Effect and safety of salt iodization to prevent iodine deficiency disorders: a systematic review with meta-analyses.

Background: Iodine deficiency one of the most prevalent micronutrient deficiencies globally is the main cause of potentially preventable mental retardation in childhood as well as a spectrum of morbidities referred to as iodine deficiency disorders. Iodization of salt is recommended to prevent and treat many of these disorders. Objective: To assess the effects and safety of consumption of iodized salt in the prevention of iodine deficiency disorders. Data sources: The following databases were searched: China National Knowledge Infrastructure the Cochrane Library EMBASE MEDLINE VIP (the register of Chinese trials developed by the Chinese Cochrane Centre) The Virtual Health Library of the Pan-American Health Organization WANFANG the World Health Organization (WHO) International Clinical Trials Platform search portal and the WHO Global Health Library regional databases. Relevant agencies were contacted and the reference lists were reviewed. Study appraisal and synthesis methods: Randomized controlled trials (RCTs) non-RCTs quasiexperimental cohort and multiple cross-sectional studies were included. All studies compared a group of individuals with exposure to iodized salt to a group without exposure to iodized salt. Two or more reviewers independently screened potential studies extracted study characteristics and outcomes and when possible conducted meta-analyses to estimate the effect of iodized salt relative to non-iodized salt. Results are presented as mean differences (MDs) risk ratios (RRs) prevalence ratios (PRs) or odds ratios (ORs) with 95% confidence intervals (CIs). Results: Two RCTs 6 non-RCTs 20 quasi-experimental studies 16 cohort studies 42 multiple crosssectional studies and 3 studies with mixed designs were included. The numbers of participants ranged from 30 in a cohort study to over 5 000 000 in a multiple cross-sectional registry study. Iodized salt significantly reduced the risk of goitre (non-RCTs RR = 0.59 [95% CI = 0.36 to 0.95]; cohort RR = 0.30 [95% CI = 0.23 to 0.41]; multiple cross-sectional PR = 0.18 [95% CI = 0.14 to 0.22]) cretinism (multiple cross-sectional OR = 0.13 [95% CI = 0.08 to 0.20]) low intelligence (quasiexperimental RR = 0.28 [95% CI = 0.21 to 0.36]; multiple cross-sectional PR = 0.24 [95% CI = 0.07 to 0.82]) and low urinary iodine excretion (multiple cross-sectional PR = 0.45 [95% CI = 0.34 to 0.59]). Iodized salt significantly increased intelligence quotient (quasi-experimental MD = 8·18 [95% CI = 6·71 to 9·65]; multiple cross-sectional MD = 10.45 [95% CI = 4·79 to 16·11]) and urinary iodine excretion (cohort MD = 59.22 [95% CI = 50.40 to 68.04]; multiple cross-sectional MD = 72·35 [95% CI = 44·54 to 100.17]). The results regarding the potential adverse effect of hypothyroidism showed no relationship (cohort OR = 1.14 [95% CI = 0.84 to 1.53]; multiple cross-sectional OR = 1.13 [95% CI = 0.94 to 1.36]) and the results for hyperthyroidism were inconsistent and dependent on study design (cohort OR = 1.36 [95% CI = 1.12 to 1.66]; multiple cross-sectional OR = 0.96 [95% CI = 0.92 to 1.00]). The quality of evidence varied from very low to moderate depending on outcome and study design. Conclusions: This review showed that iodized salt has a large effect on reducing the risk of goitre cretinism low cognitive function and iodine deficiency. Robust monitoring of salt iodization programmes is important to ensure safe and effective levels of iodine consumption especially as countries implement programmes to reduce population salt intake.