The iodine status of 8 to 10 year olds in New Zealand following the mandatory fortification of bread with iodised salt

Abstract

Background: Iodine is an essential trace element needed for the synthesis of thyroid hormones by the thyroid gland. Inadequate intakes of iodine can lead to a spectrum of detrimental outcomes at all stages of the life cycle, including cretinism, hypothyroidism, goitre, and impaired mental and physical growth. New Zealand has naturally low levels of iodine in its soils and studies have shown that iodine deficiency, seen in the early 1900s, re-emerged in the 1990s. Mandatory fortification of bread with iodised salt was introduced in 2009 to improve iodine status.

Objective: The aim of this study was to determine whether mandatory fortification of bread with iodine has improved the iodine status of 8 – 10 year old children in New Zealand.

Design: The study conducted between February and June 2015, was a cross-sectional survey of 415 children aged eight to ten living in Auckland and Christchurch. Participants were asked to provide a spot urine and fingerpick blood sample, and to answer a questionnaire that included questions on socio-demographics and a food frequency questionnaire (FFQ). Height and weight were also measured. Urine was analysed for iodine content and is reported as the median urinary iodine concentration (UIC). The iodine-specific FFQ was used to estimate the iodine intake from main food sources including bread and reported with and without the inclusion of discretionary iodised salt use. Blood samples have not been analysed and are not presented in this thesis.

Results: The median UIC for all children was 116μg/L (females: 107μg/L ; males: 131μg/L ), indicative of adequate iodine status (i.e., 100 to 199μg/L ) according to the World Health Organization (WHO), United Nations Children’s Fund (UNICEF) and International Council for the Control of Iodine Deficiency Disorders (ICCIDD) population criteria. It is a significant increase from the 66μg/L observed in children prior to mandatory fortification in 2002. Ethnicity and gender were significantly associated with UIC (P=<0.001 and 0.006, respectively). Age, school decile and use of iodised salt were not significantly associated with UIC (P=0.457, 0.705 and 0.890, respectively). The mean estimated iodine intake (with the inclusion of salt use) was 101 μg/d (females: 101 μg/d; males: 102 μg/d), with 23% of the children having an iodine intake below the Estimated average requirement of 75 μg/d. Bread contributed to 51% of iodine intake in the food-only model, providing a mean intake of 35 μg/d. Conclusion: These results are comparable to a similar study conducted in 2011 of Dunedin and Wellington children that reported a median UIC of113 μg /L and taken together with the current study, provide convincing evidence that the mandatory fortification of bread with iodine has successfully improved iodine status in New Zealand children.