The decarboxylation/oxidation and the deamination of ¹³C- and [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 ¹³CO₂ 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.