Sources of milk constituents

All or most of the major milk constituents are synthesised in the mammary gland from various precursors that are selectively absorbed from the blood. The gland also exerts this selective filtering action on certain proteins, minerals and vitamins, which are not elaborated by it but are simply transferred directly from the blood to the milk. 

Amino acids are absorbed by the mammary gland in quantities sufficient to account for the protein synthesised within it. Considerable interconversion of amino acids occurs before synthesis, and certain amino acids are important as sources of others. Thus, ornithine, which does not appear in milk protein, is absorbed and retained in large quantities by the mammary gland and has been shown to be a precursor of proline, glutamate and aspartate. Synthesis of the carbohydrate moieties of the proteins takes place in the mammary gland, as does phosphorylation of serine and threonine before their incorporation into the caseins. 

Apart from a minor fraction associated with the fat globule membrane, milk fat consists of a mixture of triacylglycerols containing a wide range of saturated and unsaturated fatty acids. The predominant saturated acid is palmitic acid and the unsaturated adds consist mainly of oleic acid with small contributions from linoleic acid and linolenic acid. The molar proportions of the fatty adds in the milk fats of different species are given in Table 16.2. 

The fats are characterised by the presence of fatty adds of medium chain length (8 0 to 12 0) that are specific to the mammary gland. Ruminant milk fats are further 

From Bickerstaff R 1970 Uptake and metabolism of fat in the lactating mammary gland. In Falconer I (ed.) Lactation, London, Butterworth. 

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