Objective: The aim of this review is to explore information available regarding iodine secretion in milk, both mothers and infants iodine nutrition during breastfeeding and to make recommendations for appropriate iodine supplementation during lactation.
Design: MEDLINE was queried for studies between 1960 and 2007 that included lactation and breastfeeding with iodine and iodine deficiency. Studies were selected if they studied (i) Secretion of iodine in breast milk; (ii) breastfeeding and iodine nutrition; (iii) factors affecting maternal iodine metabolism and (iv) recommendations for iodine supplementation during breastfeeding.
Results: Thirty-six articles met the selection criteria. The iodine content of breast milk varies with dietary iodine intake, being lowest in areas of iodine deficiency with high prevalence of goitre. Milk iodine levels are correspondingly higher when programs of iodine prophylaxis such as salt iodization or administration of iodized oil have been introduced. The small iodine pool of the neonatal thyroid turns over very rapidly and is highly sensitive to variations in dietary iodine intake. Expression of the sodium iodide symporter is up-regulated in the lactating mammary gland which results in preferential uptake of iodide. In areas of iodine sufficiency breast milk iodine concentration should be in the range of 100-150 microg/dl. Studies from France, Germany, Belgium, Sweden, Spain, Italy, Denmark, Thailand and Zaire have shown breast milk concentrations of < 100 microg/l. Adequate levels of iodine in breast milk have been reported from Iran, China, USA and some parts of Europe.
Conclusions: Adequate concentration of iodine in breast milk is essential to provide for optimal neonatal thyroid hormone stores and to prevent impaired neurological development in breast-fed neonates. In many countries of the world, low iodine content of the breast milk indicates less than optimum maternal and infant iodine nutrition. The current WHO/ICCIDD/UNICEF recommendation for daily iodine intake (250 microg for lactating mothers) has been selected to ensure that iodine deficiency dose not occur in the postpartum period and that the iodine content of the milk is sufficient for the infant's iodine requirement.