Estimation of iodine nutrition status among lactating women with adequate iodine based on fractional iodine excretion in urine.

Data on the distribution of iodine in the urine and breast milk of lactating women are limited. This study aimed to establish a formula to assess iodine status in lactating women by evaluating the fractional iodine excretion in urine and breast milk. A 3-d 24-h iodine metabolism survey in 2021-2023 was conducted on fifty-four pairs of lactating women and infants in Tianjin and Luoyang, China. We used the 24-h dietary record and salt weighing method to assess daily iodine intake (DII). Iodine excretion in breast milk and urine was measured. The median 24-h urinary iodine concentration and breast milk iodine concentration were 135·06 μg/L and 150·26 µg/L, respectively. When the DII was between 240 μg/d and 600 μg/d, the predicted value of fractional breast milk iodine excretion was 31·48 % (95 % CI: 27·16 %, 36·22 %). When the daily iodine excretion was between 258 μg/d and 476 μg/d, the fractional urine iodine excretion (59·09 %) and fractional breast milk iodine excretion (40·91 %) were stable. DII can be derived from the spot urinary iodine concentration as follows: urinary iodine concentration (μg/L) × (0·0009 L/h/kg × 24 h/d) × body weight (kg) ÷ 0·59 ÷ 0·94 = DII (μg/d). In conclusion, lactating women with adequate iodine delivered approximately 31·48 % of the DII to their infants. A stable proportion (59·09 %) of iodine excretion was discharged through urine, which was used to assess the iodine status based on the spot urinary iodine concentration of lactating women. This study was registered at ClinicalTrials.gov (NCT04492657).