Protein requirements in individuals who participate in endurance-based exercise training have been suggested to be greater than the current recommended dietary allowance (RDA).

The biological value of protein depends on its amino acid composition. Essential amino acids are believed to have main role on whole body protein metabolism. However, it remains to be determined what role non-essential amino acids (NEAA) may have in regulating protein metabolism and contributing to the increased protein requirements after endurance exercise.

The indicator amino acid oxidation (IAAO) method has clarified the individual amino acid requirement in children, normal healthy adult and clinical populations. However, the IAAO method has never been utilized for assessing the effect of NEAA on protein requirements after endurance exercise.

The objective of the current study is to investigate the importance of NEAA for endurance-trained male subjects.

Inclusion Criteria:

• endurance-trained participants who regularly run more than 40 km/week
• Participants who are categorized at least "very good" based on a study by Shvartz & Reibold, in which VO2peak is used as an index. (i.e. the participants whose VO2peak is ≥57 ml/kg/min (18-24 y), ≥54 ml/kg/min (25-29 y), ≥52 ml/kg/min (30-34 y), ≥49 ml/kg/min (35-39 y)
• Ability to complete the 16-km familiarization run in session 2

Exclusion Criteria:

• Inability to meet health and physical activity guidelines according to the physical activity readiness questionnaire (PAR-Q+)
• Inability to adhere to any of the protocol guidelines (i.e. alcohol, caffeine consumption)
• Regular tobacco use
• Illicit drug use