Effect of salt reduction on iodine status assessed by 24 hour urinary iodine excretion in children and their families in northern China: a substudy of a cluster randomised controlled trial

OBJECTIVE: To study the effect of salt reduction on iodine status and to determine whether iodine consumption was still adequate after salt reduction in a population where universal salt iodisation is mandatory. DESIGN: A substudy of a cluster randomised controlled trial, with schools randomly assigned to either the intervention or the control group. SETTING: 28 primary schools in Changzhi, northern China. PARTICIPANTS: 279 children in grade 5 of primary school (mean age: 10.1); 553 adults (age: 43.8). INTERVENTION: Children were educated about the harmful effects of salt and how to reduce salt intake using the schools' usual health education lessons. Children then delivered the message to their families. The duration was 1 school term (≈3.5 months). MAIN OUTCOME MEASURE: Difference between the intervention and control groups in the change of iodine intake as measured by repeat 24 hour urinary iodine from baseline to the end of the trial. RESULTS: At baseline, the mean salt intake was 7.0±2.5 g/day in children and 11.7±4.4 g/day in adults and the median iodine intake was 165.1 μg/day (IQR: 122.6–216.7) and 280.7 μg/day (IQR: 205.1–380.9) in children and adults, respectively. At the end of the study, salt and iodine decreased in the intervention compared with control group. The mean effect on salt for intervention versus control was −1.9 g/day (95% CI −2.6 to −1.3) in children and −2.9 g/day (95% CI −3.7 to −2.2) in adults. The mean effect on iodine was −19.3% (95% CI −29.4% to −7.7%) in children and −11.4% (95% CI −20.3% to −1.5%) in adults. CONCLUSIONS: With ≈25% reduction in salt intake, there was a significant reduction in iodine consumption in northern China where salt is iodised. Despite this, iodine intake was still adequate, and well above the estimated average requirement. Our findings indicate that reducing salt to the WHO's target—30% reduction by 2025—will not compromise iodine status. TRIAL REGISTRATION NUMBER: ClinicalTrials.gov NCT01821144.