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Background: Iodine status in populations is usually assessed by the median urinary iodine concentration (UIC). However, iodine is also excreted in breast milk during lactation ; thus, breast milk iodine concentration (BMIC) may be a promising biomarker of iodine nutrition in lactating women. Whether the mammary gland can vary fractional uptake of circulating iodine in response to changes in dietary intake is unclear.Objective: We evaluated UIC and BMIC as biomarkers for iodine status in lactating women with a wide range of iodine intakes.Methods: We recruited 866 pairs of lactating mothers and exclusively breastfed infants from 3 iodine- sufficient study sites: Linfen, China (n = 386) ; Tuguegarao, Philippines (n = 371) ; and Zagreb, Croatia (n = 109). We also recruited iodine- deficient lactating women from Amizmiz, Morocco (n = 117). We collected urine and breast milk samples and measured UIC and BMIC.Results: In the 3 iodine-sufficient sites, a pooled regression analysis of the estimated iodine excretion revealed higher fractional iodine excretion in breast milk than in urine at borderline low iodine intakes. In contrast, in the iodine-deficient site in Morocco, a constant proportion (∼33%) of total iodine was excreted into breast milk.Conclusions: In iodine-sufficient populations, when iodine intake in lactating women is low, there is increased partitioning of iodine into breast milk. For this reason, maternal UIC alone may not reflect iodine status, and BMIC should also be measured to assess iodine status in lactating women. Our data suggest a BMIC reference range (2.5th and 97.5th percentiles) of 60-465 μg/kg in exclusively breastfeeding women. This trial was registered at clinicaltrials.gov as NCT02196337.

Background: Optimal iodine intake during infancy is critical for brain development, but no estimated average requirement (EAR) is available for this age group. Objective: We measured daily iodine intake, excretion, and retention over a range of iodine intakes in early infancy to determine the minimum daily intake required to achieve iodine balance. Design: In a dose-response crossover study, we randomly assigned healthy infants (n = 11; mean ± SD age 13 ± 3 wk) to sequentially consume over 33 d 3 infant formula milks (IFMs) containing 10.5, 19.3, and 38.5 μg I/100 kcal, respectively. Each IFM was consumed for 11 d, consisting of a 6-d run-in period followed by a 4-d balance period and 1 run-out day. Results: Iodine intake (mean ± SD: 54.6 ± 8.1, 142.3 ± 23.1, and 268.4 ± 32.6 μg/d), excretion (55.9 ± 8.6, 121.9 ± 21.7, and 228.7 ± 39.3 μg/d), and retention (−1.6 ± 8.3, 20.6 ± 21.6, and 39.8 ± 34.3 μg/d) differed among the low, middle, and high iodine IFM groups (P < 0.001 for all). There was a linear relation between daily iodine intake and both daily iodine excretion and daily iodine retention. Zero balance (iodine intake = iodine excretion, iodine retention = 0 μg/d) was achieved at a daily iodine intake of 70 μg (95% CI: 60, 80 μg). Conclusion: Our data indicate the iodine requirement in 2- to 5-mo-old infants is 70 μg/d. Adding an allowance for accumulation of thyroidal iodine stores would produce an EAR of 72 μg and a recommended dietary allowance of 80 μg. This trial was registered at clinicaltrials.gov as NCT02045784., The American Journal of Clinical Nutrition, 104 (3), ISSN:0002-9165, ISSN:1938-3207

Background: Optimal iodine intake during infancy is critical for brain development, but no estimated average requirement (EAR) is available for this age group. Objective: We measured daily iodine intake, excretion, and retention over a range of iodine intakes in early infancy to determine the minimum daily intake required to achieve iodine balance. Design: In a dose-response crossover study, we randomly assigned healthy infants (n = 11; mean ± SD age 13 ± 3 wk) to sequentially consume over 33 d 3 infant formula milks (IFMs) containing 10.5, 19.3, and 38.5 μg I/100 kcal, respectively. Each IFM was consumed for 11 d, consisting of a 6-d run-in period followed by a 4-d balance period and 1 run-out day. Results: Iodine intake (mean ± SD: 54.6 ± 8.1, 142.3 ± 23.1, and 268.4 ± 32.6 μg/d), excretion (55.9 ± 8.6, 121.9 ± 21.7, and 228.7 ± 39.3 μg/d), and retention (−1.6 ± 8.3, 20.6 ± 21.6, and 39.8 ± 34.3 μg/d) differed among the low, middle, and high iodine IFM groups (P < 0.001 for all). There was a linear relation between daily iodine intake and both daily iodine excretion and daily iodine retention. Zero balance (iodine intake = iodine excretion, iodine retention = 0 μg/d) was achieved at a daily iodine intake of 70 μg (95% CI: 60, 80 μg). Conclusion: Our data indicate the iodine requirement in 2- to 5-mo-old infants is 70 μg/d. Adding an allowance for accumulation of thyroidal iodine stores would produce an EAR of 72 μg and a recommended dietary allowance of 80 μg. This trial was registered at clinicaltrials.gov as NCT02045784. ISSN:0002-9165 ISSN:1938-3207