Breast-feeding protects against diarrheal ailments in three ways. Infants who nurse are not drinking formula, which is a perfect medium for bacteria if it's not refrigerated and if it is reconstituted with contaminated water, as is typically the case in nonindustrialized countries. And if infants are exclusively breast-fed during the vulnerable first six months of life, they're also not taking in contaminated food. Finally, the maternal antibodies in a baby's gut deactivate swallowed bacteria and viruses that might otherwise infect the cells that line the intestine or penetrate the lining. Most often this prevents bacterial or viral gastroenteritis, which presently kills 1.6 million children worldwide a year.
Fifteen years ago, the Innocenti Declaration in support of breast-feeding was adopted internationally. Since then, breast-feeding has increased dramatically around the globe. In 2005, UNICEF estimated that 6 million infant lives worldwide (most in the developing world) are saved annually by policies that have replaced formula with breast-feeding. It is likely that a substantial fraction of the 1.6 million infants who still die of diarrheal disease could be saved by breast-feeding as well.
What should we make of the facts about the immunobiology of lactation? First, it bears repeating that even if the immunological benefits are often overstated, there is clear and obvious benefit to breast-feeding in most of the developing world. Second, though it is harder to demonstrate in a scientifically satisfying way, there are probably other biological benefits. And there are surely economic reasons to give babies human milk instead of formula, which costs between $1,500 and $3,000 a year. In the developing world, the economic case against formula-feeding might be as potentially lifesaving as the immunological one: Money stolen from a poor family's budget for formula will not be available for food, housing, education—or even soap.
In the end, though, I find myself falling back on the same logic (or lack of logic) that appealed to me when my babies were born. Biologically speaking, it seems as if breast-feeding ought to be better for babies. At the same time, I am strongly convinced that there are two kinds of nutrition, physical and psychological, and that both are equally important. This conviction persuades me that it's better for a mother to formula-feed her baby pleasurably than to breast-feed and hate it. Fortunately, the majority of mothers enjoy nursing. But not all. Some women don't like to nurse, and others, even with the best help, find it physically difficult or daunting or intolerably uncomfortable. Sometimes, also, babies just aren't good nursers. In the end, I always encourage a mother to choose the feeding method that is most satisfying to her.