Maternal nutrition and the regulation of milk synthesis.

Recent research has demonstrated the unsurpassed value of breast-feeding in promoting the optimal nutrition of the human neonate. Breast milk has been shown to have a uniquely appropriate composition for the neonate at a time when growth and development are occurring at a near-maximal rate, yet when many of the body's systems (digestive, hepatic and renal) are relatively immature. The N required for growth is both qualitatively and quantitatively appropriate for the immaturity of these systems, and the bioavailability of many vitamins and minerals to the neonate is enhanced in breast milk. Trophic factors in human milk (peptides, growth factors, hormones, amino acids, glycoproteins, polyamines and nucleotides) promote the development of the immature gastrointestinal tract, and breast milk contains a number of enzymes (e.g. bile saltstimulated lipase (EC 3.1.1.3)) with the potential to augment the infant's gastrointestinal digestive capacity (Harzer & Haschke, 1989). These diverse observations illustrate the advantages of human milk and provide the rationale for the recommendation that breast milk should be the infant's only source of nutriment for the first 4-6 months of life (National Health and Medical Research Council, 1992). In contrast to the nutrition of the breast-fed infant, less definitive information is available on the relationship between maternal nutrition and the well-being of the lactating woman. Mothers who breast-feed their infants have prolonged postpartum amenorrhoea (Short et al. 1991) and this may assist in the maintenance of maternal Fe status. Breast-feeding may assist maternal weight loss, or on the other hand it may increase the risk of obesity (due to the failure to lose fat stored during pregnancy). An increased risk of osteoporosis is also a possible consequence. Since the energy output in milk represents a significant proportion of the energy intake in the maternal diet of most mammals (approximately 25% for women exclusively breast-feeding their infants), it is reasonable to expect that the level of food intake would regulate milk yield. This expectation has received experimental support from research in dairy animals; however, a comprehensive review by Prentice et al. (1986) questioned its validity for lactating women. Prentice et al. (1986) demonstrated that milk yield from poorly nourished mothers in developing countries was similar to that from wellnourished mothers in developed countries and that improving maternal nutrition did not increase milk yield of women in developing countries. They concluded, therefore, that the most important determinant of milk yield in women from both groups of countries