Hepatic lipid metabolism in exercise and training.

The liver plays a central role in the metabolism of fat. The available data, though sometimes controversial, clearly indicate that muscular exercise affects almost every aspect of fat metabolism in this organ. Neither acute exercise nor training affects total lipid, phospholipid, or cholesterol concentrations in the liver of rats fed chow or low fat diets. However, exercise training reduces accumulation of total hepatic fat and cholesterol in rats fed a fat-rich diet. In addition, training seems to increase both the synthesis and catabolism of cholesterol in the liver in rats fed a chow diet. Production of ketones by the liver increases both during prolonged exercise and during recovery from exercise. Acute prolonged exercise reduces the activities of the enzymes involved in the synthesis of fatty acids and increases oxidation of fatty acids by the liver. This type of work also increases the esterification of fatty acids with the subsequent accumulation of triacylglycerols in this organ. Training does not affect triacylglycerol concentration in the liver of rats fed a chow diet but attenuates its accumulation after a fat-rich diet. Training reduces the postheparin plasma hepatic lipase activity. Finally, it reduces production of triacylglycerols and increases production of high density lipoprotein cholesterol by the liver. A large body of descriptive information has been published indicating that exercise has a dramatic effect upon hepatic lipid metabolism. The next step in this work is the identification of the molecular mechanisms responsible for these exercise-induced alterations.