The use of breast milk iodine concentration in the first week of lactation as a biomarker of iodine status in breastfeeding women

Breast milk iodine concentration (BMIC) is a promising indicator of iodine status in lactating women. However, there are limited data on its usefulness to reflect maternal iodine deficiency. Therefore, the aim of our study was to assess iodine concentration in breast milk and urine samples in exclusively breast-feeding women. Eligible pregnant women undergoing routine antenatal care in a large hospital in Shaanxi Province, China, were followed up from the third trimester of pregnancy until the first week of lactation. Urine samples (20 ml) were collected during pregnancy and lactation. Iodine concentration in samples was measured based on Sandell-Kolthoff reaction. Breast milk samples (5 ml) were provided during lactation. A receiver operating curve (ROC) was constructed to determine the diagnostic performance of BMIC. An iodine-specific FFQ was completed twice during pregnancy and lactation. A total of 200 women completed the study. The overall median BMIC was 89 μg/l, indicating iodine sufficiency (i.e. BMIC reference range between 60 and 465 μg/l). Women reported similar median urinary iodine concentration (UIC) during pregnancy and lactation (112 and 113 μg/l, respectively), but their iodine status differed - mild-to-moderate iodine deficiency during pregnancy and iodine sufficiency during lactation. The ROC for BMIC using UIC as a reference standard was 0·755 (95 % CI: 0·644, 0·866). In conclusion, this study demonstrated that women were iodine sufficient in the first week of lactation as assessed by UIC, which was consistent with BMIC. These findings suggested that BMIC is a useful biomarker to assess iodine status in lactating women.