Effects of increased plasma fatty acids on glycogen utilization and endurance

The purpose of this study was to test the hypothesis that increased availability of fatty acids could increase endurance by slowing the rate of glycogen depletion. Rats were given corn oil by stomach tube, and 3 h later an injection of heparin was given to raise their plasma free fatty acids (FFA). The rats with raised FFA were able to run approximately 1 h longer than otherwise comparable control animals before becoming exhausted (181 +/- 8 vs. 118 +/- 8 min, P less than 0.001). At the point of exhaustion, both groups were hypoglycemic and had low muscle glycogen concentrations. The fall in blood glucose occurred less rapidly in the animals with raised FFA; these rats also had significantly higher blood glycerol and beta-hydroxybutyrate concentrations than the controls. Glycogen concencentration decreased less rapidly in all three types of skeletal muscle and in liver in the animals with raised FFA than in the controls. We conclude that increased availability of fatty acids delays the development of exhaustion in rats subjected to prolonged running. It appears likely that the carbohydrate-sparing effect of fatty acids is largely responsible for the increase in endurance.