Factors Affecting Cla Content Of Foods

Processing has little effect on CLA; thus, its content in food products is related to the concentration in the original milk and meat fat.[2] Most investigations of factors that influence CLA have examined milk fat synthesis in the dairy cow.

Diet is the most significant factor affecting the CLA in ruminant fat and the concentration can be increased several-fold by dietary means.[5,6] Increasing the dietary supply of 18-carbon PUFA substrates by addition of seeds or plant oils high in linoleic and/or linolenic acids (e.g., soybeans or sunflowers) results in an increase in rumen

Fig. 1 Structures of two isomers of conjugated linoleic acid (CLA) and linoleic acid (cis 9, cis 12 18:2). Arrows indicate double bond position. (From Ref. 5.) (View this art in color at www.dekker.com.)

trans-10, c/s-12 CLA c/s-9, irans-11 CLA linoleic acid

Fig. 1 Structures of two isomers of conjugated linoleic acid (CLA) and linoleic acid (cis 9, cis 12 18:2). Arrows indicate double bond position. (From Ref. 5.) (View this art in color at www.dekker.com.)

output of VA, and to a lesser extent CLA, which allows for greater endogenous synthesis of CLA. Dietary factors that affect rumen bacteria involved in biohydrogenation, either directly or indirectly via changes in rumen environment, can also affect the CLA content of ruminant fat. In general, no single bacterium carries out the complete biohydrogenation process; rather, one group carries out the steps to convert linoleic and linolenic acids to VA and then another bacteria group carries out the final step to form stearic acid.[3] Alterations in the rumen environment can differentially affect these two groups so that the rates and extent of biohydrogenation are altered.

In particular, increasing the forage/concentrate ratio increases the CLA concentration by this means.

A combination of dietary supply of PUFA and modification of the rumen environment can be especially effective to increase the CLA content of ruminant fat. Dietary supplements of fish oil and grazing pasture are two examples. Fresh pasture results in a two- to three-fold increase in the CLA content of milk fat, but the effect diminishes as the pasture matures. Increases in CLA observed with fish oils and feeding fresh pasture cannot be fully explained in terms of their PUFA content. Therefore, other factors or components of these feeds must affect

Rumen

Tissues

Rumen

Linolenic acid

Linoleic acid

X

I

trans-11, cis-15 18:2

1

X

18:0

cis-9, trans-11 CLA t

I A9-desaturase trans-1! 18:1

Fig. 2 Pathways for the biosynthesis of cis 9, trans 11 conjugated linoleic acid (CLA) in ruminants. (From Ref. 4.)

rumen bacteria involved in biohydrogenation, thereby promoting rumen production of VA and CLA. Dietary supplements of CLA or VA can also be used to enhance the CLA content of ruminant fat. To be effective, these must be protected from rumen biohydrogenation, and dose-dependent increases in milk fat CLA content have been achieved.

Physiological factors that affect milk-fat content of CLA have also been examined, and differences among individuals are particularly striking.[4] Even when diet and other physiological variables are similar, there is still a three-fold range among individuals in the milk fat concentration of CLA. The CLA-desaturase index is a proxy for A9-desaturase that is based on the relationship between substrate and product for endogenous synthesis of CLA, and it also varies about three-fold among cows regardless of diet. To a large extent, milk fat differences in CLA content and CLA desaturase index among individuals are maintained throughout lactation and across dietary shifts. This suggests a genetic basis related to rumen output of VA (and to a lesser extent CLA) or the enzyme A9-desaturase. Examination of other physiological factors has established that milk fat content of CLA and CLA-desaturase index has little relation to milk or milk fat yield. Further, parity, stage of lactation, and breed have minimal effects when compared to the variation among individuals or the effect of diet.

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