The female human body manufactures milk that is a perfect food for a human newborn in several ways. Human milk is rich in the lipid needed for rapid brain growth, and it is low in protein. Cow's milk is the reverse, with three times as much protein as human milk. Much of this protein is casein, which is fine to spur a calf's rapid muscle growth but forms hard-to-digest curds in a human baby's stomach. The protein in human milk has a better balance of essential amino acids than does the protein in cow's milk.

Human milk protects a newborn from many infections. For the first few days after giving birth, a new mother's breasts produce colostrum, which has less sugar and fat than mature milk but more protein, and is rich in antibodies. The antibodies protect the baby from such infections as Salmonella poisoning and polio. When the milk matures by a week to 10 days, it has antibodies, enzymes, and white blood cells from the mother that continue the infection protection introduced in the colostrum. A milk protein called lactoferrin binds iron, making it unavailable to microorganisms that might use it to thrive in the newborn's digestive tract. Another biochemical in human milk, bifidus factor, encourages the growth of the bacteria Lactobacillus bifidus, which manufactures acids in the baby's digestive system that kill harmful bacteria.

A breast-fed baby typically nurses until he or she is full, not until a certain number of ounces have been drunk, which may explain why breast-fed babies are less likely to be obese than bottle-fed infants. Babies nurtured on human milk are also less likely to develop allergies to cow's milk.

But breast-feeding is not the choice for all women. It may be impossible to be present for each feeding or to provide milk. Also, many drugs taken by the mother enter breast milk and can affect the baby. A nursing mother must eat about 500 calories per day more than usual to meet the energy requirements of milk production--but she also loses weight faster than a mother who bottle-feeds, because the fat reserves set aside during pregnancy are used to manufacture the milk. Another disadvantage of breast-feeding is that the father cannot do it.

An alternative to breast-feeding is infant formula, which is usually cow's milk plus fats, proteins, carbohydrates, vitamins, and minerals added to make it as much like breast milk as possible. Although infant formula is nutritionally sound, the foul-smelling and bulkier bowel movements of the bottle-fed baby compared to the odorless, loose, and less abundant feces of a breast-fed baby indicate the breast milk is a more digestible first food than infant formula.

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