The presence of CLA in ruminant milk and meat is related to rumen fermentation. The rumen contains many types of bacteria, and a portion converts dietary PUFA to saturated fatty acids. This process is referred to as biohydrogenation and is the basis for the greater content of saturated fat in ruminants compared to nonruminants. The most common plant PUFA consumed by ruminants are linoleic and linolenic acids, and their biohydrogenation is shown in Fig. 2. Cis-9, trans-11 CLA is an intermediate formed during the biohydrogenation of linoleic acid, whereas trans-11 18:1 (vaccenic acid; VA) is an intermediate from both linoleic and linolenic acids.
Initially it was assumed that the CLA in milk fat and body fat of ruminants originated from incomplete biohydrogenation in the rumen. However, studies revealed that CLA represented only a transitory product during biohydrogenation, and VA was the major intermediate that accumulated in the rumen. This led to more extensive investigations and it was discovered that cis-9, trans-11 CLA in milk and meat fat originates mainly from endogenous synthesis by the animal's own tissues; only a minor portion comes from production in the rumen. The substrate used to form this CLA isomer is VA, produced as an intermediate during rumen biohydrogenation; the enzyme that catalyzes the reaction is A9-desaturase (Fig. 2). Endogenous synthesis occurs primarily in the mammary gland during lactation and in adipocytes during the growth phase.
The second most prevalent CLA isomer in ruminant fat is trans-7, cis-9 CLA (- 5 10% of total CLA) and this also arises from endogenous synthesis involving A9-desaturase with the precursor being trans-7 18:1 produced in the rumen. Other individual CLA isomers originate from rumen production and are present in only trace amounts in milk and body fat.
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