Effect of iodine exposure on blood pressure among female children and women in Kathonzweni, Makueni, Kenya

Abstract

High or raised blood pressure (HBP) is a global public health issue and a major cardiovascular risk factor. While the Kenya StepWise survey of 2015 confirmed that more than half (56%) of Kenyans have never been screened for BP, renewed interests in hypertension in children and adolescents has resulted from the recognition that its presence in adults often has its roots at a younger age. It is also still controversial whether mild thyroid dysfunction affects BP. To understand if iodine status affects blood pressure, A two- arm treatment; two periods’ double-blind cluster randomized crossover study comparing the effect of low and high iodine in salt (within the iodization regulation levels) in women of reproductive age and school age girls (8-12years). Participants in clusters randomized to one of the two sequence groups: Arm1-(H2L) started with high iodine(50mg) in salt and changed to low iodine in salt after 3 weeks) while Arm2-(L2H) (started with low iodine (30mg) in salt and changed to high iodine in salt after 3 weeks – without wash out period). 171 women-girl pair participants were assessed for family history of chronic disease, age, level of education and anthropometrics. Urinary iodine, sodium, potassium, lithium and blood pressure were assessed weekly. Data were analysed using SPSS version 20.0. Student unpaired two sample t-test was used to compare the difference in BP levels while differences in the distribution of independent variables (low (30mg) and high (50mg) iodine intake) between groups was determined using Pearson’s chi-square or Fisher’s exact test for categorical variables. Coefficient of dispersion was used to understand the spread of lithium as a marker of intake while analysis of covariance using univariate General Linear Model was used to estimate the effect of the treatment adjusting for specific covariates identified to be significantly different between the two intervention arm sequences at baseline. In women, the total effect due to high dose iodine in salt was equivalent to -1.1 [95% CI: -4.0 to 1.9]; constant -2.7, (P=0.474) for Systolic Blood Pressure (SBP) and -3.7 [95% CI: -6.0 to -1.3] constant 1.03 (P =0.003) for Diastolic Blood Pressure (DBP). Regression analysis of treatment on BP showed a significant adjusted effect of high dose iodine in salt on DBP (P=0.001) but not SBP (P=0.474). The variability of the difference in SBP and DBP attributable to treatment was 0.4% and 8.1% respectively. In school age girls, the total effect due to high dose iodine in salt was equivalent to -1.68 [95% CI -4.49 to 1.12], (P=0.237) for SBP and -4.48 [95% CI -7.66 to -1.29] (P =0.006) for DBP. Univariate covariance analysis in General Linear Modelling showed statistically significant effect of treatment on DBP f=6.83, (P=0.010) but not for SBP f=1.38, (P=0.242) in the girls. General Linear regression on the net adjusted effect attributable to the treatment showed that the high dose of iodine was negatively associated with both SBP and DBP resulting in a positive role in lowering blood pressure in both girls and women. While salt is the main source of iodine, it is also known that sodium in the edible salt is also responsible for HBP worldwide. These findings highlight an inverse relationship between BP and Iodine intake. High iodine intake decreased BP. There is need for further in-depth research on the iodine and BP linkage and to understand whether restricting iodized salt intake may cause a decrease in iodine intake and could worsen HBP, rather than control it.