Metabolism of Uniformly Labeled Glucose-C14 Introduced into the Rumen of a Lactating Cow. I. Transfer of C14 to Respired Air, Volatile Fatty Acids, and Major Milk Constituents1,2

Abstract

Summary To study the metabolic fate of dietary glucose in the lactating cow, uniformly C 14 -labeled glucose (G-U-C 14 ) was introduced through a fistula into the rumen. Continuous samples of respired CO 2 were collected and assayed for carbon-14; in addition, samples of rumen and duodenal contents, jugular and ruminal vein blood, and milk were collected at frequent intervals and assayed for Carbon-14. Following the administration of G-U-C 14 , the specific activity reached a maximum in respired CO 2 at 45min and in blood volatile fatty acids (VFA) at 15min, but the maximum was not reached in plasma glucose until 2 hr had elapsed. The first sample of rumen VFA, collected 1 hr after G-U-C 14 administration, had the highest specific activity and later samples decreased in specific activity by approximately one-half each 4–5 hr. Carbon-14 was not detected in rumen VFA after 24 hr. The specific activity of milk constituents after rumen administration of G-U-C 14 was different from that observed after intravenous injection of G-U-C 14 , but was similar to results obtained after intravenous injection of C 14 -labeled short-chain fatty acids. These results indicate that glucose was rapidly converted to short-chain fatty acids in the rumen before its absorption and utilization in the tissues of the cow for synthesis of milk components and oxidation to carbon dioxide. This study presents new information on the difference in metabolic fate of dietary glucose compared to intravenously administered glucose in the ruminant and shows the rapid transfer of carbon from dietary glucose to blood VFA, which had not been previously demonstrated.