The differences in the digestibility and availability of human and cow's milk by infants, according to Finkelstein and Meyer,1 are not due to qualitative differences in the constituents of the milk. Casein causes no disturbances of digestion. Fat and sugar have no pathologic action unless the intestinal functions have been injured. The fat and sugar of human milk can act in the same way if the injury to the intestine is severe enough. The action of the whey from human and from cow's milk is, however, different. A baby can digest the various food elements when they are in human whey but cannot when they are in the whey of cow's milk. Any other medium than human milk interferes with the functionating of the intestinal epithelium. They have, however, in spite of many experiments, been unable to find a better menstruum than the whey of cow's milk.
MORSE JL. THE USE OF MALT SUGAR AND HIGH PERCENTAGES OF CASEIN IN INFANT FEEDING. Am J Dis Child. 1911;II(5):315-328. doi:10.1001/archpedi.1911.04100110024003