A Dose‐Response Balance Study to Establish the Daily Dietary Iodine Requirement in Infancy
Authors: Dold Susanne , Zimmermann Michael , Baumgartner Jeannine , Braegger Christian , Andersson Maria 
Published: November 2023

Introduction

Iodine is needed for thyroid hormone production and is an essential nutrient for normal growth and development. Infants are born with limited iodine stores and are entirely dependent on the iodine intake from breast milk or infant formula milk (IFM). The physiological requirement for dietary iodine at this critical age is poorly defined. The US Institute of Medicine recommends a daily iodine intake of 110 μg/day for 0–6 months old infants, but this is only an adequate intake (AI). The estimated average requirement (EAR) and recommended dietary allowance (RDA) during the first 6 months of life have not been defined for iodine due to lacking data.

Objectives

To assess the iodine retention and to determine the daily amount of dietary iodine required to achieve positive iodine balance in 2–4 months old infants.

Design

We conducted a dose‐response metabolic iodine balance study in 2–4 months old healthy, full‐term, euthyroid, formula fed infants in Switzerland (n=11). Infants were randomly allocated to three 11‐day cross‐over periods of exclusive consumption of IFM providing 60 μg I/day (50% of the AI), 110 μg I/day (100% of the AI) and 220 μg I/day (200% of the AI). The iodine intake was monitored by recording the amount of IFM fed; iodine excretion in urine and feces was measured from complete collections of diapers for 3 × 4 days. The daily iodine retention was calculated as the difference between intake and excretion. The iodine content of IFM samples and individually homogenized diapers was determined after alkaline extraction with tetramethylammonium hydroxide (TMAH) by multi collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS), using isotope dilution analysis with 129I and Tellurium for mass bias correction.

Results

Mean±SD iodine intakes (n=11) for the three iodine levels in IFM were 8.0±1.1 μg/kg bodyweight (BW), 21.3±2.1 μg/kg BW and 39.2±4.6 μg/kg BW (p<0.001), respectively and mean±SD iodine excretions were 8.2±1.0 μg/kg BW, 18.3±2.6 μg/kg BW and 33.4±5.4 μg/kg BW (p<0.001) (Figure 1). The mean±SD iodine retentions were −0.2±1.3 μg/kg BW, 3.0±3.1 μg/kg BW and 5.8±4.9 μg/kg BW (p<0.001) (Figure 2). We evaluated the relation between daily iodine intake, excretion and retention by linear and nonlinear models, suggesting a linear relationship between iodine intake and excretion (R2=0.92, p<0.001), with an iodine excretion rate of 80%. Null balance (intake = excretion, iodine retention = 0 μg/day) was achieved at a daily iodine intake of 59 μg (R2=0.42, p<0.001).

Conclusion

Our results suggest the minimum daily iodine intake requirement in 2–4 months old infants is approximately 60 μg. However, higher intakes may be beneficial to build up thyroidal iodine stores.

Potential impact

Our findings provide new data for the scientific evidence base needed to establish the EAR for iodine during infancy, important for monitoring iodine intake and iodine status in infants worldwide.

Support or Funding Information

The study was funded by the ETH Zurich, Switzerland, and the Swiss National Fund.

We thank Hochdorf Nutritec for producing and supplying the infant formula with three different iodine contents free of charge.