Validation and reproducibility of a new iodine specific food frequency questionnaire for assessing iodine intake in Norwegian pregnant women

Abstract

BackgroundIodized salt is not mandatory in Norway, and the permitted level of iodine in table salt is low (5 μg/g). Thus, milk and dairy products, fish and eggs are the main dietary sources of iodine in Norway. Mild-to-moderate iodine deficiency in pregnant women has been described in several European countries, including Norway. There are few validated tools available to assess iodine intake in an efficient manner. The aim of the current study was to assess the validity and reproducibility of a new iodine-specific food frequency questionnaire (I-FFQ) in Norwegian pregnant women.MethodsAn I-FFQ consisting of a total of 60 food items and the use of supplements was developed to assess iodine intake and was administrated to 137 pregnant women at gestational week 18–19. Reference methods were a structured 6-days iodine specific food diary, urinary iodine concentration (UIC) (pooled sample of spot UIC from six consecutive days), and thyroid function tests. Correlation analyses, Cohen’s weighted kappa, Bland-Altman plots, and linear regression analyses were used to assess validity. Reproducibility of the I-FFQ was assessed in a subgroup (n = 47) at gestational week 35–36.ResultsThere was a strong correlation between estimated iodine intake from the I-FFQ and food diary (r = 0.62, P < 0.001) and an acceptable correlation between the I-FFQ and UIC (r = 0.21, P = 0.018). There was no significant association between the I-FFQ and thyroid function tests. The I-FFQ estimated higher iodine intake compared to the food diary with a mean absolute difference 33 μg/day. The limits of agreement from the Bland-Altman plots were large, however few participants fell outside the limits of agreement (5.2–6.5%). There was no difference between the estimated iodine intake from the I-FFQ assessed at gestational week 18–19, and gestational week 35–36 (P = 0.866), and there was a strong correlation between the two time points (r = 0.63, P < 0.001).ConclusionIn summary, this study suggests that the I-FFQ can be used as a valid tool to estimate and rank iodine intake among Norwegian pregnant women. We further suggest that this I-FFQ may also be valid in other populations with similarly dietary patterns and where salt is not iodized.Trial registrationThe study is registered in ClinicalTrials.gov (NCT02610959).

Iodine Nutrition and Iodine Supplement Initiation in Association with Thyroid Function in Mildly-to-Moderately Iodine-Deficient Pregnant and Postpartum Women

“…One spot urine sample at 3 and 6 mo postpartum. 3 Estimated total iodine intake from foods and supplements from a validated iodine-specific FFQ ( 35 ). 4 Supplement user defined as taking iodine-containing supplements >2 times/wk.…”

“…One spot urine sample at 3 and 6 mo postpartum. 3 Estimated total iodine intake (from foods and supplements) from a validated iodine-specific FFQ ( 35 ). 4 Log2-transformed values of TSH owing to skewed data.…”

“… 3 Estimated total iodine intake (from foods and supplements) from a validated iodine-specific FFQ ( 35 ). …”

“…Iodine intake and use of iodine supplements were self-reported and estimated from a validated electronic iodine-specific FFQ which was developed specifically for this study ( 35 ). The FFQ was completed by the participants at GWs 18 and 36, and 3 and 6 mo postpartum.…”

“…Total iodine intake (μg/d) was summarized and estimated from the food items and dietary supplements. More information regarding the FFQ and calculation of iodine intake has been published elsewhere ( 35 ).…”