Abstract
Background: Iodine is an essential micronutrient that plays an important role in growth and development. Due to the low iodine content in New Zealand soils, iodine deficiency was common in the 1900s. The introduction of iodised salt in 1939 helped to improve with iodine status until the 1990s when iodine re-emerged again. The mandatory fortification of bread with iodised salt in 2009 has again shown an obvious improvement in the iodine status in children and adults. However, most studies have not directly measured iodine intake and identify the primary dietary sources that contribute to iodine intakes.
Objective: To determine the adequacy and sources of dietary iodine intake of adolescents aged 15 to 18 years old, and to compare intake and food sources between male and female adolescents.
Design: This was a cross-sectional study of adolescents aged between 15 and 18 years old in eight towns or cities across New Zealand. Data for females were collected in 2019 while data for males were collected in 2020. Participants completed online questionnaires, anthropometric measurements (including height and weight), and two 24-hour diet recalls. FoodWorks was used to convert foods to nutrients using the New Zealand Food Composition Tables. The usual iodine intake from diet only was determined using the Multiple Source Method programme. For those who self-reported iodised salt use in cooking or at table, 48µg iodine was added to their usual iodine intake. The contribution of major food groups to iodine intake, based on the Adult Nutrition Survey (ANS) food group classification, was also determined.
Results: Overall 266 female participants and 135 male participants were involved in the study. The majority of the adolescents were aged between 16 and 17 years old. Most identified as NZ European and Other ethnicity (57% in males and 78% in females). Most fell in the healthy weight range based on the BMI z-score (67% in males and 65% in females. The mean iodine intake from food alone for male adolescents was 107 µg/day and for female adolescents was 82 µg/day; the mean (95% CI) difference in iodine intake between the males and females was 25 (19, 32) µg /day. When iodine from the use of iodised salt was included, the mean iodine intake for males increased to 133 µg/day while for female adolescents increased to 103 µg/day. The mean energy intake for male adolescents was 10077kJ while for female adolescents was 7959kJ. When mean iodine intake was adjusted by energy intake, the difference in iodine intake between males and females was 0.42 (-0.39, 1.24) µg/day for food only, and 0.26 (-0.92, 1.45) µg/day for food plus iodised salt. Bread and bread-based dishes were the food groups contributing the largest proportion of iodine for both male and female adolescents, at 31% and 29%, respectively.
Conclusion: Iodine intake was higher for males than for females, which was due to male adolescents eating significantly more energy, and thus more foods, than female adolescents. Because females eat less food than males, the use of iodised salt makes an important and necessary contribution to an adequate iodine intake. If salt is used by adolescents, or by parents who cook for them, iodised salt is recommended.