Scientist Peter Laurberg and his colleagues at the Aalborg Hospital in Denmark studied cotinine (a nicotine metabolite) levels in urine and serum to identify whether healthy pregnant women who were admitted for delivery were smokers or nonsmokers.
Both groups had identical urinary iodine on the fifth day after giving birth, but smoking was associated with reduced iodine content in breast milk (26.0 vs. 53.8 µg/litre; P<0.001) and in the infants' urine (33.3 vs. 50.4 µg/litre; P=0.005).
Statistical analysis showed that the odds ratio for smoking vs. nonsmoking mothers to have lower iodine content in breast milk than urine was 8.4. In smokers, iodine transfer into breast milk was inversely related to urinary cotinine concentration.
The scientists report that 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,' conclude the researchers.
Full findings of the study 'Iodine Nutrition in Breast-Fed Infants Is Impaired by Maternal Smoking' by Peter Laurberg, Susanne B. Nøhr, Klaus M. Pedersen, and Ebbe Fuglsang are published in this month's issue of the Journal of Clinical Endocrinology and Metabolism (2004) 89, 181-187.