Digestion and utilisation of fatty acids by ruminants

Abstract

This review deals with the quantitative and qualitative changes in fatty acids (FA) throughout the total digestive tract of ruminants. Special attention is paid to the causes of variation in the extent to which different mechanisms contribute to the ruminal metabolism and intestinal digestion of FA. Most results obtained with diets not supplemented with lipids show that the FA flow leaving the rumen is higher than FA intake. This is due to bacterial synthesis of FA in the rumen. With diets supplemented with lipids, the FA balance at the end of the rumen is often negative. The cause of this apparent disappearance of FA is not known. In the rumen, lipids are first hydrolysed to a very large extent; then unsaturated FA are hydrogenated. Hydrogenation is almost complete for linolenic acid, and amounts to between 60 and 95% for linoleic acid. This proportion decreases when the level of concentrates increases in the diet. Digestibility of FA in the small intestine ranges from 70 to 90% and is not related to the level of FA intake. Contrary to the situation in monogastric animals, there are only moderate differences in the digestibility of individual FA. It appears to be higher for palmitic and stearic acids than for other saturated FA, and for oleic and linoleic acids than for stearic and linolenic acids. In the large intestine, there is synthesis of FA which are probably not absorbed.