Breast milk offers some immunity to some infections. It also contains enzymes that help the digestive system to develop. The act of sucking on the breast – which is quite different and much harder work than sucking from a bottle – helps to develop the mouth and jaw. Research has also shown that breastfeeding mothers have a more acute awareness of the baby's position when co-sleeping. This why co-sleeping is not recommended for bottle-fed babies even if all other factors (e.g. non-smoking parents, not overweight parents, etc) are positive. The "recipe" of breast milk changes with the baby's needs and more milk is produced during growth spurts when the baby is hungrier.
Baby formula is a safe substitute for breast milk, but does not provide anything like the same benefits.
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