Regulation of nutrient partitioning by visceral tissues in ruminants

Together, tissues of the portal-drained viscera and liver account for 35 to 53% of body oxygen uptake in ruminants, and therefore have a substantial impact on the partition of metabolizable energy between heat loss and production. As proposed more than a century ago, these tissues are principal determinants of heat increment of feeding and increases in heat resulting from increased fiber digestion. The metabolism of these tissues also has a profound impact on the structure and quantity of absorbed nutrients ultimately available for utilization by peripheral tissues. Substantial amounts of absorbed volatile fatty acids and amino acids are oxidized or transformed during their absorption and never reach the portal vein in the form in which they were absorbed. In addition, the liver utilizes large quantities of these nutrients to support glucose, urea and protein synthesis. Ruminants absorb large amounts of ammonia which must be converted to urea by the liver and portal-drained viscera absorption of ammonia and liver urea production are highly correlated with nitrogen intake, but portal-drained viscera absorption of alpha-amino and urea nitrogen is poorly correlated with nitrogen intake. The portal-drained viscera and liver also effect nutrient partitioning by regulating amounts of insulin and glucagon released to peripheral tissues.