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?

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 3-10y and their parents (n=9) aged 30-47y. Interventions: The volunteers collected 1729 spot urine samples (5.1996-5.1998). UIC data from winter (UICW: December-February) and summer seasons (UICs: 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 = 1.56, significant), followed by seven school children (w/s UIC group average = 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- and Rentenanstalt' and the 'Schupbach Foundation of the University of Bern'.