Biosynthesis of conjugated linoleic acid in ruminants

Food products from ruminants are the major dietary source of conjugated linoleic acids (CLA) for humans. The uniqueness of CLA in ruminant fat relates to the biohydrogenation of dietary unsaturated fatty acids by rumen bacteria. The CLA are intermediates in the biohydrogenation, and a portion escape the rumen and are incorporated into milk fat and body fat. In addition, the animal itself synthesizes cis-9, trans-11 CLA from trans-11 octadecenoic acid, another intermediate in ruminal biohydrogenation that is absorbed. This involves ∆-desaturase, which is present in mammary tissue (lactation) and adipose tissue (growth). Investigations to alter the content of CLA have typically involved lactating cows (milk fat); fewer data from growing cattle (body fat) are available. Dietary factors that alter the content of CLA because of effects on the rumen biohydrogenation processes include unsaturated fatty acid substrates and altered rumen environment. The cis-9, trans-11 CLA isomer is the major isomer found in ruminant fat; this isomer typically represents 80 to 90% of the total CLA in milk fat, but its proportion in beef fat is less. Under certain dietary conditions the proportion of the trans-10, cis-12 CLA isomer increases. Thus, dietary factors also alter the direction of the biohydrogenation pathways in the rumen. The CLA possess anticarcinogenic effects, which relates to the cis-9, trans-11 CLA isomer, as evident from results with mammary tumors in a rat model. Lipid accretion and nutrient partitioning are also altered by CLA in several species. Recent work demonstrates that this relates primarily to the trans-10, cis-12 CLA isomer, as evident by effects on milk fat synthesis in lactating cows and body fat accretion in growing mice. Overall, consideration of functional foods containing CLA represents an exciting area of potential importance in producing food products derived from ruminants.