Each species of mammal produces milk to support the optimal growth of that species. Since no two have the same needs, each species' milk differs. Cow milk suits the need of a calf to grow quickly and to walk immediately after birth. Human milk contains three times less protein than cow milk because of the human infant's longer period of development. Humans do not reach their full physical size for at least 12 years, while most cows do so in their first year.
Studies show that specific constituents in human milk play an important role in brain development.
The immunological protection passed to the infant via human milk is unique to the needs of a human baby. The substances which offer this protection are present in colostrum, the first milk baby gets after birth, and continue to be present in the mother's milk as long as the baby nurses.