Endurance Training

Since proteins serve either a structural or functional role within the cell, chronic endurance exercise training would be expected to achieve adaptations that would attenuate the oxidation of protein for energy. This would also be predicted based on the sparing of muscle glycogen that accompanies chronic endurance training, which would tend to attenuate BCOAD activation. Early work by Gontzea et al.73 showed that untrained persons who started endurance training were in a negative nitrogen balance, but as they continued to train, the nitrogen balance became less negative.

To date, human data have not yielded consistent findings on the effects of chronic exercise training on protein metabolism. Following endurance exercise training, whole-body protein synthesis at rest is increased.69 143 There is also a greater proportion of leucine flux at rest diverted toward oxidation in the untrained vs. trained athlete.69 However, differences in leucine turnover between trained and untrained subjects disappeared when the data were expressed relative to lean mass.42 These findings are not consistent with the hypothesis that endurance exercise training attenuates glycogen use and spares protein oxidation. For this reason we designed an experiment to train sedentary individuals for 38 days and to measure their leucine oxidation and BCOAD activation during exercise, before and after the training.40 We found that leucine oxidation during exercise was lower after training, as was BCOAD activation.40 However, consequent to the increase in total mitochondrial content, the absolute capacity of BCOAD enzyme activity was higher in the trained state.40 Our findings were confirmed in a recent study that showed that resting amino acid oxidation was lower after endurance exercise training.144 Taken together, data from the latter two studies confirm that chronic endurance training results in a sparing of amino acid oxidation. With the greater total amount of BCOAD activity after endurance exercise training,40 the maximal capacity for amino acid oxidation would be higher in the trained state. However, it is likely that only top sport athletes training for long hours and at a high relative intensity or during periods of nutritional stress (i.e., low energy or CHO intake) could ever strain metabolic capacity such that the daily amount of amino acid oxidation would exceed that in the untrained or moderately trained individual.

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