Lactation is a continuation of intrauterine gestation. In both processes, maternal diet plays an active role in the provision of nutrients, maternal nutritional stores and endocrine adaptations serve to buffer the short-term variations in maternal nutritional intake, blood flow plays an overriding role in nutrient transfer to the fetus and newborn infant, and the nutrient demands of the recipient are the highest of any stage in human development.
Human milk is remarkable in its variability. Recent data suggest that the variability often improves the nutrient composition as part of a complex adaptation to the infant's specific needs. A comprehensive survey of the literature on lactation and human milk is provided in two review articles.1,2
NUTRIENTS
Lipids
Milk lipids provide the major fraction of calories in human milk, yet they are the most variable constituent.3 Preceding a nursing, the fluid phase of milk stored within the gland resembles skimmed milk. During the course of a nursing, the contraction of smooth muscle launches the fat droplets. This draught reflex is essential for caloric adequacy for the breast-fed infant.2
Women living under unfavorable socioeconomic conditions have reduced total milk lipid.4-6 There is evidence that supplementing the diets of these women leads to increased milk fat. Under controlled metabolic ward conditions, a high-caloric, high-fat diet can be demonstrated to increase milk fat production.7 The distribution of the spectrum of fatty acids in human milk also is responsive to dietary changes.7-13
Women who are malnourished also produce an excess of 12:0 and 14:0 fatty acids.14
trans Fatty acids in human milk are inversely associated with concentrations of essential all-cis n−6 and n−3 fatty acids and determine trans, but not n−6 and n−3, fatty acids in plasma lipids of breast-fed infants
