While breastfeeding usually means feeding from a mother’s nipple, breast pumping is quite popular in the U.S. There is little research comparing how feeding babies expressed (pumped) milk differs from actual breastfeeding in terms of health benefits. A paper in the American Journal of Public Health noted that when expressed milk is stored in the refrigerator or freezer, various immune factors and vitamins can be reduced. And babies who are fed expressed milk from a bottle are more likely to overconsume the milk compared with when they breastfeed.
Furthermore, when studying breastfeeding, researchers should distinguish between women who breastfeed exclusively compared with those who also supplement with formula. Many existing studies have lumped the two together.