Cow's milk is inappropriate for babies, and the American Academy of Pediatrics recommends that cow's milk not be used as a replacement for breast milk or formula for the first year of life. Infants fed whole cow's milk have low intakes of iron, linoleic acid and vitamin E and excessive intakes of sodium, potassium and protein. The protein molecule in cow's milk is large and can permeate the intestine of the infant undigested, causing intestinal bleeding. This intestinal bleeding may not be visible to the naked eye but would be detected on exam. This intestinal bleeding contributes to the poor iron status of the infant. The protein level in cow's milk is too high, causing stress on the kidneys as they try to dilute the nitrogenous waste products from the protein. This can lead to dehydration as the baby's body draws on water stores.
Although cow's milk protein is used in infant formula, it has been modified to make it more digestible. The amount of protein used is in smaller concentrations than in cow's milk, and more closely matches the level in breast milk.