Study will investigate the impact of training volume on protein metabolism to estimate the impact on daily protein requirements of endurance athletes.

Inclusion Criteria:

Healthy, male, endurance-trained participants who regularly run more than 40 km/week or perform endurance exercise for more than 4.5 hours/week

• Participants who have a self-reported 5-km run time of ≤23 min
• Participants who are categorized as at least "very good" based on a study by Shvartz & Reibold (Aviat Space Environ Med. 1990 Jan;61(1):3-11), 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 on Session 2
• Aged 18-40 years old

Exclusion Criteria:

• - Inability to meet health and physical activity guidelines according to the PAR-Q+.
• Female: hormonal fluctuations associated with the menstrual cycle have been reported to alter protein metabolism during exercise and may influence specific amino acid requirements at rest; as such, the present study will test the effect of endurance exercise on protein requirements in males only to ensure a stable hormonal environment and to increase the homogeneity of the physiological response. (NOTE: we plan to test females in a future trial based off of the results obtained herein)
• Inability to adhere to any of the protocol guidelines (i.e. alcohol, caffeine consumption)
• Regular tobacco use
• Illicit drug use (e.g. growth hormone, testosterone, etc.)