).…”

“…However, a highly significant difference was observed between I/cr or I/L in spot samples or 24 h collections and I/24h in 24 h collections. For evident reasons of feasibility, spot samples are used in most studies on iodine supply as well in industrialized countries as in developing countries (Follis et al, 1962;Bourdoux, 1988;Bürgi et al, 1999;Als et al, 2003). The fact that all spot samples were obtained at the beginning of each 24 h collection may represent a methodological bias.…”

“…When precise 24 h urine collections could be obtained, iodine output was presented as an excretion in mg I/day or better mg I/24h. In field studies, quantification of iodine has been most often deduced from readily available spot samples and expressed as a concentration in mg I/g creatinine (mg I/g cr, mmol I/mmol cr) or, lately, in mg I/dL (Follis et al, 1962;Vought et al, 1963;Jolin & Escobar del Rey, 1965;Bourdoux et al, 1985;Bourdoux, 1988Bourdoux, , 1998Delange et al, 1997;Als et al, 2003). However, many authors still confusedly use the term excretion instead of concentration or try to simply extrapolate the former from the latter or report mean values for data that are not normally distributed.…”