Effects of glycerol feeding before and after exhausting exercise in rats

Rats were fed either 1 g of glycerol or 1 g of glucose following exercise to exhaustion. Glucose feeding resulted in a rapid increase in blood glucose, with attainment of peak hyperglycemia within 1 h and of peak muscle glycogen in about 2 h. Feeding 1 g of glycerol resulted in marked elevation of glycerol concentration in blood, liver, and muscle; the highest values, measured 1 h after glycerol feeding, ranged from 29 to 42 mumol/g tissue. Blood glucose and muscle glycogen increased slowly in the glycerol-fed rats and peaked much later than after glucose feeding. In perfused rat hindlimb muscles, negligible amounts of [14C]glycerol were incorporated into glycogen. These findings suggest that muscle is not able to utilize glycerol to a significant extent and that glucose derived from glycerol was the major substrate for muscle glycogen synthesis. Animals fed glycerol before a run to exhaustion were able to exercise significantly longer than control rats (153 +/- 11 min vs. 116 +/- 6 min). The glycerol-fed rats depleted their muscle and liver glycogen stores less rapidly than the controls and were protected against development of hypoglycemia during the exercise.