Milk fat synthesis

As can be seen in Table 4.2, the fatty acids are not randomly distributed among the three positions of the TG in bovine milk. Control of esterification is not understood, but there are several factors known to affect it. The presence of glucose is known to stimulate the synthesis of milk TG (Dimmena and Emery 1981 Rao and Abraham 1975). In the mouse, Rao and Abraham concluded that glucose was supplying factors other than NADPH or acylglycerol precursors that stimulated milk fat synthesis. The fatty acid that is esterified is known to be affected by the concentration of the acyl donors present (Marshall and Knudsen 1980 Bickerstaffe and Annison 1971). However, in studies under various conditions, palmitic acid was consistently esterified at a greater rate than other fatty acids (Bauman and Davis 1974 Moore and Christie 1978 Smith and Abraham 1975). 

Dimick, P. S., McCarthy, R. D. and Patton, S. 1970. Milk fat synthesis. In Physiology of Digestion and Metabolism in Ruminant, Ed. A.T. Phillipson (Editor). Oriel Press, Newcastle on Tyne, p. 534. 

Modern concepts of milk fat synthesis developed rapidly in the 1950s with the carefully-designed physiological studies in lactating goats by Popjak et al. (1951) which showed unequivocally the de novo synthesis of short-chain fatty acids from 14C-labelled acetate. Also, the incorporation of tritium-labelled stearic acid into milk fat was demonstrated by Glascock et al. (1956). From empirical calculations of the quantity of dietary fat and the recovery of label in milk fat, the latter authors estimated that dietary fat contributed a maximum 25% of the weight of milk fat. 

In addition to long-chain fatty acids from plasma, the major nutrients utilized for milk fat synthesis are glucose, acetate and 0-hydroxybutyrate. Kinetics for the uptake of these from blood were reported by Miller et al. (1991). Glucose is absolutely required for milk synthesis, being a precursor for lactose or other carbohydrates, or both, in all terrestrial mammals (Oftedal and Iverson, 1995). 

Annison, E.F., Linzell, J.L., Fazakerley, S., Nichols, B.W. 1967. The oxidation and utilization of palmitate, stearate, oleate and acetate by the mammary gland of the fed goat in relation to their overall metabolism, and the role of plasma phospholipids and neutral lipids in milk-fat synthesis. Biochem. J. 102, 637-647. 

Baumgard, L.H., Corl, B.A., Dwyer, D.A., Saebo, A., Bauman, D.E. 2000. Identification of the conjugated linoleic acid isomer that inhibits milk fat synthesis. Am. J. Physiol. 278, R179-184. 

Jones, C.S., Parker, D.S. 1978. Uptake of substrates for milk-fat synthesis by lactating-rabbit mammary-gland. Biochem. J. 174, 291-296. 

McClymont, G.L., Vallance, S. 1962. Depression of blood glycerides and milk fat synthesis by glucose infusion. Proc. Nutr. Soc. 21(2), R41. 

Peterson, D.G., Matitashvili, E.A., Bauman, D.E. 2003. Diet-induced milk fat depression in dairy cows results in increased nans-10, cis-12 CLA in milk fat and coordinate suppression of mRNA abundance for mammary enzymes involved in milk fat synthesis. J. Nutr. 133, 3098-3102. 

Popjak, G., French, T.H., Folley, S.J. 1951. Utilization of acetate for milk-fat synthesis in the lactating goat. Biochem. J. 48, 411-416. 

Trans-10, cis-12 CLA is another CLA isomer in milk fat which can affect lipid metabolism. It is generally present at low concentrations in milk fat (typically <0.2% of CLA) under some dietary conditions, a portion of the rumen biohydrogenation shifts to produce more of this isomer, although it is still only a minor portion of total CLA. These dietary conditions are associated with milk fat depression and as little as 2 g/d of tram-10, cis-12 leaving the rumen will reduce milk fat synthesis by 20%. Because of the potency and specificity of this CLA isomer, it is being developed as a dairy management tool to allow for a controlled reduction in milk fat output. 

A broad overview of the biological effects of CLA is presented elsewhere in this volume (Chapter 17), so the emphasis in the following section will be two-fold. Firstly, the biology of trans-10, cis-12 CLA in the dairy cow will be summarized because under certain dietary conditions, production of this isomer in the rumen can profoundly affect milk fat synthesis. Secondly, the biological effects of RA when supplied as a natural component of the diet will be reviewed because this CLA isomer represents a functional component of milk fat that has potential health benefits. Although other CLA isomers are present in milk fat, they are present at concentrations much too low to have a significant effect. 

Investigations in which the transfer of CLA to milk fat in dairy cows was examined showed that supplementation of mixed isomers of CLA resulted in a dramatic reduction in milk fat secretion (Loor and Herbein, 1998 Chouinard et al., 1999a, b). Decreases of up to 50% in milk fat yield occurred and the effects were reversed when supplementation was terminated. Furthermore, effects were specific for milk fat with the yield of milk and other milk components being relatively unaffected. Initial investigations were of short duration (<7 days) and the CLA supplement was infused abomasally as a convenient experimental method to avoid possible alterations during rumen fermentation. However, subsequent long-term studies (20 weeks) demonstrated that the reduction in milk fat synthesis was... 

Relationships between trans-10, cz.y-12 CLA and milk fat synthesis have been examined. There is a curvilinear relationship between the reduction in milk fat yield and the abomasal infusion dose of trans-10, cis-12 CLA (Figure 3.4). Trans-10, cis-12 CLA is a very potent inhibitor of milk fat synthesis in dairy cows a dose of 2.0 g/d (<0.01% of dry matter intake) reduced milk fat synthesis by 20%. Trans-10, cis-12 CLA is also incorporated into milk fat and in this case the relationship is linear (Figure 3.4) a summary of seven studies showed that the transfer efficiency of aboma-sally-infused trans-10, cis-12 CLA into milk fat averaged 22% (de Veth et al., 2004). The linear relationship in transfer to milk fat is remarkable when one considers that the yield of milk fat is simultaneously decreased as the abomasal dose of trans-10, cis-12 CLA is increased. This suggests that the mechanisms which coordinate the CLA-induced decrease in the use of preformed fatty acids for milk fat synthesis have a less pronounced effect on the mammary uptake and incorporation of trans-10, cis-12 CLA into milk fat, but the basis for this difference is unknown. 

Baumgard, L.H., Sangster, J.K., Bauman, D.E. 2001. Milk fat synthesis in dairy cows is progressively reduced by increasing supplemental amounts of trans-10, cis-12 conjugated linoleic acid (CLA). J. Nutr. 131, 1764-1769. 

Bickerstaffe, R., Johnson, A.R. 1972. The effect of intravenous infusions of sterculic acid on milk fat synthesis. Br. J. Nutr. 27, 561-570. 

Moore, C.E., Hafliger, H.C., Mendivil, O.B., Sanders, S.R., Bauman, D.E., Baumgard, L.H. 2004. Increasing amounts of conjugated linoleic acid (CLA) progressively reduces milk fat synthesis immediately postpartum. J. Dairy Sci. 87, 1886-1895. 

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