Conjugated linoleic acids alter milk fatty acid composition and inhibit milk fat secretion in dairy cows.

Conjugated linoleic acids (CLA) have positive health effects in experimental models. Our objective was to determine the effect of CLA supplementation on milk of dairy cows. A commercial source of CLA was infused abomasally to by-pass rumen fermentation. The supplement contained 61.2% CLA; the major CLA isomers were cis/trans 8,10, cis/trans 9,11, cis/trans 10,12 and cis/trans 11,13. Four Holstein cows were used in a 4 x 4 Latin square design. Treatments were 5-d infusions of 0, 50, 100 and 150 g/d of CLA supplement. Infusion increased milk fat content of CLA from 6.8 mg/g fat (zero dose) to 63.6 mg/g fat (highest dose). All of the major CLA isomers in the supplement were transferred to milk fat in a dose-dependent manner. Apparent efficiency of transfer to milk fat was 22.5, 22.5, 10.2 and 26.3% for cis/trans 8,10, cis/trans 9,11, cis/trans 10,12 and cis/trans 11,13, respectively. CLA infusion had no effect on milk protein and little effect on milk yield (21.5, 20.4, 20.9 and 18.3 kg/d for 0, 50, 100 and 150 g/d CLA supplement, respectively). However, CLA infusion dramatically reduced milk fat. On average, the content and yield of milk fat were reduced by 52 and 55%, respectively. The role of specific CLA isomers and mechanism(s) for the reduction in milk fat have not been established, although the pattern of milk fatty acids demonstrated effects were most pronounced on de novo fatty acid synthesis and the desaturation process. Overall, dietary supplemention of CLA increased milk fat content of CLA, altered milk fatty acid composition and markedly reduced the content and yield of milk fat.