Protein Content of Human Milk, from Colostrum to Mature Milk

By definition, mammals are those animals that suckle their young and they comprise the highest class of vertebrates. Thus lactation is one important distinguishing characteristic of this class of animals. Mammals are born at markedly different stages of maturity and if it is assumed that their nutritive requirements correspond to their physiological maturity, one may reason Ideologically that the milk of a given species is best adapted to the nutritional needs of the young of that same species. The nutritional adequacy of mother's milk for the young depends not only on the composition of the milk, but also on the quantity of milk produced. Linzell's comparative studies (1) of milk production in 22 different species indicate a mean daily yield of 0.126 kg milk/kg body weight and a daily energy output of 140 kcal/kg. The milk yield is closely related to the body size of the lactating mother. In the human, however, there seems to be a great individual variation in the milk yield and it may therefore be more common in humans than in animals for the milk production to be insufficient for optimal growth. Protein in milk is the source of amino acids needed for protein synthesis and growth of the young. The protein content of milk produced by various species ranges from about 1% in humans to about 20% in rabbits. At present the compositions of milks of about 150 different mammalian species are known. For six species in addition to humans (horse, goat, cow, sheep, and reindeer) the data are extremely comprehensive and detailed (Table 1), due to the fact that the milks of all these animals, particularly the cow and goat, have commonly been used for human consumption. The protein content of milk has been related to the growth rate of the young. Bernhart (2) suggested that there is a direct correlation between milk protein and the time required to double the weight of the young, based on studies of nine different species: humans, horse, cow, goat, pig, sheep, dog, cat, and rabbit. The data range from humans, with 125 days to double birthweight and 7% of energy intake from protein, to the rabbit, with 7 days to double birthweight and nearly 30% of the energy