Milk Composition

The major macronutrients in milk are (a disaccharide unique to milk); lipids lactose proteins, including casein, a-lactalbumin, lactoferrin, secretory immunoglobulin A (sIgA), and many others present at much lower concentrations; and minerals such as sodium, chloride, calcium, and magnesium. Minor nutrients in milk are enzymes, vitamins, trace elements, and growth factors. The lipid content of milk varies considerably between species. In human and cow's milk, the fat accounts for approximately 4% of milk volume, whereas in whales and seals it can account for as much as 60% of milk volume. Milk fat is primarily composed of triglycerides, a major source of neonatal calories, but it also contains cholesterol and phos-pholipids, essential for early neonatal development. Casein micelles form a separate phase that can be pelleted by high-speed centrifugation or acidification. These micelles have a high calcium and phosphate content. The aqueous fraction of milk, often called whey, is a true solution that contains all the milk sugar as well as the major milk proteins lac-toferrin, a-lactalbumin, and sIgA and nonprotein nitrogen compounds (mostly urea); the monovalent ions sodium, potassium, and chloride; citrate; calcium; free phosphate; and most of the water-soluble minor components of milk.

The casein fraction from cow's milk, usually obtained by rennin precipitation, is used in cheese making, whereas the whey has a multiplicity of uses, most notably as the base for infant formula. Urea and other nonprotein nitrogen components of milk are a source of nitrogen for amino acid and protein synthesis. Isotope utilization studies indicate that on average 10-20% of urea nitrogen is converted into protein by breast-fed infants. Significantly higher utilization rates, however, have been measured in children recovering from infection, suggesting that alterations in urea nitrogen utilization may be a homeostatic response. Human and bovine milk differ primarily in their concentrations of lactose, mono- and divalent ions, and casein levels and the existence of antiinfectious agents in human milk (Table 1). These differences are related to the specific needs of these species. Human milk, for example, possesses higher concentrations of lactose and lower divalent ion concentrations than cow's milk. The high lactose concentration provides a large amount of 'free water,' via osmotic regulation, that serves as a reserve for temperature regulation via sweating in human infants. Human milk also contains a number of agents that protect against gastrointestinal and respiratory infections, including oligosacchar-ides that interact specifically with pathogen receptors, lactoferrin and sIgA. Bovine milk, on the other hand, contains high concentrations of casein, which provides protein and associated calcium and phosphate needed to support rapid growth of young calves.

Table 1 Comparison of the macronutrient contents of human and bovine milk




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