Utilization of Volatile Fatty Acids in Ruminants. I. Removal of Them from Portal Blood by the Liver

Abstract

Removal of volatile fatty acids from portal blood by the liver was studied in 20 ewes and 5 goats. When animals were fed hay, practically all of the propionate but only a small part of the acetate present in portal blood was removed by the liver. However, on a molar basis more acetate than propionate was taken up by the liver. In animals fed grain for extended periods, or grain plus fish oil, the proportion of propionate in the rumen fluid increased markedly and more propionate than acetate was taken up by the liver. The liver showed a greater capacity for removing acetate than either propionate or butyrate from the portal blood when these acids were added to the rumen. Butyrate was not present in significant amounts in right rumen vein blood or portal blood. Only acetate and propionate are absorbed from the rumen as such and presented to extraruminal tissues for metabolism. This work confirms the conclusion by others (4, 6) that propionate is metabolized mainly by the liver and acetate by extrahepatic tissues. The manner in which butyrate is utilized by ruminant tissues under normal conditions remains to be elucidated. © 1965, American Dairy Science Association. All rights reserved.