Upgrading the lipid fraction of foods of animal origin by dietary means: rumen activity and presence of trans fatty acids and CLA in milk and meat

Abstract

The recent literature dealing with the effect of the diet on the quality of milk and meat fat is reviewed. Some aspects ofthe rumen metabolism of lipids are dealt with: lipolysis, bio-hydrogenation, synthesis of microbial fatty acids and inhibitionmechanisms on fermentation. Firstly, the influence of forage is considered. Pasture is the best forage, better if highhill pasture, as compared to hay and silage: short chain fatty acids (SCFA) (shorter than C10) are increased, mediumchain fatty acids (MCFA) (C 12 through C 16 ) are decreased, oleic (OA), linoleic (LA) and linolenic (LNA) acids are increasedand so are the conjugated linoleic acid pool of isomers (CLA) and n-3 polyunsaturated fatty acids (n-3 PUFA). Secondly,the energy supplementation of diets with fats is looked at. Animal fats depress milk yield and SCFA, while OA is increasedbecause of the enhanced activity of mammary Δ 9 desaturase. Fish oil depresses milk yield as well, but promotes CLA andn-3 PUFA. If animal fats are protected against rumen bacteria, milk yield and milk fat depression are avoided. Vegetablefats are richer in unsaturated fatty acids (UFA), thus more susceptible to the rumen bio-hydrogenation. As calcium soapsor inside whole seeds, plant fats are protected and CLA is increased. CLA is an important component of fat. In ruminantsit comes from the desaturation of vaccenic acid (VA) both in rumen and udder; and the yield of VA depends on the dietquality. In conclusion, simple directions are given on how to improve the quality of animal fat by dietary means, withoutaffecting yield.