mother is in energy balance, fatty acids derived directly from the diet account for approximately 30% of the fatty acids found in the milk.

There is no evidence that the concentrations of cholesterol and phospholipid in human milk can be altered by changes in the maternal diet. Indeed, milk cholesterol remains at 100 to 150 mg/liter even in hypercholesterolemic women and increases only in severe cases of pathologic hypercholesterolemia (Jensen, 1989). Since both cholesterol and phospholipids are integral components of the milk fat globule membrane, their secretion rates relate to the total quantity of fat secreted in milk, which is apparently not influenced by diet.

Studies conducted in communities where maternal undernutrition is prevalent have furnished evidence indicating that the percentage of maternal body fat may influence the concentration of fat in milk. Milk fat concentrations in The Gambia (Prentice et al., 1981) and Bangladesh (Brown et al., 1986b) were positively correlated with maternal skinfold thickness and decreased over the course of lactation. This positive relationship (R = .46) between milk fat concentration and body fat (as a percentage of ideal body weight) was likewise noted in U.S. women in late (6 to 12 months) lactation but not in early lactation (Nommsen et al., in press). Prentice and associates (1989) report that high- (>10) parity Gambian women had a decreased capacity for total milk fat synthesis and, thus, lower milk fat concentrations.

Carbohydrates

The principal carbohydrate in human milk is lactose, a disaccharide that consists of galactose joined by a β linkage to glucose. In human milk, lactose is present in an average concentration of 70 g/liter and is second only to water as a major constituent. In all species of mammals studied, milk is isotonic with plasma, which helps keep the energy cost of milk secretion low. Lactose exerts 60 to 70% of the total osmotic pressure of milk. Compared with glucose, lactose provides nearly twice the energy value per molecule (per unit of osmotic pressure). The concentrations of lactose in human milk are remarkably similar among women, and there is no convincing evidence that they can be influenced by maternal dietary factors. However, Hartmann and Prosser (1982) noted that lactose concentration in human milk decreased from 78 to 60 g/liter both 5 to 6 days before and 6 to 7 days after ovulation. Other carbohydrates and their complexes are discussed below in the section ''Nonlactose Carbohydrates in Human Milk".

Vitamins

A major factor influencing the vitamin content of human milk is the mother's vitamin status. In general, when maternal intakes of a vitamin are chronically low, the levels of that vitamin in human milk are also low. As maternal intakes of the vitamin increase, levels in milk also increase, but for



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