Regulation of lipid metabolism in the rumen.

Lipid added to animal diets can affect productive efficiency through a combination of caloric and regulatory effects. Any attempt to improve productive efficiency of ruminants by modifying the amount or composition of lipid added to the diet must take into account ruminal metabolism of lipids. The caloric value of lipid is changed little as it passes through the rumen, but the greater concern is the possible negative effect of lipid on the energy value of the basal diet. Limiting unsaturated fatty acids fed to ruminants to just a few percent of the diet to avoid digestive disturbances, combined with extensive hydrogenation by ruminal microbes, maintains only small quantities of unsaturated fatty acids in duodenal contents. However, in some studies, lipid added to ruminant diets has invoked metabolic changes that were not attributable to caloric effects. These metabolic changes suggest that the rumen does not maintain digesta flow with constant lipid composition in all circumstances. Variations in rumen outflow of biohydrogenation intermediates and microbial lipids are as great, or greater in some situations, than unsaturated fatty acids. Diet characteristics, such as amount of grain or unsaturated lipid, can substantially alter the nature of fatty acids absorbed from the duodenum and deposited in animal tissues.