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-day 24-h iodine metabolism survey in 2021-2023 was conducted on 54 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 UIC and BMIC were 135.06μg/L and 150.26µg/L, respectively. When the DII was between 240μg/d and 600μg/d, the predicted value of fractional breast milk iodine excretion (FBMIE)was 31.48% (95%CI: 27.16-36.22%). When the daily iodine excretion (DIE) was between 258μg/d and 476μg/d, the fractional urine iodine excretion (FUIE) (59.09%) and FBMIE (40.91%) were stable. DII can be derived from the spot UIC as follows: UIC(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 UIC of lactating women. This study was registered at ClinicalTrials.gov (NCT04492657).