Maternal smoking, breastfeeding, and risk of overweight and growth during infancy

Breastfeeding provides infants with nutrients that protect against risk of overweight but can also expose the infant to significant amounts of tobacco compounds when the mother smokes. Thus, a question arises regarding the joint effect of maternal smoking and breastfeeding on risk of overweight and growth during infancy. Methods - We used data (N=23,571) from the Collaborative Perinatal Project to conduct the first study of the net effect of exposure to tobacco compounds via breast milk on growth and risk of overweight during infancy. We stratified infants by maternal smoking and feeding type, and fit interaction terms to isolate the net effect of exposure to tobacco compounds via breast milk from exposure in uterus and in ambient air after birth. Results - The risk of overweight at 1 year of age associated with maternal smoking among breastfed infants was similar with that among bottle-fed infants. The association between maternal smoking and infancy weight gain was marginally greater among breastfed infants (adjusted mean difference, 0.23 kg [95% CI, 0.08 to 0.38]) than among bottle-fed infants (0.14, [95% CI, 0.09 to 0.19]). Conclusions - Among this sample maternal smoking and breastfeeding each appear to be associated primarily with the risk of overweight through influencing the upper tail of the BMI distribution rather than shifting the entire BMI distribution. Implications of these findings are discussed in terms of catch-up growth, confounding by SES, and potential biologic pathways.