The nutritive and metabolic advantages of homologous milk.

In discussing whether or not homologous milk is an advantage to the young (Le., whether feeding the young of a species milk of the same species confers an advantage over a substitute derived from milk of another species), we are still largely reduced to the old teleological arguments: the milk of a species is appropriate for the growth of the young of that species, therefore, growth and development on homologous milk are the optima and the standards by which alternatives are to be judged. To some extent the development of scientific interest in human milk and breastfeeding must be credited to early attempts at making substitutes for maternal milk. This is true, also, for other species, such as those held in zoos, and thoroughbred racehorses. The primary function of mammalian milks is to feed the young of the species, but the recognition that milk contains numerous important, non-nutritive substances, including immune, growth and regulatory factors, has led to a tremendous increase in investigations, especially of human milk, its biological functions, and the lactation process, in the last 10-15 years (Peaker et uf. 1984; Hamosh & Goldman, 1986; Howell ez af . 1986; Goldman et al. 1987). The interest in functions of human milk, other than its nutritional properties, has led to it being probably the best documented of all milks. With the exception of the major domestic dairy animals (cow, goat, sheep, buffalo, camel), the information base of milk composition is very poor. Composition information is available for about 200 of the more than 4000 mammalian species; in only about fifty-five of these have more than ten milk samples been analysed, often with poor documentation of sampling, so that nothing can be said about variability in composition (Oftedal, 1984; Gittleman & Oftedal, 1987; Oftedal & Jenness, 1988). The effects of milk on the young have been studied in even fewer species, so that most of the discussion relates to the effect of human milk on the human infant.