Iodine intake assessment in the staff of a Porto region university (Portugal): the iMC Salt trial

Abstract

Purpose: Iodine deficiency disorder (IDD) is an ongoing worldwide recognized problem with over two billion individuals having insufficient iodine intake. School-aged children and pregnant women are often target groups for epidemiological studies, but there is a lack of knowledge on the general adult population. The aim of this study was to assess the iodine status among a Portuguese public university staff as a proxy for the adult working population.

Methods: The population study covered 103 adults within the iMC Salt randomized clinical trial, aged 24-69 years. Urinary iodine concentration was measured spectrophotometrically using the Sandell-Kolthoff reaction. Iodine food intake was assessed using a 24-h dietary recall. The contribution of discretionary salt to the iodine daily intake was assessed through 24-h urinary sodium excretion (UIE) and potentiometric iodine determination of household salt.

Results: The mean urine volume in 24 h was 1.5 L. The median daily iodine intake estimated from 24-h UIE was 113 µg/day, being lower among women (p < 0.05). Only 22% of participants showed iodine intake above the WHO-recommended cutoff (150 µg/day). The median daily iodine intake estimated from the 24-h dietary recall was 58 µg/day (51 and 68 µg/day in women and men, respectively). Dairy, including yoghurt and milk products, were the primary dietary iodine source (55%). Iodine intake estimated from 24-h UIE and 24-h dietary recall was moderately correlated (Spearman rank correlation coefficient r = 0.34, p < 0.05). The average iodine concentration in household salt was 14 mg I/kg, with 45% of the samples below the minimum threshold preconized by WHO (15 mg I/kg). The contribution of discretionary salt to the daily iodine intake was around 38%.

Conclusion: This study contributes new knowledge about iodine status in Portuguese working adults. The results revealed moderate iodine deficiency, particularly in women. Public health strategies and monitoring programs are needed to ensure iodine adequacy in all population groups.

Keywords: 24-h dietary recall; 24-h urinary iodine excretion; Adult population; Iodine intake; Public health; Salt intake.

Fig. 1

Bland–Altman plot, mean iodine intake estimated from 24-h urine iodine excretion (UIE) and the 24-h dietary recall (24-h DR) versus the difference between the methods (24-h DR—UIE). The red line indicates the mean difference in iodine intake estimates, whereas the red dashed line represents the limits of agreement from − 1.96 SD to + 1.96 SD

Fig. 2

The contribution (%) from food groups to the iMC Salt participant’s total iodine intake by sex

Fig. 3

Iodine levels (mg I/kg) found in household salt samples from the iMC Salt intervention group. Green circles—salt samples complying with the WHO and the Portuguese criteria; orange circle—salt samples complying only with the WHO criteria; gray circles—salt samples that did not comply with WHO or the Portuguese criteria; gray dashed line—iodine range according to the Portuguese legal criteria for salt iodization (19–27 mg I/kg); red solid line—WHO-recommended level of iodine for salt iodization (15 mg I/kg)