Iodine status of New Zealand children post-fortification of bread with iodised salt

Abstract

Abstract Background: Iodine is an essential dietary nutrient needed for thyroid hormone synthesis. Thyroid hormones function to support the reproduction, growth and development of tissues, and control Basal Metabolic Rate (BMR). Low dietary intakes of iodine can result in iodine deficiency. Iodine Deficiency Disorders (IDD) is the collective term used to define the adverse health effects iodine deficiency inflicts on the body. Iodine deficiency is of particular concern in children and adolescents, as it can impair cognitive development and function. One method used to improve iodine intakes is iodine fortification (i.e. the addition of iodine to food and/or water). Until recently, iodine fortification in the form of iodised salt has been the only strategy implemented in New Zealand (NZ) to combat iodine deficiency. In 2009, mandatory fortification of bread with iodised salt was introduced in NZ to address the re-emergence of mild iodine deficiency. Objective: The objective of the present study was to assess the impact of mandatory fortification on the iodine status of NZ schoolchildren. Design: This was a cross-sectional survey of 445 schoolchildren aged 8-10 years living in Christchurch and Auckland, New Zealand, conducted from March-April 2015. Children completed an iodine-specific food frequency questionnaire (FFQ), which also included demographic questions. The FFQ was used to derive estimated iodine intakes from food groups, as well as an estimate of total daily iodine intake, with and without the inclusion of discretionary iodised salt. Participants provided a casual spot urine and finger-prick blood sample for the measurement of Urinary Iodine Concentration (UIC) μg /L and Thyroglobulin (Tg) μ/L, respectively, to determine iodine status. Results: The median UIC (25th, 75th percentile) of the children (n=415) was 116 (82, 158) μg /L, falling between the 100-199μg/L indicating adequate iodine status according to the WHO/ UNICEF/ ICCIDD population criteria. Furthermore, 5% of children had UIC <50μg/L and 39% had a UIC <100μg/L. Geometric mean UIC significantly differed for sex and ethnicity, but not age or salt type. Males had a higher UIC than females (126 vs. 107μg/L; P<0.001). Asian children had a higher UIC than Māori and NZEO (both 151 vs. 110μg/L; P<0.006 and P<0.002, repectivly). Estimated mean iodine intakes for two scenarios: ‘Food only’ and ‘Food plus iodised salt’ were 65μg/day and 101μg/day, respectively. The percentage of participants with intakes below the EAR for iodine (65μg/day and 75μg/day for 8 and 9-10 year olds, respectively) from ‘Food only’ was 72% and from ‘Food plus iodised salt’ was 23%. Fortified bread, bread products and bread based dishes were the primary contributors to iodine intakes in schoolchildren providing 52% of total iodine intake, or 27μg iodine/day. Conclusion: This study showed that compared to the median UIC of NZ schoolchildren reported pre-mandatory iodine fortification (68μg/L in 2002), the median UIC in this representative sample has increased to 116μg/L indicative of adequate iodine intakes. This study provides evidence that the introduction of mandatory iodine fortification of bread and bread products has improved the iodine status of NZ schoolchildren and indicates that NZ schoolchildren are now iodine sufficient.