Utilization of skeletal muscle triacylglycerol during postexercise recovery in humans

The utilization of muscle triacylglycerols was studied during and after prolonged bicycle ergometer exercise to exhaustion in eight healthy young men. Two days before exercise and in the postexercise recovery period, subjects were fed a carbohydrate-rich diet (65-70% of energy from carbohydrates). Exercise decreased muscle glycogen concentrations from 533 +/- 18 to 108 +/- LO mmol/kg dry wt, whereas muscle triacylglycerol concentrations were unaffected (49 +/- 5 before vs. 49 +/- 8 mmol/kg dry wt after exercise). During the first 18 h after exercise, muscle glycogen concentrations were restored to 409 +/- 20 mmol/kg dry wt. In contrast, muscle triacylglycerol concentrations decreased (P < 0.05) to a nadir of 38 +/- 5 mmol/kg dry wt, and muscle lipoprotein lipase activity increased by 72% compared with values before exercise. Pulmonary respiratory exchange ratio values of 0.80-0.82 indicated a relatively high fractional lipid combustion despite the high carbohydrate intake. From 18 to 42 h of recovery, muscle glycogen synthesis was slow and muscle triacylglycerol concentrations and lipoprotein lipase activity were restored to the preexercise values. It is concluded that muscle triacylglycerol concentrations are not diminished during exhaustive glycogen-depleting exercise. However, in the postexercise recovery period, muscle glycogen resynthesis has high metabolic priority, resulting in postexercise lipid combustion despite a high carbohydrate intake. It is suggested that muscle triacylglycerols, and probably very low density lipoprotein triacylglycerols, are important in providing fuel for muscle metabolism in the postexercise recovery period.