Splanchnic metabolism of long-chain fatty acids in ruminants

The liver and portal-drained viscera (PDV) work in series to deliver dietary nutrients and coordinate nutrient use by the rest of the body. The PDV tissues are generally net suppliers of nonesterified fatty acids (NEFA), but may utilize some triacylglycerol (TAG) and NEFA during feed deprivation. The liver takes up more NEFA than the PDV releases, resulting in the total splanchnic tissues being a "sink" for NEFA during mobilization. Production of ketone bodies from NEFA generally is well-matched to NEFA uptake. Hepatic synthesis of very-low-density lipoproteins (VLDL) for export of TAG by nature is not a major fate of NEFA in ruminants. Hepatic lipid accumulation occurs in periparturient dairy cows because of intensive NEFA mobilization. While much has been learned about regulation of hepatic NEFA metabolism, causes of extreme fatty liver and ketosis remain unclear. The capacity of liver tissue to metabolize NEFA via oxidation or esterification changes with physiological state, and may be able to be manipulated to prevent TAG accumulation and to optimize production and health. Strategies to increase oxidative disposal of NEFA may be more successful than attempts to increase VLDL output. Interrelationships of PDV-associated adipose tissues and the liver are fertile areas for future investigation.