The present invention alters mammary synthesis of fat to improve milk quality. These changes in milk composition represent improvements in nutritional quality consistent with contemporary dietary recommendations. Of special importance is the disclosure of new data relating to specific conjugated linoleic acids (CLA), potent naturally occurring anti-carcinogens. In the course of an investigation to enhance milk content of conjugated linoleic acid, it was discovered that abomasal infusion of a single TFA isomer caused a marked milk fat depression. This observation was unexpected because the prior art has consistently shown that body fat and milk fat always show reciprocal changes in lactating cows and indicated that CLA's generally reduced body fat in growing animals. The current disclosure demonstrates that an increase in milk fat content of a specific TFA isomer, trans-10 C18:1. (Griinari et al., 1997, 1998) causes MFD. This observation is in conflict with the prior art that taught that an increase in total TFA caused MFD. These results are applicable to other domestic lactating mammals (e.g., pigs). Upon the infusion of CLA, a portion of the CLA is transferred to the mammary gland and incorporated into milk fat. Hence, the methods disclosed increase the levels of CLA found in milk, thereby improving the nutritional benefits to human health associated with CLA.