During this time, the amount of protein and immune factors in the milk gradually decrease while fat, lactose, and calories in the milk increase. By the tenth day after birth, the mother produces mature breast milk. Colostrum and human milk are rich in antibodies and have anti-infective factors. These help protect the newborn infant from viruses and bacteria that the infant was exposed to in the birth canal. They help to protect the infant's immature gut from infection.
Breast milk promotes the growth of bacteria that all humans normally have in the digestive tract.
Breast milk contains immune factors to help an infant fight infection and help prevent the infant from developing possible food allergies. Breast milk from most women has the same nutrients. There could be small differences, based on what a woman eats and how her body produces breast milk. If the mother eats a poor diet, this can decrease both the amount of milk produced and the nutrients found in the milk. Women who are breast-feeding should consume an extra 500 calories per day above their maintenance calories.