A meta-analysis combining parallel and cross-over randomized controlled trials to assess impact of iodine fortified foods on urinary iodine concentration among children.

The purpose of this analysis was to combine evidence from parallel and cross-over randomized controlled trials to assess the impact of iodine fortified foods on urinary iodine concentration (UIC) in children. A structured search for studies on iodine intervention studies on MEDLINE, Pro Quest, and the Cochrane Library from Jan, 1990 to Dec, 2012 was carried out. Carry-over effect was estimated by general linear model. We explored two methods to pool continuous outcomes in a meta-analysis by combining parallel and cross-over trial designs. The standard mean difference was calculated for net change in UIC. Fixed or random-effects models were used to summaries fortified food response data. Meta-regression and covariate meta-analysis were performed to explore the influ-ence of confounders on the net pooled effect on UIC. The overall pooled estimate, which combined parallel with cross-over trials in the absence of carry-over effect of UIC from 9 studies, showed a significant increase in the fortified group compared with the control group (n=3448; standard mean difference=2.02 μg/L; 95% CI: 1.30, 2.73; I2=99%, τ2=1.81, p<0.01). Meta-regression analysis indicated that dose of the feeding was positively related to the effect size (regression coefficient=0.014; 95% CI: 0.003, 0.026; p<0.019). The net pooled effect size after removing the confounders was 1.59 (95% CI: 0.953, 2.23) μg/L. There was an association between intakes of io-dine fortified foods and UIC in children. These results suggest that we can combine parallel with cross-over trials for meta-analysis for nutrients such as iodine when absorption is high.