Identification of the conjugated linoleic acid isomer that inhibits milk fat synthesis

Conjugated linoleic acids (CLA) are octadecadienoic fatty acids that have p rofound effects on lipid metabolism. Our previous work showed that CLA (mix ture of isomers) markedly reduced milk fat synthesis. In this study, our ob jective was to evaluate the effects of specific CLA isomers. Multiparous Ho lstein cows were used in a 3 x 3 Latin square design, and treatments were 4 -day abomasal infusions of 1) skim milk (control), 2) 9,11 CLA supplement, and 3) 10,12 CLA supplement. CLA supplements provided 10 g/day of the speci fic CLA isomer (cis-9,trans-11 or trans-10,cis-12). Treatments had no effec t on intake, milk yield, or milk protein yield. Only the 10,12 CLA suppleme nt affected milk fat, causing a 42 and 44% reduction in milk fat percentage and yield, respectively. Milk fat composition revealed that de novo synthe sized fatty acids were extensively reduced. Increases in ratios of C-14:0 t o C-14:1 and C-18:0 to C-18:1 indicated the 10,12 CLA supplement also alter ed Delta(9)-desaturase. Treatments had minimal effects on plasma concentrat ions of glucose, nonesterified fatty acids, insulin, or insulin-like growth factor-I. Overall, results demonstrate that trans-10,cis-12 CLA is the iso mer responsible for inhibition of milk fat synthesis.