Differential metabolic fate of the carbon skeleton and amino-N of [C]alanine and [N]alanine ingested during prolonged exercise

Korach-André, M., Y. Burelle, F. Péronnet, D. Massicotte, C. Lavoie, and C. Hillaire-Marcel. Differential metabolic fate of the carbon skeleton and amino-N of 13Cand [15N]alanine ingested during prolonged exercise. J Appl Physiol 93: 499–504, 2002. First published April 5, 2002; 10.1152/japplphysiol.01195.2001.—The decarboxylation/oxidation and the deamination of 13Cand [15N]alanine ingested (1 g/kg or 73.7 2 g) during prolonged exercise at low workload (180 min at 53 2% maximal O2 uptake) was measured in six healthy male subjects from V̇CO2 at the mouth and [15N]urea excretion in urine and sweat. Over the exercise period, 50.6 3.5 g of exogenous alanine were oxidized (68.7 4.5% of the load), providing 10.0 0.6% of the energy yield vs. 4.8 0.4, 47.6 4.3, and 37.4 4.7% for endogenous proteins, glucose, and lipids, respectively. Alanine could have been oxidized after conversion into glucose in the liver and/or directly in peripheral tissues. In contrast, only 13.0 3.2 mmol of [15N]urea were excreted in urine and sweat (10.6 0.4 and 2.4 0.5 mmol, respectively), corresponding to the deamination of 2.3 0.3 g of exogenous alanine (3.1 0.4% of the load). These results confirm that the metabolic fate of the carbon skeleton and the amino-N moiety of exogenous alanine ingested during prolonged exercise at low workload are markedly different. The large positive nitrogen balance (8.5 0.3 g) suggests that in this situation protein synthesis could be increased when a large amount of a single amino acid is ingested.