A reduced lactate mass explains much of the glycogen sparing associated with training

Endurance training is associated with glycogen (Gly) sparing, generally attributed to less carbohydrate (CHO) oxidation. However, untrained individuals commit a greater fraction of CHO to lactate (La), accounting for a portion of the Gly 'spared.' We examined the effects of training (running 1 h/day at 30 m/min up an 8° grade) on whole body CHO distribution and oxidation. Female Long Evans rats (n = 27) were assigned to control (Untr) and trained (Tr) groups. Two days before the experiment, animals were chronically catheterized. On the day of the experiment, animals ran for 20 min at a speed of 28 m/min and were killed with an overdose of pentobarbital sodium injection while running. Whole carcasses were then promptly freeze- clamped with a liquid N₂-cooled press. Whole body carcass powder was assayed for La, Gly, and glucose. Resting whole body values were not different between groups (La = 0.78 ± 0.06 vs. 0.83 ± 0.07, Gly = 4.46 + 0.62 vs. 3.77 ± 0.35, glucose = 0.19 ± 0.07 vs. 0.23 ± 0.09 mmol/body for Tr and Untr rats, respectively). However, postexercise La was higher in Untr vs. Tr group (2.01 ± 0.28 vs. 1.13 ± 0.09 mmol/body), and Gly was lower in the Untr vs. Tr rats (1.58 ± 0.25 vs. 3.42 ± 0.43 mmol/body). Similarly, Untr animals displayed higher epinephrine levels than Tr at the end of the exercise bout (4.9 ± 1.0 vs. 1.7 ± 0.4 ng/ml). Differences between groups in La and glucose masses (postexercise minus rest data) accounted for 60% of the Gly differences. Gly spared from oxidation and replaced by increased fat oxidation only accounted for 40% of the differences in Gly levels between Tr and Untr animals. We conclude that untrained mammals commit a significant portion of their CHO pool to La, which accounts for almost one-half of the apparent Gly spared during moderate-intensity exercise in the trained state.