Iodine status of New Zealand adults post mandatory fortification of bread with iodine

Abstract

Iodine is an essential nutrient and key component of the thyroid hormones, which play a role in the normal growth and development of the human body. Poor dietary iodine intakes can lead to a spectrum of iodine deficiency disorders including goitre, cretinism, hypothyroidism, congenital abnormalities and impaired mental function; all of which may affect growth and development at different life stages. In both developing and developed countries world-wide, a range of iodine fortification strategies have been introduced to combat the prevalence of iodine deficiency. In September 2009, mandatory fortification of bread with iodine was introduced in New Zealand to address the re-emergence of mild iodine deficiency in children and adults that has occurred over the past two decades. The aim of this study was to measure the iodine status and iodine intake of New Zealand adults 18-64 years of age following mandatory fortification of bread with iodine. The study was a cross-sectional survey of 301 adult men and women aged 18-64 years living in Dunedin and Wellington during February and November 2012. Participants were asked to complete a 24-hour urine collection, a combined demographic questionnaire and iodine-specific food frequency questionnaire (FFQ), and attend a clinic where height and weight was measured. The 24-hour urine collection was analysed for iodine and reported as median urinary iodine concentration (UIC) µg/L and median urinary iodine excretion (UIE) µg/day. The iodine-specific FFQ was used to derive an estimate of iodine intake with and without discretionary iodised salt use. The median UIC for all adults was 73µg/L (females: 67µg/L and males: 80µg/L), indicative of mild iodine deficiency (50 - 99µg/L) according to the World Health Organization (WHO)/United Nations Children’s Fund (UNICEF)/International Council for the Control of Iodine Deficiency Disorders (ICCIDD) population iodine status criteria. The mean urinary volume was 2.0L. As an estimate of iodine intake, the median UIE was 127µg/day. Estimated iodine intake using the FFQ, that included discretionary salt use, was 132µg/day. Iodine intake was found to be associated with UIC (p=0.040) and UIE (p=0.003); however bread iodine intake and iodised salt use were not associated with either UIC or UIE. Bread provides around 35µg iodine per day, and up to 50% of the total iodine intake for New Zealand adults. The iodine status of New Zealand adults has improved (from 53 to 73µg/L) since the introduction of mandatory iodine fortification, however not enough to meet the target of >100µg/L as recommended by WHO/UNICEF/ICCIDD. Since the WHO/UNICEF/ICCIDD criteria were based on healthy school-aged children over six years with a mean urinary volume half of that found in this study, caution should be taken in applying these criteria to the New Zealand adult population. The UIE is a better estimate of iodine in this population and suggests that that the iodine status of New Zealand adults is now adequate. Continued monitoring by New Zealand government agencies is critical to prevent a re-emergence of mild iodine deficiency in New Zealand adults.