Iodine nutrition in breast-fed infants is impaired by maternal smoking

Lack of iodine for thyroid hormone formation during the fetal stage and/or the first years of life may lead to developmental brain damage. During the period of breastfeeding, thyroid function of the infant depends on iodine in maternal milk. We studied healthy, pregnant women admitted for delivery and their newborn infants. Cotinine in urine and serum was used to classify mothers as smokers (n = 50) or nonsmokers (n = 90). Smoking and nonsmoking mothers had identical urinary iodine on d 5 after delivery, but smoking was associated with reduced iodine content in breast milk (smokers 26.0 micro g/liter vs. nonsmokers 53.8 micro g/liter; geometric mean, P < 0.001) and in the infants' urine (smokers 33.3 micro g/liter, vs. nonsmokers 50.4 micro g/liter, P = 0.005). Results were consistent in multivariate linear models and by logistic regression analysis. The odds ratio for smoking vs. nonsmoking mothers to have lower breast milk than urinary iodine content was 8.4 (95% confidence interval, 3.5-20.1). In smokers, iodine transfer into breast milk correlated negatively to urinary cotinine concentration. Smoking mothers had significantly higher serum levels of thiocyanate, which may competitively inhibit the sodium-iodide symporter responsible for iodide transport in the lactating mammary gland. Smoking during the period of breastfeeding increases the risk of iodine deficiency-induced brain damage in the child. Women who breastfeed should not smoke, but if they do, an extra iodine supplement should be considered.