Economics of visceral energy metabolism in ruminants: Toll keeping or internal revenue service?

Abstract

Measurements across a range of productive states show that the portal-drained viscera and liver, or the total splanchnic tissues, account for 40 to 50% of body oxygen consumption or heat. This high rate of metabolism is in part attributable to high rates of protein turnover and thus, AA utilization, as well as other “service” functions supporting nutrient assimilation and “waste management.” This metabolic intensity and the anatomic position of absorptive and liver tissues have led to the assumption that the tissues that assimilate and process incoming nutrients from the diet exact a toll in payment for their entry. This “toll” is believed to reduce the extent to which absorbed nutrients gain admission to the arterial blood pool and reach “productive” organs such as the mammary gland or skeletal muscle. Measurements of net nutrient flux generally support this concept of splanchnic metabolism “restricting entry” and, thus, dictating supply. On a net basis the appearance of the major carbon-based nutrients absorbed into the portal vein is typically low compared to their rate of disappearance from the gut lumen. An alternative interpretation is that this low net recovery of absorbed nutrients across splanchnic tissues is attributable to extensive metabolism of nutrients from the arterial pool, which masks true rates of absorption. In this scenario, any tax to support community services is paid using internal funds. Measurements of nutrient kinetics, based on isotopic labeling, support the latter scenario. In the case of the liver, catabolism of AA is driven in part by supply and demand, with overpopulation dealt with by deportation, restructuring, or the metabolic equivalent of cremation. Similarly, relative rates of AA metabolism by the gut and mammary gland vary with requirement. Metabolism of many energy-yielding nutrients varies with supply, demand, and the need for community services such as waste management.