Association between urinary iodine concentrations and insulin resistance in US adults
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Background: Iodine intake within the US population has declined in recent years. Mild iodine deficiency increases the risk of inadequate thyroid hormone production and there is growing evidence that sub-clinical hypothyroidism may give rise to outcomes disruptive to metabolic health including insulin resistance (IR). Objective: To investigate the association between urinary iodine concentrations (UIC), a measurement of iodine status, and IR in US adults. Method: Data from 1286 US adults (≥ 20years) in the NHANES 2011-2012 were used in the study. Two subgroups (low= UIC < 100μg/L and normal=UIC ≥ 100μg/L) were compared for measures of IR, including fasting blood glucose (FBG), hyperinsulinemia, and glycated hemoglobin. Chi-square test, simple linear regressions and multiple logistic regressions were used to test for statistical differences between the groups. Results: Median UIC (mUIC) of adults was optimal although women, high income earners, age groups 40-59yrs and non-iodine supplements users had significantly lower mUIC (P < 0.05). In males, no significant associations were noted between measures of IR and UIC. Females with low UIC were at greater risk for FBG ≥5.6mmol/L (adjusted odd ratio (AOR) = 1.73, 95% confidence interval (CI) = 1.09 – 2.72) while those with normal UIC were at greater risk for HbA1C ≥5.7% (OR= 0.56, 95% CI = 0.34 – 0.90) and HOMA-IR ≥ 2.6 (AOR= 0.56, 95% CI = 0.32 – 0.99). Conclusion: Our results partially support our hypothesis that UIC is associated with the odds of IR. Adult females with low UIC had a greater risk for elevated FBG, a marker of prediabetes, while those with normal UIC had greater risks for elevated HbA1C and HOMA-IR. Further investigations are warranted to elucidate the casual relationship between iodine status and IR.