Glycogen metabolism and post-exercise ketosis in carbohydrate-restricted trained and untrained rats

Liver and muscle glycogen, and blood 3-hydroxybutyrate concentrations were studied during and for 2 h after treadmill running for 1 h, in 144 carbohydrate-starved trained and untrained rats. The resting liver glycogen concentration of the trained animals was 227 +/- 8 (mean +/- S.E.M.) mumol glucosyl units/g wet mass, compared with 162 +/- 12 mumol/g in the untrained animals. The muscle glycogen levels were 42 +/- 1 and 28 +/- 1 mumol/g respectively. Exercise reduced muscle and liver glycogen concentrations by approximately the same absolute amounts in both animal groups, leaving the trained rats with nearly 3 times as much residual glycogen as the untrained animals. There was very little resynthesis of muscle glycogen recovery, but the trained animals replenished approximately 43% of the liver glycogen used during exercise. The blood 3-hydroxybutyrate concentrations were negatively correlated with the simultaneous liver glycogen concentration of our experimental animals (r = -0.55; P less than 0.001). It is concluded that trained animals primarily owe their resistance to post-exercise ketosis to their large stores of glycogen.