milk (Rana and Sanders, 1986). Taurine functions in bile acid conjugation and may also function as an inhibitory neurotransmitter and as a membrane stabilizer.

A broad spectrum of nucleotides occurs in human milk (Janas and Picciano, 1982), but the effects of maternal nutrition on the concentrations of these nucleotides have not yet been reported.

Lipids

The lipids in milk are contained within membrane-enclosed milk fat globules, the core of which consists of triglycerides—the major energy source in milk. The globule membrane is composed mainly of phospholipids, cholesterol, and proteins.

Although there is no compelling evidence that changes in maternal fat intake influence the total quantity of milk fat, it has been shown repeatedly that the nature of the fat consumed by the mother will influence the fatty acid composition of milk (Jensen, 1989). For example, milk from four complete vegetarian women in Great Britain was found to contain five times as much C18:2 fatty acids as milk from four nonvegetarian women (31.9 and 6.9%, respectively) (Sanders et al., 1978). Finley et al. (1985) noted that, as lactation progressed, milk from both vegetarian and nonvegetarian women contained more fatty acids principally synthesized in the mammary gland (C8:0, C10:0, C12:0, C14:0) and less from the diet and adipose tissue. Chappell et al. (1985a) reported that the trans fatty acid content of human milk was directly related to maternal intake of partially hydrogenated fats and oils; in women experiencing postpartum weight loss, fat mobilized from adipose tissue also contributed trans fatty acids to human milk fat independently of current dietary intake.

In the classic study of a single subject by Insull and colleagues (1959), both the total energy and fat contents of the diet were altered. Their results demonstrated that mammary lipid synthesis was influenced by energy balance as well as by the type and amount of fat in the diet. When the subject was fed excess energy as a low-fat, high-carbohydrate diet, the investigators found that 40 to 60% of the fatty acids in milk fat had carbon chain lengths of less than 16. On a very high fat diet (70% of kilocalories as corn oil) that was adequate in energy, the combined linoleic and linolenic acid content of the milk fatty acids increased from approximately 2 to 45%, and there was a corresponding drop in the content of shorter-chain saturated fatty acids. When a low-fat, calorie-restricted diet was fed, C16 or longer-chain saturated fatty acids predominated in the milk, indicating that stored body fat was utilized for milk fat synthesis. Effects of such changes on infant health have not been studied.

Using stable isotope methodology, Hachey and colleagues (1987, 1989) confirmed the results of the study of Insull et al. (1959) showing that diet composition affects milk fat synthesis. Hachey et al. estimate that when the



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