The impact of training volume on protein requirements in endurance trained males was investigated with indicator amino acid oxidation (IAAO) methodology on a recovery day (REST) or after a 10- or 20-km run while consuming a single suboptimal protein intake (0.93 g/kg/d). Phenylalanine excretion (F13CO2; inverse proxy for whole body protein synthesis) was greatest and phenylalanine net balance was lowest on REST compared to post-exercise recovery with no difference between training volumes. Single point F13CO2 was indistinguishable from past IAAO studies using multiple protein intakes. Our results suggest protein requirements may be greatest on recovery days but are not influenced by moderate training volumes in endurance athletes.