Introduction
Conjugated linoleic acid is a collective term for a mixture of positional and geometric isomers of linoleic acid with conjugated double bonds. It has received considerable attention in recent years because it possesses potential anticarcinogenic effects based on research with laboratory animals (Ip et al., 1994; Parodi, 1994, 1997). Dairy products are one of the major dietary sources of CLA, of which cis-9, trans-11-18:2 is the major isomer (Chin et al., 1992; Lin et al. 1995). Besides the potential anticarcinogenic effect, CLA may work as an antioxidant and play beneficial roles in coronary heart disease. It may protect against catabolic effects of immune stimulation and enhance immune function. It may also affect growth and body composition by diminishing fat deposition, while promoting lean mass growth (McGuire et al., 1997).
It is now well established that CLA can be formed both ruminally by microbial activity during biohydrogenation (BH) of linoleic acid (LA) (Kepler et al., 1966) and endogenously from vaccenic acid (VA), which also is an intermediate product of ruminal BH by the activity of D9-desaturase (Griinari et al., 2000). During the BH of LA, it is first isomerized into CLA (cis-9, trans-11-C18:2, specifically), the CLA is then quickly hydrogenated into VA (trans-11-C18:1), and the VA can be further hydrogenated into stearic acid. It is known that the first two steps are not rate limiting and take a relatively short time to complete, while the last step is rate limiting and usually takes a longer time to complete. Under certain conditions, particularly with high concentration of LA, reduction of VA seems to be inhibited (Noble et al., 1974).
Considerable research efforts have been made through nutritional practice to enhance the CLA content of milk (Dhiman, 2000; Palmquist, 2000). However, not nearly as much has been done to elucidate the contribution of different factors on the production of CLA and VA in the rumen.
We hypothesized that the production of CLA in the rumen is affected by solid dilution rate (SDR), dietary source and level of fat, and ruminal pH. Two experiments were conducted to investigate these effects.