Characterization of the acute lactational response to trans-10, cis-12 conjugated linoleic acid.

Trans-10, cis-12 conjugated linoleic acid (CLA) is a potent inhibitor of milk fat synthesis in the dairy cow. The decrease in milk fat yield during abomasal infusion of CLA reaches a nadir after 3 to 5 d. The acute responses to CLA were evaluated using 4 cows in a crossover design. Cows were milked with the aid of oxytocin every 4h from -28 to 80h and every 6h from 86 to 116h relative to the initiation of abomasal CLA infusion. An initial priming dose of 7.5g of CLA was given at time zero followed by infusion of 2.5g every 4h for 72h. Plasma CLA reached a near-steady-state concentration by 4h, and initial plasma enrichments were greatest in the triglyceride and nonesterified fatty acid fractions. Milk CLA concentration peaked at 6h and reached steady state by 22h. At termination of the infusion, decreases in milk CLA concentration and yield and plasma CLA concentration were best fit by a reciprocal-linear function. Milk fat percentage decreased progressively after 2h and was significant by 14h. Milk fatty acid profile was initially unchanged, but between 18 and 36h after initiation of the CLA dose the proportions of fatty acids progressively shifted, resulting in an increase in fatty acids >C16 and a decrease in fatty acids <C16 by 38 to 46h. In contrast, changes in the desaturase index were immediate, with a significant decrease by 6h and a near-maximal decrease by 10h. Thus, stearoyl desaturase enzyme was more acutely responsive to CLA than other enzymes in milk fat synthesis. The initial decrease in milk fat synthesis involved an equal depression of short- and long-chain fatty acid pathways and was followed thereafter by a more pronounced decrease in the synthesis of de novo fatty acids.